I have a CDS photocell. The data sheet specifies a minimum of 10k-100k
ohms at 10 lux. Is the most likely value the arithmetic mean or the
geometric mean at 10 lux? Or neither?

On Wednesday, May 19, 2021 at 2:31:52 PM UTC-7, John S wrote:
> I have a CDS photocell. The data sheet specifies a minimum of 10k-100k
> ohms at 10 lux. Is the most likely value the arithmetic mean or the
> geometric mean at 10 lux? Or neither?

Arithmetic, surely (the resistance is proportional to the illumination, not exponential).
More important, though, is the COLOR of the illumination, since photon energy
determines the interactions that free those conduction electrons. It's possible that
the minimum for a given sample is much of that range, depending on the light color.

There's an order of magnitude between the response to IR and blue, for CdS sensors.

John S wrote:

> I have a CDS photocell. The data sheet specifies a minimum of 10k-100k
> ohms at 10 lux. Is the most likely value the arithmetic mean or the
> geometric mean at 10 lux? Or neither?

Hard to say. CdS (and especially CdSSe) is a truly horrible material,
whose conductivity can vary 5:1 depending on its previous history. Its
main virtue is a gigantic photoconductive gain.(1) Long ago, it was
considered superior in camera light meters because its spectral
sensitivity curve is sharply peaked in the blue, making it a good match
for orthochromatic B&W film.

Unlike photodiodes, photoconductors are unipolar--there's no PN
junction, just a chunk of doped amorphous or polycrystalline
semiconductor that functions as a very sensitive light-dependent
resistor (LDR).

Cheers

Phil Hobbs

(1) When light hits a photoconductor, it produces carrier pairs that
drift apart under the applied E field. If the carrier lifetime (tau) is
long, as it is in PbS (milliseconds iirc), it may be much longer than
the transit time t_T.

The photoconductor then exhibits gain, so that the charge transferred
per detection event is

q = e tau/t_T,

which in most PbS detectors is a big number (thousands).(2) You'd think
that this would be great--a poor man's photomultiplier. In some
respects it is, but it comes with a huge cost in speed and flakiness.

(2) Perceptive readers may wonder how this can be--the carriers all
recombine when they reach the metallic contacts, so even if

tau = 500 t_T,

how can that work? The physics is that the electron and hole don't
arrive at the contacts simultaneously, so that there's a net charge
imbalance for a bit, leading to carriers being emitted by the contacts
to maintain overall charge neutrality.

Maxwell's demon is still on vacation, so this doesn't happen
carrier-by-carrier, but rather stochastically. Nevertheless, on average
one detected photon increases the current for a time tau rather than
T_t. The stochastic nature of the recombination process doubles the
shot noise in photoconductors compared with photodiodes, where the
recombination is deterministic and only the generation is stochastic.

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510

http://electrooptical.net
http://hobbs-eo.com

On 5/19/2021 5:23 PM, whit3rd wrote:
> On Wednesday, May 19, 2021 at 2:31:52 PM UTC-7, John S wrote:
>> I have a CDS photocell. The data sheet specifies a minimum of 10k-100k
>> ohms at 10 lux. Is the most likely value the arithmetic mean or the
>> geometric mean at 10 lux? Or neither?
>
> Arithmetic, surely (the resistance is proportional to the illumination, not exponential).
> More important, though, is the COLOR of the illumination, since photon energy
> determines the interactions that free those conduction electrons. It's possible that
> the minimum for a given sample is much of that range, depending on the light color.
>
> There's an order of magnitude between the response to IR and blue, for CdS sensors.
>

Helpful info. Thank you!

On 5/19/2021 6:17 PM, Phil Hobbs wrote:
> John S wrote:
>
>> I have a CDS photocell. The data sheet specifies a minimum of 10k-100k
>> ohms at 10 lux. Is the most likely value the arithmetic mean or the
>> geometric mean at 10 lux? Or neither?
>
> Hard to say.  CdS (and especially CdSSe) is a truly horrible material,
> whose conductivity can vary 5:1 depending on its previous history.  Its
> main virtue is a gigantic photoconductive gain.(1) Long ago, it was
> considered superior in camera light meters because its spectral
> sensitivity curve is sharply peaked in the blue, making it a good match
> for orthochromatic B&W film.
>
> Unlike photodiodes, photoconductors are unipolar--there's no PN
> junction, just a chunk of doped amorphous or polycrystalline
> semiconductor that functions as a very sensitive light-dependent
> resistor (LDR).
>
> Cheers
>
> Phil Hobbs
>
> (1) When light hits a photoconductor, it produces carrier pairs that
> drