For my sins I'm monitoring the temperature and relative humidity in
the main document room of our local archive. There are any number of
measuring devices available, some of them cheap single-point things,
others more sophisticated data-loggers that can give a semi-continuous
recording so that they cover nights and weekends etc. I am using one
of the latter, a TZone TZ-TempU03 https://tinyurl.com/nhzszvew.

I am interested in how any of these devices actually measure humidity
and how they convert that into %RH. I assume there's some sort of
sensor, possible a resistor whose resistance is sensitive to the
moisture content in the surrounding air. That variable resistance has
been previously calibrated, and the resistance is then converted to a
figure corresponding to the absolute moisture content in the air,
g/cu.m. Another module then takes that value and using look-up tables
for the saturated moisture content at the temperature given by the
temperature sensor, calculates the %RH.

That is all my speculation BTW - they may not work anything like that.

Because relative humidity is temperature sensitive, I wanted to know
what the absolute moisture content was in the document room to see if
that was changing. I have been calculating the reverse of the above
process, taking the figures for the %RH and the temperature from the
meter and back-calculating to give me the absolute moisture content
g/cu.m., using the equation from here https://tinyurl.com/2prscp3u

I'm not entirely sure I believe the results I'm getting, so my
curiosity was aroused as to how the TZone unit, or any other
temperature and relative humidity unit for that matter, actually
works. Can anyone enlighten me?

--
Chris

On 19/12/2022 11:19, Chris Hogg wrote:
> For my sins I'm monitoring the temperature and relative humidity in
> the main document room of our local archive. There are any number of
> measuring devices available, some of them cheap single-point things,
> others more sophisticated data-loggers that can give a semi-continuous
> recording so that they cover nights and weekends etc. I am using one
> of the latter, a TZone TZ-TempU03 https://tinyurl.com/nhzszvew.
>
> I am interested in how any of these devices actually measure humidity
> and how they convert that into %RH. I assume there's some sort of
> sensor, possible a resistor whose resistance is sensitive to the
> moisture content in the surrounding air. That variable resistance has
> been previously calibrated, and the resistance is then converted to a
> figure corresponding to the absolute moisture content in the air,
> g/cu.m. Another module then takes that value and using look-up tables
> for the saturated moisture content at the temperature given by the
> temperature sensor, calculates the %RH.
>
> That is all my speculation BTW - they may not work anything like that.
>
> Because relative humidity is temperature sensitive, I wanted to know
> what the absolute moisture content was in the document room to see if
> that was changing. I have been calculating the reverse of the above
> process, taking the figures for the %RH and the temperature from the
> meter and back-calculating to give me the absolute moisture content
> g/cu.m., using the equation from here https://tinyurl.com/2prscp3u
>
> I'm not entirely sure I believe the results I'm getting, so my
> curiosity was aroused as to how the TZone unit, or any other
> temperature and relative humidity unit for that matter, actually
> works. Can anyone enlighten me?

I would expect they are using an off the shelf sensor "chip". Most of
these measure temperature and humidity in the same device and are self
calibrating. The oldest examples like the DHT11 had a resistive element
for sensing RH, but the newer ones use a capacitive one.

DHT22/AM2302 are newer and more accurate and can work in a wider range
of temp and RH.

https://components101.com/sensors/dht22-pinout-specs-datasheet

--
Cheers,

John.

/=================================================================\
| Internode Ltd - http://www.internode.co.uk |
|-----------------------------------------------------------------|
| John Rumm - john(at)internode(dot)co(dot)uk |
\=================================================================/

On 19/12/2022 11:42, John Rumm wrote:
> On 19/12/2022 11:19, Chris Hogg wrote:

>
> DHT22/AM2302 are newer and more accurate and can work in a wider range
> of temp and RH.
>
> https://components101.com/sensors/dht22-pinout-specs-datasheet
>
>

Another BigClive video - including checking accuracy

https://www.youtube.com/watch?v=ae8jxXXGWU0

--
mailto : news {at} admac {dot} myzen {dot} co {dot} uk

On 19/12/2022 11:42, John Rumm wrote:
> On 19/12/2022 11:19, Chris Hogg wrote:
>> For my sins I'm monitoring the temperature and relative humidity in
>> the main document room of our local archive. There are any number of
>> measuring devices available, some of them cheap single-point things,
>> others more sophisticated data-loggers that can give a semi-continuous
>> recording so that they cover nights and weekends etc. I am using one
>> of the latter, a TZone TZ-TempU03 https://tinyurl.com/nhzszvew.
>>
>> I am interested in how any of these devices actually measure humidity
>> and how they convert that into %RH. I assume there's some sort of
>> sensor, possible a resistor whose resistance is sensitive to the
>> moisture content in the surrounding air. That variable resistance has
>> been previously calibrated, and the resistance is then converted to a
>> figure corresponding to the absolute moisture content in the air,
>> g/cu.m. Another module then takes that value and using look-up tables
>> for the saturated moisture content at the temperature given by the
>> temperature sensor, calculates the %RH.
>>
>> That is all my speculation BTW - they may not work anything like that.
>>
>> Because relative humidity is temperature sensitive, I wanted to know
>> what the absolute moisture content was in the document room to see if
>> that was changing. I have been calculating the reverse of the above
>> process, taking the figures for the %RH and the temperature from the
>> meter and back-calculating to give me the absolute moisture content
>> g/cu.m., using the equation from here https://tinyurl.com/2prscp3u
>>
>> I'm not entirely sure I believe the results I'm getting, so my
>> curiosity was aroused as to how the TZone unit, or any other
>> temperature and relative humidity unit for that matter, actually
>> works. Can anyone enlighten me?
>
> I would expect they are using an off the shelf sensor "chip". Most of
> these measure temperature and humidity in the same device and are self
> calibrating. The oldest examples like the DHT11 had a resistive element
> for sensing RH, but the newer ones use a capacitive one.
>
> DHT22/AM2302 are newer and more accurate and can work in a wider range
> of temp and RH.
>
> https://components101.com/sensors/dht22-pinout-specs-datasheet
>
>

The Lidl temp+humidity meter I bought recently showed 69%
in my 3rd bedroom/office/computer room. MY ebac dehumidifier
got that down to 50% but the recent cold spell brought it
down to 43%. Now it is back up 54%.

On 19/12/2022 11:19, Chris Hogg wrote:
> For my sins I'm monitoring the temperature and relative humidity in
> the main document room of our local archive. There are any number of
> measuring devices available, some of them cheap single-point things,
> others more sophisticated data-loggers that can give a semi-continuous
> recording so that they cover nights and weekends etc. I am using one
> of the latter, a TZone TZ-TempU03 https://tinyurl.com/nhzszvew.
>
> I am interested in how any of these devices actually measure humidity
> and how they convert that into %RH. I assume there's some sort of
> sensor, possible a resistor whose resistance is sensitive to the
> moisture content in the surrounding air. That variable resistance has
> been previously calibrated, and the resistance is then converted to a
> figure corresponding to the absolute moisture content in the air,
> g/cu.m. Another module then takes that value and using look-up tables
> for the saturated moisture content at the temperature given by the
> temperature sensor, calculates the %RH.
>
> That is all my speculation BTW - they may not work anything like that.
>
> Because relative humidity is temperature sensitive, I wanted to know
> what the absolute moisture content was in the document room to see if
> that was changing. I have been calculating the reverse of the above
> process, taking the figures for the %RH and the temperature from the
> meter and back-calculating to give me the absolute moisture content
> g/cu.m., using the equation from here https://tinyurl.com/2prscp3u
>
> I'm not entirely sure I believe the results I'm getting, so my
> curiosity was aroused as to how the TZone unit, or any other
> temperature and relative humidity unit for that matter, actually
> works. Can anyone enlighten me?
>
FWIW around £20 gets you a Raspberyy Pi Pico W, a BME280 sensor and a
couple of AA batteries to log temperature, humidity (+/- 3% RH accuracy)
and pressure for weeks.

(The "W" so you can sync the Pico's internal clock from wifi to
timestamp the data).

A fun little DIY project, especially if, like me, you've never worked
with microcontrollers before.

--
Reentrant

On Mon, 19 Dec 2022 13:02:39 +0000, alan_m <junk@admac.myzen.co.uk>
wrote:

>On 19/12/2022 11:42, John Rumm wrote:
>> On 19/12/2022 11:19, Chris Hogg wrote:
>
>>
>> DHT22/AM2302 are newer and more accurate and can work in a wider range
>> of temp and RH.
>>
>> https://components101.com/sensors/dht22-pinout-specs-datasheet
>>
>>
>
>Another BigClive video - including checking accuracy
>
>https://www.youtube.com/watch?v=ae8jxXXGWU0

Thanks both. I'm familiar with saturated salt solutions for checking
the accuracy of these things, and have already checked my device in
just the way Big Clive did, in a sealed sandwich box but using several
salts in sequence.
https://www.engineeringtoolbox.com/salt-humidity-d_1887.html

But what interests me are the stages between measuring the resistance
or capacitance of the sensor and arriving at the % relative humidity,
because I'm pretty sure it can't just be a simple case of a direct
calculation or calibration. Somewhere in there the saturated humidity
at the temperature being recorded must be available, in order to
calculate the relative humidity. Even if the absolute humidity (gms of
water per cu.m. air) remained constant, the relative humidity will
change with temperature.

But knowing they can use either resistive or capacitative sensors set
me off on another search, and I've found a web site of a Mfr who seems
to accept questions, so I'll ask there.
https://blog.servoflo.com/humidity-sensors-capacitive-vs-resistive

--
Chris

On 19/12/2022 15:01, Chris Hogg wrote:
> On Mon, 19 Dec 2022 13:02:39 +0000, alan_m <junk@admac.myzen.co.uk>
> wrote:
>
>> On 19/12/2022 11:42, John Rumm wrote:
>>> On 19/12/2022 11:19, Chris Hogg wrote:
>>
>>>
>>> DHT22/AM2302 are newer and more accurate and can work in a wider range
>>> of temp and RH.
>>>
>>> https://components101.com/sensors/dht22-pinout-specs-datasheet
>>>
>>>
>>
>> Another BigClive video - including checking accuracy
>>
>> https://www.youtube.com/watch?v=ae8jxXXGWU0
>
> Thanks both. I'm familiar with saturated salt solutions for checking
> the accuracy of these things, and have already checked my device in
> just the way Big Clive did, in a sealed sandwich box but using several
> salts in sequence.
> https://www.engineeringtoolbox.com/salt-humidity-d_1887.html
>
> But what interests me are the stages between measuring the resistance
> or capacitance of the sensor and arriving at the % relative humidity,
> because I'm pretty sure it can't just be a simple case of a direct
> calculation or calibration. Somewhere in there the saturated humidity
> at the temperature being recorded must be available, in order to
> calculate the relative humidity. Even if the absolute humidity (gms of
> water per cu.m. air) remained constant, the relative humidity will
> change with temperature.
>
> But knowing they can use either resistive or capacitative sensors set
> me off on another search, and I've found a web site of a Mfr who seems
> to accept questions, so I'll ask there.
> https://blog.servoflo.com/humidity-sensors-capacitive-vs-resistive

The one Clive was looking at there was a more basic device - basically
just the sensor element itself, and it relies on external circuitry to
drive it, also to read temperature, and compute the RH.

The ones I linked to ealier have their own built in microcontroller,
that handle all the reading, calibration and outputs the final value.
Those rely on a proprietary protocol that you need to manually "bit
bang" to communicate with. There are also some that are similar but they
support I2C which makes them much simpler to interface with[1]

[1] Well kind of, there are standard libraries that do all the bit
banged IO to the other type - so from an application developer's point
of view there is probably not much in it!

--
Cheers,

John.

/=================================================================\
| Internode Ltd - http://www.internode.co.uk |
|-----------------------------------------------------------------|
| John Rumm - john(at)internode(dot)co(dot)uk |
\=================================================================/

On Mon, 19 Dec 2022 13:55:36 +0000, Reentrant
<reentrant@invalid.org.uk> wrote:

>On 19/12/2022 11:19, Chris Hogg wrote:
>> For my sins I'm monitoring the temperature and relative humidity in
>> the main document room of our local archive. There are any number of
>> measuring devices available, some of them cheap single-point things,
>> others more sophisticated data-loggers that can give a semi-continuous
>> recording so that they cover nights and weekends etc. I am using one
>> of the latter, a TZone TZ-TempU03 https://tinyurl.com/nhzszvew.
>>
>> I am interested in how any of these devices actually measure humidity
>> and how they convert that into %RH. I assume there's some sort of
>> sensor, possible a resistor whose resistance is sensitive to the
>> moisture content in the surrounding air. That variable resistance has
>> been previously calibrated, and the resistance is then converted to a
>> figure corresponding to the absolute moisture content in the air,
>> g/cu.m. Another module then takes that value and using look-up tables
>> for the saturated moisture content at the temperature given by the
>> temperature sensor, calculates the %RH.
>>
>> That is all my speculation BTW - they may not work anything like that.
>>
>> Because relative humidity is temperature sensitive, I wanted to know
>> what the absolute moisture content was in the document room to see if
>> that was changing. I have been calculating the reverse of the above
>> process, taking the figures for the %RH and the temperature from the
>> meter and back-calculating to give me the absolute moisture content
>> g/cu.m., using the equation from here https://tinyurl.com/2prscp3u
>>
>> I'm not entirely sure I believe the results I'm getting, so my
>> curiosity was aroused as to how the TZone unit, or any other
>> temperature and relative humidity unit for that matter, actually
>> works. Can anyone enlighten me?
>>
>FWIW around £20 gets you a Raspberyy Pi Pico W, a BME280 sensor and a
>couple of AA batteries to log temperature, humidity (+/- 3% RH accuracy)
>and pressure for weeks.
>
>(The "W" so you can sync the Pico's internal clock from wifi to
>timestamp the data).
>
>A fun little DIY project, especially if, like me, you've never worked
>with microcontrollers before.

Unnecessarily complicated. Those little circular guages you get from
pet shops for reptiles are cheap as chips, use no battery and are
surprisingly accurate and sensitive. I keep one in the room where I
work on a lot of static sensitive test equipment. If it drops below
75%, I know I need to put an anti-static bracelet on. I'm terrible for
generating static and blowing things up.

On Mon, 19 Dec 2022 22:19:01 +1100, Chris Hogg <me@privacy.net> wrote:

> For my sins I'm monitoring the temperature and relative humidity in
> the main document room of our local archive. There are any number of
> measuring devices available, some of them cheap single-point things,
> others more sophisticated data-loggers that can give a semi-continuous
> recording so that they cover nights and weekends etc. I am using one
> of the latter, a TZone TZ-TempU03 https://tinyurl.com/nhzszvew.
>
> I am interested in how any of these devices actually measure humidity
> and how they convert that into %RH. I assume there's some sort of
> sensor, possible a resistor whose resistance is sensitive to the
> moisture content in the surrounding air.

Normally now. Previously it could be hair whose length changes with
humidity.

Wet and dry bulb is also used but that needs a water
reservoir that needs to be topped up so you only see
that with proper scientific instruments now.

There are also chilled mirror and gravimetric systems.
https://en.wikipedia.org/wiki/Humidity#Measurement

> That variable resistance has
> been previously calibrated, and the resistance is then converted to a
> figure corresponding to the absolute moisture content in the air,
> g/cu.m. Another module then takes that value and using look-up tables
> for the saturated moisture content at the temperature given by the
> temperature sensor, calculates the %RH.
>
> That is all my speculation BTW - they may not work anything like that.
>
> Because relative humidity is temperature sensitive, I wanted to know
> what the absolute moisture content was in the document room to see if
> that was changing. I have been calculating the reverse of the above
> process, taking the figures for the %RH and the temperature from the
> meter and back-calculating to give me the absolute moisture content
> g/cu.m., using the equation from here https://tinyurl.com/2prscp3u
>
> I'm not entirely sure I believe the results I'm getting, so my
> curiosity was aroused as to how the TZone unit, or any other
> temperature and relative humidity unit for that matter, actually
> works. Can anyone enlighten me?

It shouldnt be hard to check it visually. It will be the sensor whose
resistance changes with relative humidity.

On 19/12/2022 16:43, Cursitor Doom wrote:
> On Mon, 19 Dec 2022 13:55:36 +0000, Reentrant
> <reentrant@invalid.org.uk> wrote:
>
>> On 19/12/2022 11:19, Chris Hogg wrote:
>>> For my sins I'm monitoring the temperature and relative humidity in
>>> the main document room of our local archive. There are any number of
>>> measuring devices available, some of them cheap single-point things,
>>> others more sophisticated data-loggers that can give a semi-continuous
>>> recording so that they cover nights and weekends etc. I am using one
>>> of the latter, a TZone TZ-TempU03 https://tinyurl.com/nhzszvew.
>>>
>>> I am interested in how any of these devices actually measure humidity
>>> and how they convert that into %RH. I assume there's some sort of
>>> sensor, possible a resistor whose resistance is sensitive to the
>>> moisture content in the surrounding air. That variable resistance has
>>> been previously calibrated, and the resistance is then converted to a
>>> figure corresponding to the absolute moisture content in the air,
>>> g/cu.m. Another module then takes that value and using look-up tables
>>> for the saturated moisture content at the temperature given by the
>>> temperature sensor, calculates the %RH.
>>>
>>> That is all my speculation BTW - they may not work anything like that.
>>>
>>> Because relative humidity is temperature sensitive, I wanted to know
>>> what the absolute moisture content was in the document room to see if
>>> that was changing. I have been calculating the reverse of the above
>>> process, taking the figures for the %RH and the temperature from the
>>> meter and back-calculating to give me the absolute moisture content
>>> g/cu.m., using the equation from here https://tinyurl.com/2prscp3u
>>>
>>> I'm not entirely sure I believe the results I'm getting, so my
>>> curiosity was aroused as to how the TZone unit, or any other
>>> temperature and relative humidity unit for that matter, actually
>>> works. Can anyone enlighten me?
>>>
>> FWIW around £20 gets you a Raspberyy Pi Pico W, a BME280 sensor and a
>> couple of AA batteries to log temperature, humidity (+/- 3% RH accuracy)
>> and pressure for weeks.
>>
>> (The "W" so you can sync the Pico's internal clock from wifi to
>> timestamp the data).
>>
>> A fun little DIY project, especially if, like me, you've never worked
>> with microcontrollers before.
>
> Unnecessarily complicated. Those little circular guages you get from
> pet shops for reptiles are cheap as chips, use no battery and are
> surprisingly accurate and sensitive. I keep one in the room where I
> work on a lot of static sensitive test equipment. If it drops below
> 75%, I know I need to put an anti-static bracelet on. I'm terrible for
> generating static and blowing things up.

Wrong forum if you're not DIYing a solution!

But surely it depends on what your requirements are, I was curious how
the temperature and humidity changed in our garage overnight during the
freeze. The Pico logs timestamped temperature, humidity and pressure at
5-minute intervals in a CSV file.

I'd assumed the humidity part of the sensor worked via capacitance. As
it also knows T and P it can work out RH.

--
Reentrant

On Tue, 20 Dec 2022 04:03:33 +1100, chop <chop654@gmail.com> wrote:

>
>It shouldnt be hard to check it visually. It will be the sensor whose
>resistance changes with relative humidity.

I don't think so. The resistance changes with the absolute moisture
content, and the device then goes on to calculate the relative
humidity from the measured temperature and the saturation humidity for
that temperature. The latter at least would require some sort of
look-up table I would think.

--
Chris

On 19/12/2022 16:43, Cursitor Doom wrote:
> On Mon, 19 Dec 2022 13:55:36 +0000, Reentrant
> <reentrant@invalid.org.uk> wrote:
>
>> On 19/12/2022 11:19, Chris Hogg wrote:
>>> For my sins I'm monitoring the temperature and relative humidity in
>>> the main document room of our local archive. There are any number of
>>> measuring devices available, some of them cheap single-point things,
>>> others more sophisticated data-loggers that can give a semi-continuous
>>> recording so that they cover nights and weekends etc. I am using one
>>> of the latter, a TZone TZ-TempU03 https://tinyurl.com/nhzszvew.
>>>
>>> I am interested in how any of these devices actually measure humidity
>>> and how they convert that into %RH. I assume there's some sort of
>>> sensor, possible a resistor whose resistance is sensitive to the
>>> moisture content in the surrounding air. That variable resistance has
>>> been previously calibrated, and the resistance is then converted to a
>>> figure corresponding to the absolute moisture content in the air,
>>> g/cu.m. Another module then takes that value and using look-up tables
>>> for the saturated moisture content at the temperature given by the
>>> temperature sensor, calculates the %RH.
>>>
>>> That is all my speculation BTW - they may not work anything like that.
>>>
>>> Because relative humidity is temperature sensitive, I wanted to know
>>> what the absolute moisture content was in the document room to see if
>>> that was changing. I have been calculating the reverse of the above
>>> process, taking the figures for the %RH and the temperature from the
>>> meter and back-calculating to give me the absolute moisture content
>>> g/cu.m., using the equation from here https://tinyurl.com/2prscp3u
>>>
>>> I'm not entirely sure I believe the results I'm getting, so my
>>> curiosity was aroused as to how the TZone unit, or any other
>>> temperature and relative humidity unit for that matter, actually
>>> works. Can anyone enlighten me?
>>>
>> FWIW around £20 gets you a Raspberyy Pi Pico W, a BME280 sensor and a
>> couple of AA batteries to log temperature, humidity (+/- 3% RH accuracy)
>> and pressure for weeks.
>>
>> (The "W" so you can sync the Pico's internal clock from wifi to
>> timestamp the data).
>>
>> A fun little DIY project, especially if, like me, you've never worked
>> with microcontrollers before.
>
> Unnecessarily complicated.

Did you miss the fun DIY project criterion?

> Those little circular guages you get from
> pet shops for reptiles are cheap as chips, use no battery and are
> surprisingly accurate and sensitive. I keep one in the room where I
> work on a lot of static sensitive test equipment. If it drops below
> 75%, I know I need to put an anti-static bracelet on. I'm terrible for
> generating static and blowing things up.

So how do you get that to log data then?

--
Cheers,

John.

/=================================================================\
| Internode Ltd - http://www.internode.co.uk |
|-----------------------------------------------------------------|
| John Rumm - john(at)internode(dot)co(dot)uk |
\=================================================================/

On Tue, 20 Dec 2022 04:03:33 +1100, chop wrote:

> Wet and dry bulb is also used but that needs a water reservoir that
> needs to be topped up so you only see that with proper scientific
> instruments now.

I bought a basic one a few years ago, not from an instrument supplier.
Works pretty well; it's right beside me now, on the wall.

--
My posts are my copyright and if @diy_forums or Home Owners' Hub
wish to copy them they can pay me £1 a message.
Use the BIG mirror service in the UK: http://www.mirrorservice.org
*lightning surge protection* - a w_tom conductor

I don't think they are officially called that I thought it was hygrometers,
but I do recall they must now be electronic ones as a clock sold by the
Science Museum some time ago had one as well as other sensors and it could
speak them as well.
Brian

--

--:
This newsgroup posting comes to you directly from...
The Sofa of Brian Gaff...
briang1@blueyonder.co.uk
Blind user, so no pictures please
Note this Signature is meaningless.!
"Chris Hogg" <me@privacy.net> wrote in message
news:2ve0qhd3j4inopueola5ss5ccrnucp886c@4ax.com...
> For my sins I'm monitoring the temperature and relative humidity in
> the main document room of our local archive. There are any number of
> measuring devices available, some of them cheap single-point things,
> others more sophisticated data-loggers that can give a semi-continuous
> recording so that they cover nights and weekends etc. I am using one
> of the latter, a TZone TZ-TempU03 https://tinyurl.com/nhzszvew.
>
> I am interested in how any of these devices actually measure humidity
> and how they convert that into %RH. I assume there's some sort of
> sensor, possible a resistor whose resistance is sensitive to the
> moisture content in the surrounding air. That variable resistance has
> been previously calibrated, and the resistance is then converted to a
> figure corresponding to the absolute moisture content in the air,
> g/cu.m. Another module then takes that value and using look-up tables
> for the saturated moisture content at the temperature given by the
> temperature sensor, calculates the %RH.
>
> That is all my speculation BTW - they may not work anything like that.
>
> Because relative humidity is temperature sensitive, I wanted to know
> what the absolute moisture content was in the document room to see if
> that was changing. I have been calculating the reverse of the above
> process, taking the figures for the %RH and the temperature from the
> meter and back-calculating to give me the absolute moisture content
> g/cu.m., using the equation from here https://tinyurl.com/2prscp3u
>
> I'm not entirely sure I believe the results I'm getting, so my
> curiosity was aroused as to how the TZone unit, or any other
> temperature and relative humidity unit for that matter, actually
> works. Can anyone enlighten me?
>
> --
>
> Chris

On 19/12/2022 23:38, Bob Eager wrote:
> On Tue, 20 Dec 2022 04:03:33 +1100, chop wrote:
>
>> Wet and dry bulb is also used but that needs a water reservoir that
>> needs to be topped up so you only see that with proper scientific
>> instruments now.
>
> I bought a basic one a few years ago, not from an instrument supplier.
> Works pretty well; it's right beside me now, on the wall.
>
>
>

But they only work correctly when there is an airflow across
the wet bulb and a specific speed. Kept indoors and away
from drafts they give a higher wet bulb reading than
expected. I have a Brannen greenhouse mercury wet/dry
thermometer in the house and even in that heatwave the
difference between dry and wet never exceeded 4C

On Mon, 19 Dec 2022 23:22:49 +0000, John Rumm
<see.my.signature@nowhere.null> wrote:

>On 19/12/2022 16:43, Cursitor Doom wrote:
>> On Mon, 19 Dec 2022 13:55:36 +0000, Reentrant
>> <reentrant@invalid.org.uk> wrote:
>>
>>> On 19/12/2022 11:19, Chris Hogg wrote:
>>>> For my sins I'm monitoring the temperature and relative humidity in
>>>> the main document room of our local archive. There are any number of
>>>> measuring devices available, some of them cheap single-point things,
>>>> others more sophisticated data-loggers that can give a semi-continuous
>>>> recording so that they cover nights and weekends etc. I am using one
>>>> of the latter, a TZone TZ-TempU03 https://tinyurl.com/nhzszvew.
>>>>
>>>> I am interested in how any of these devices actually measure humidity
>>>> and how they convert that into %RH. I assume there's some sort of
>>>> sensor, possible a resistor whose resistance is sensitive to the
>>>> moisture content in the surrounding air. That variable resistance has
>>>> been previously calibrated, and the resistance is then converted to a
>>>> figure corresponding to the absolute moisture content in the air,
>>>> g/cu.m. Another module then takes that value and using look-up tables
>>>> for the saturated moisture content at the temperature given by the
>>>> temperature sensor, calculates the %RH.
>>>>
>>>> That is all my speculation BTW - they may not work anything like that.
>>>>
>>>> Because relative humidity is temperature sensitive, I wanted to know
>>>> what the absolute moisture content was in the document room to see if
>>>> that was changing. I have been calculating the reverse of the above
>>>> process, taking the figures for the %RH and the temperature from the
>>>> meter and back-calculating to give me the absolute moisture content
>>>> g/cu.m., using the equation from here https://tinyurl.com/2prscp3u
>>>>
>>>> I'm not entirely sure I believe the results I'm getting, so my
>>>> curiosity was aroused as to how the TZone unit, or any other
>>>> temperature and relative humidity unit for that matter, actually
>>>> works. Can anyone enlighten me?
>>>>
>>> FWIW around £20 gets you a Raspberyy Pi Pico W, a BME280 sensor and a
>>> couple of AA batteries to log temperature, humidity (+/- 3% RH accuracy)
>>> and pressure for weeks.
>>>
>>> (The "W" so you can sync the Pico's internal clock from wifi to
>>> timestamp the data).
>>>
>>> A fun little DIY project, especially if, like me, you've never worked
>>> with microcontrollers before.
>>
>> Unnecessarily complicated.
>
>Did you miss the fun DIY project criterion?
>
>> Those little circular guages you get from
>> pet shops for reptiles are cheap as chips, use no battery and are
>> surprisingly accurate and sensitive. I keep one in the room where I
>> work on a lot of static sensitive test equipment. If it drops below
>> 75%, I know I need to put an anti-static bracelet on. I'm terrible for
>> generating static and blowing things up.
>
>So how do you get that to log data then?

Dunno.

On Wed, 21 Dec 2022 06:16:42 +1100, Andrew
<Andrew97d-junk@mybtinternet.com> wrote:

> On 19/12/2022 23:38, Bob Eager wrote:
>> On Tue, 20 Dec 2022 04:03:33 +1100, chop wrote:
>>
>>> Wet and dry bulb is also used but that needs a water reservoir that
>>> needs to be topped up so you only see that with proper scientific
>>> instruments now.
>> I bought a basic one a few years ago, not from an instrument supplier.
>> Works pretty well; it's right beside me now, on the wall.

> But they only work correctly when there is an airflow across
> the wet bulb and a specific speed.

That's bullshit with the specific speed claim. They were in
fact the standard instrument used in standard stevenson
screens used in stardard weather stations for more than
a century.

> Kept indoors and away
> from drafts they give a higher wet bulb reading than
> expected. I have a Brannen greenhouse mercury wet/dry
> thermometer in the house and even in that heatwave the
> difference between dry and wet never exceeded 4C

That's because the humidity was so high.

On 19/12/2022 13:55, Reentrant wrote:
> FWIW around £20 gets you a Raspberyy Pi Pico W, a BME280 sensor and a
> couple of AA batteries to log temperature, humidity (+/- 3% RH accuracy)
> and pressure for weeks.
>
> (The "W" so you can sync the Pico's internal clock from wifi to
> timestamp the data).
>
> A fun little DIY project, especially if, like me, you've never worked
> with microcontrollers before.

I built several raspberry pi based environmental controllers over the
years. Many of the cheaper humidity sensors are only accurate in the
middle of the range. I wanted something that was accurate up nearer
100%RH which is more taxing.

I went for the Sensirion SHT75 which is very accurate from 10-90%RH, and
outside that the accuracy only tails off by a factor of 2, so still
quite good. They were expensive at around £25 each. They also sold the
ones which failed to meet spec as SHT71 for half the price, which was
good enough for one of my applications. They are a proprietary
interface, a bit like I2C without addressing, which meant I had to
bit-bang them, but that worked fine. I've had some of these operating
continuously for 10 years now, including outdoors, and they've been rock
solid. Sadly, Sensirion stopped producing them a few years ago because
they couldn't get the cost any lower - they were very expensive to
manufacture. I think I'm down to one left in my sock now.

I bought an SHT31 which is a new design from them, and is properly I2C
so I don't have to write a bit-banging driver, just software to use the
standard I2C API. This works fine. It's almost as accurate as the SHT75
was, but because the accuracy doesn't vary between 0-100%RH, it's
actually much more accurate at the extremes. It's cheaper too, but being
surface mount, is a bit more fiddly to deal with at the DIY level (not
just the mounting issues, but also that if just mounted on a normal
board, it's in the dead air space which wouldn't be good for accurately
measuring temperature and humidity, so it needs raising off the board).

Andrew

On 20/12/2022 22:25, Andrew Gabriel wrote:
> On 19/12/2022 13:55, Reentrant wrote:
>> FWIW around £20 gets you a Raspberyy Pi Pico W, a BME280 sensor and a
>> couple of AA batteries to log temperature, humidity (+/- 3% RH
>> accuracy) and pressure for weeks.
>>
>> (The "W" so you can sync the Pico's internal clock from wifi to
>> timestamp the data).
>>
>> A fun little DIY project, especially if, like me, you've never worked
>> with microcontrollers before.
>
> I built several raspberry pi based environmental controllers over the
> years. Many of the cheaper humidity sensors are only accurate in the
> middle of the range. I wanted something that was accurate up nearer
> 100%RH which is more taxing.
>
> I went for the Sensirion SHT75 which is very accurate from 10-90%RH, and
> outside that the accuracy only tails off by a factor of 2, so still
> quite good. They were expensive at around £25 each. They also sold the
> ones which failed to meet spec as SHT71 for half the price, which was
> good enough for one of my applications. They are a proprietary
> interface, a bit like I2C without addressing, which meant I had to
> bit-bang them, but that worked fine. I've had some of these operating
> continuously for 10 years now, including outdoors, and they've been rock
> solid. Sadly, Sensirion stopped producing them a few years ago because
> they couldn't get the cost any lower - they were very expensive to
> manufacture. I think I'm down to one left in my sock now.
>
> I bought an SHT31 which is a new design from them, and is properly I2C
> so I don't have to write a bit-banging driver, just software to use the
> standard I2C API. This works fine. It's almost as accurate as the SHT75
> was, but because the accuracy doesn't vary between 0-100%RH, it's
> actually much more accurate at the extremes. It's cheaper too, but being
> surface mount, is a bit more fiddly to deal with at the DIY level (not
> just the mounting issues, but also that if just mounted on a normal
> board, it's in the dead air space which wouldn't be good for accurately
> measuring temperature and humidity, so it needs raising off the board).

It is a small device. I was going to say that with a small soldering
iron, flux and some copper braid it's shouldn't be difficult to mount.

However, why not use something like:
https://www.adafruit.com/product/2857

On 20/12/2022 22:36, Fredxx wrote:
> On 20/12/2022 22:25, Andrew Gabriel wrote:
>> On 19/12/2022 13:55, Reentrant wrote:
>>> FWIW around £20 gets you a Raspberyy Pi Pico W, a BME280 sensor and a
>>> couple of AA batteries to log temperature, humidity (+/- 3% RH
>>> accuracy) and pressure for weeks.
>>>
>>> (The "W" so you can sync the Pico's internal clock from wifi to
>>> timestamp the data).
>>>
>>> A fun little DIY project, especially if, like me, you've never worked
>>> with microcontrollers before.
>>
>> I built several raspberry pi based environmental controllers over the
>> years. Many of the cheaper humidity sensors are only accurate in the
>> middle of the range. I wanted something that was accurate up nearer
>> 100%RH which is more taxing.
>>
>> I went for the Sensirion SHT75 which is very accurate from 10-90%RH,
>> and outside that the accuracy only tails off by a factor of 2, so
>> still quite good. They were expensive at around £25 each. They also
>> sold the ones which failed to meet spec as SHT71 for half the price,
>> which was good enough for one of my applications. They are a
>> proprietary interface, a bit like I2C without addressing, which meant
>> I had to bit-bang them, but that worked fine. I've had some of these
>> operating continuously for 10 years now, including outdoors, and
>> they've been rock solid. Sadly, Sensirion stopped producing them a few
>> years ago because they couldn't get the cost any lower - they were
>> very expensive to manufacture. I think I'm down to one left in my sock
>> now.
>>
>> I bought an SHT31 which is a new design from them, and is properly I2C
>> so I don't have to write a bit-banging driver, just software to use
>> the standard I2C API. This works fine. It's almost as accurate as the
>> SHT75 was, but because the accuracy doesn't vary between 0-100%RH,
>> it's actually much more accurate at the extremes. It's cheaper too,
>> but being surface mount, is a bit more fiddly to deal with at the DIY
>> level (not just the mounting issues, but also that if just mounted on
>> a normal board, it's in the dead air space which wouldn't be good for
>> accurately measuring temperature and humidity, so it needs raising off
>> the board).
>
> It is a small device. I was going to say that with a small soldering
> iron, flux and some copper braid it's shouldn't be difficult to mount.

That's exactly what I did do to test it:
http://www.cucumber.me.uk/SHT31.jpg
(The thicker grey wires are just guards to prevent it being knocked.)
Soldering to a chip which is only 2.5mm square with 8 inset/recessed
terminals is quite a challenge, even though I only needed 5 connections
to it. It took a while.

I modified someone else's driver to make full use of its features,
adding actually checking the CRC and calculating the dew point, and
measuring its response time (out of curiosity):
https://github.com/andrew-gabriel/SHT31

> However, why not use something like:
> https://www.adafruit.com/product/2857

I see the Adafruit board does attempt to break the dead air space issue
with the board cutouts.

Andrew

On Mon, 19 Dec 2022 11:19:01 +0000, Chris Hogg <me@privacy.net> wrote:

>I'm not entirely sure I believe the results I'm getting, so my
>curiosity was aroused as to how the TZone unit, or any other
>temperature and relative humidity unit for that matter, actually
>works. Can anyone enlighten me?

From memory -- and I can look some things up if you need hard data:

Capacitive sensor work with a finger arrangement, and a dielectric between
these: the dielectric absorbs moisture form the air, and sin Water has a high
epsilon_r, this changes the capacitance greatly. ISTR these measure rH, so the
amount of water in the dielectric varies with the relative humidity. (Though
pretty much any and every sensor is temperature-dependent). Capacitive sensors
are often used to determine the frequency of an oscillator, and this can then
easily be measured by a processor.

The variation will be nonlinear. In a more expensive measuring unit, I found a
calibration function for two set points, ISTR 13 % rh and 76 % rh, corresponding
to saturated salt solutions of lithium chloride and sodium chloride. Both at
somewhere around 20 °C. In a cheaper one, the function will be a
one-size-fits-all calculated from the mfg data sheet using typical values.

Resistive sensors may be linearized using a parallel and series resistor, to
give a nearly-flat response of part of the range.

There's also an industrial sensor, accurate and measures absolute humidity.
Downside is it needs a lot of power (relatively) and it must be continuously
on: Lithium chloride cells. A sock of lithium chloride solution is heated, and
then the steady-state temperature is measured. The LiCl absorbs water, it is
hygroscopic und ~13% rh, and the heat drives the water off. The temperature is a
measure of the dew point. A problem is that if left in humid air, and not
heated, the LiCl can drip off. You may be able to get one used, cheaply?

Then there's the variations on "hair in air": various materials that change
their length with (usually) relative humidity. Some of the cheaper "humidistats"
work like that: a material expands and contracts, and operates a click-bendy
spring to open and close a contact, knob to adjust the switch point. Lots of
hysteresis included for free -- good thing if running a compressor dehumidifier.

Thomas Prufer

On 21/12/2022 11:00, Andrew Gabriel wrote:
> On 20/12/2022 22:36, Fredxx wrote:
>> On 20/12/2022 22:25, Andrew Gabriel wrote:
>>> On 19/12/2022 13:55, Reentrant wrote:
>>>> FWIW around £20 gets you a Raspberyy Pi Pico W, a BME280 sensor and
>>>> a couple of AA batteries to log temperature, humidity (+/- 3% RH
>>>> accuracy) and pressure for weeks.
>>>>
>>>> (The "W" so you can sync the Pico's internal clock from wifi to
>>>> timestamp the data).
>>>>
>>>> A fun little DIY project, especially if, like me, you've never
>>>> worked with microcontrollers before.
>>>
>>> I built several raspberry pi based environmental controllers over the
>>> years. Many of the cheaper humidity sensors are only accurate in the
>>> middle of the range. I wanted something that was accurate up nearer
>>> 100%RH which is more taxing.
>>>
>>> I went for the Sensirion SHT75 which is very accurate from 10-90%RH,
>>> and outside that the accuracy only tails off by a factor of 2, so
>>> still quite good. They were expensive at around £25 each. They also
>>> sold the ones which failed to meet spec as SHT71 for half the price,
>>> which was good enough for one of my applications. They are a
>>> proprietary interface, a bit like I2C without addressing, which meant
>>> I had to bit-bang them, but that worked fine. I've had some of these
>>> operating continuously for 10 years now, including outdoors, and
>>> they've been rock solid. Sadly, Sensirion stopped producing them a
>>> few years ago because they couldn't get the cost any lower - they
>>> were very expensive to manufacture. I think I'm down to one left in
>>> my sock now.
>>>
>>> I bought an SHT31 which is a new design from them, and is properly
>>> I2C so I don't have to write a bit-banging driver, just software to
>>> use the standard I2C API. This works fine. It's almost as accurate as
>>> the SHT75 was, but because the accuracy doesn't vary between
>>> 0-100%RH, it's actually much more accurate at the extremes. It's
>>> cheaper too, but being surface mount, is a bit more fiddly to deal
>>> with at the DIY level (not just the mounting issues, but also that if
>>> just mounted on a normal board, it's in the dead air space which
>>> wouldn't be good for accurately measuring temperature and humidity,
>>> so it needs raising off the board).
>>
>> It is a small device. I was going to say that with a small soldering
>> iron, flux and some copper braid it's shouldn't be difficult to mount.
>
> That's exactly what I did do to test it:
> http://www.cucumber.me.uk/SHT31.jpg
> (The thicker grey wires are just guards to prevent it being knocked.)
> Soldering to a chip which is only 2.5mm square with 8 inset/recessed
> terminals is quite a challenge, even though I only needed 5 connections
> to it. It took a while.

I think using something like this would have been much easier:
https://www.hobbytronics.co.uk/smd-adapter-boards

I have attached wires to small devices but as you found it is tricky and
also fragile.

> I modified someone else's driver to make full use of its features,
> adding actually checking the CRC and calculating the dew point, and
> measuring its response time (out of curiosity):
> https://github.com/andrew-gabriel/SHT31
>
> > However, why not use something like:
> >    https://www.adafruit.com/product/2857
>
> I see the Adafruit board does attempt to break the dead air space issue
> with the board cutouts.

I don't understand the issue?

On 21/12/2022 11:29, Fredxx wrote:
> On 21/12/2022 11:00, Andrew Gabriel wrote:
>> On 20/12/2022 22:36, Fredxx wrote:
>>> On 20/12/2022 22:25, Andrew Gabriel wrote:
>>>> On 19/12/2022 13:55, Reentrant wrote:
>>>>> FWIW around £20 gets you a Raspberyy Pi Pico W, a BME280 sensor and
>>>>> a couple of AA batteries to log temperature, humidity (+/- 3% RH
>>>>> accuracy) and pressure for weeks.
>>>>>
>>>>> (The "W" so you can sync the Pico's internal clock from wifi to
>>>>> timestamp the data).
>>>>>
>>>>> A fun little DIY project, especially if, like me, you've never
>>>>> worked with microcontrollers before.
>>>>
>>>> I built several raspberry pi based environmental controllers over
>>>> the years. Many of the cheaper humidity sensors are only accurate in
>>>> the middle of the range. I wanted something that was accurate up
>>>> nearer 100%RH which is more taxing.
>>>>
>>>> I went for the Sensirion SHT75 which is very accurate from 10-90%RH,
>>>> and outside that the accuracy only tails off by a factor of 2, so
>>>> still quite good. They were expensive at around £25 each. They also
>>>> sold the ones which failed to meet spec as SHT71 for half the price,
>>>> which was good enough for one of my applications. They are a
>>>> proprietary interface, a bit like I2C without addressing, which
>>>> meant I had to bit-bang them, but that worked fine. I've had some of
>>>> these operating continuously for 10 years now, including outdoors,
>>>> and they've been rock solid. Sadly, Sensirion stopped producing them
>>>> a few years ago because they couldn't get the cost any lower - they
>>>> were very expensive to manufacture. I think I'm down to one left in
>>>> my sock now.
>>>>
>>>> I bought an SHT31 which is a new design from them, and is properly
>>>> I2C so I don't have to write a bit-banging driver, just software to
>>>> use the standard I2C API. This works fine. It's almost as accurate
>>>> as the SHT75 was, but because the accuracy doesn't vary between
>>>> 0-100%RH, it's actually much more accurate at the extremes. It's
>>>> cheaper too, but being surface mount, is a bit more fiddly to deal
>>>> with at the DIY level (not just the mounting issues, but also that
>>>> if just mounted on a normal board, it's in the dead air space which
>>>> wouldn't be good for accurately measuring temperature and humidity,
>>>> so it needs raising off the board).
>>>
>>> It is a small device. I was going to say that with a small soldering
>>> iron, flux and some copper braid it's shouldn't be difficult to mount.
>>
>> That's exactly what I did do to test it:
>> http://www.cucumber.me.uk/SHT31.jpg
>> (The thicker grey wires are just guards to prevent it being knocked.)
>> Soldering to a chip which is only 2.5mm square with 8 inset/recessed
>> terminals is quite a challenge, even though I only needed 5
>> connections to it. It took a while.
>
> I think using something like this would have been much easier:
>   https://www.hobbytronics.co.uk/smd-adapter-boards
>
> I have attached wires to small devices but as you found it is tricky and
> also fragile.
>
>> I modified someone else's driver to make full use of its features,
>> adding actually checking the CRC and calculating the dew point, and
>> measuring its response time (out of curiosity):
>> https://github.com/andrew-gabriel/SHT31
>>
>>  > However, why not use something like:
>>  >    https://www.adafruit.com/product/2857
>>
>> I see the Adafruit board does attempt to break the dead air space
>> issue with the board cutouts.
>
> I don't understand the issue?

If you want to measure dynamic properties of air such as temperature and
humidity, you get inaccurate results if you do this close to a surface
(such as a PCB). This is because you get a layer of dead air against a
surface which doesn't move, so it may be stale and not have the same
properties as the bulk of the air which is what you intended to measure.
Sensors therefore need to be spaced away from expanses of circuit board
to give accurate results. There can be other reasons for doing this too,
such as to avoid conducting heat generated by other components to the
sensor, and FR4 which most PCBs are made of does absorb and release some
moisture, so this would also contaminate the dead air space which will
be relevant to humidity sensing.

If you look at the design of the older SHT75 sensor, this gets around
these issues by being available in what they call a pin format, which
means the sensor is easily positioned about an inch away from the board,
and the pin board it comes mounted on is no wider than the sensor.
Sensirion did say at the time the SHT75 was discontinued that they'd
look at making some of their other sensors available in pin format, but
I haven't looked to see if they did.

Andrew

On 21/12/2022 11:27, Thomas Prufer wrote:
> Then there's the variations on "hair in air": various materials that change
> their length with (usually) relative humidity. Some of the cheaper "humidistats"
> work like that: a material expands and contracts, and operates a click-bendy
> spring to open and close a contact, knob to adjust the switch point. Lots of
> hysteresis included for free -- good thing if running a compressor dehumidifier.

I have a big elaborate one of these I saved from a 1970's computer room.
It has a tall perforated tube coming out of the top with horses hair
stretched in it (like a violin bow). The case under this with a glass
front has a circular paper graph and a clockwork drive which will rotate
it once a day or once a week, and a pen plots the humidity based on the
varying length of the horse hair. Another pen does same for temperature.

Never used it, but I couldn't bare the thought of it going in a skip,
which was where it was destined to go. At the time, I did establish that
the graphing paper disks were still available, but that was decades ago.
Nowadays, I could print them of course.

Andrew

On 21/12/2022 12:25, Andrew Gabriel wrote:
> If you want to measure dynamic properties of air such as temperature and
> humidity, you get inaccurate results if you do this close to a surface
> (such as a PCB).

LOL. The hot wire air mass sensors in some F1 car engines were a
disaster in full wet conditions.

The pitot tubes in the Air France Airbus that crashed into the
Atlantic, didn't work when covered in ice.

--
“It is hard to imagine a more stupid decision or more dangerous way of
making decisions than by putting those decisions in the hands of people
who pay no price for being wrong.”

Thomas Sowell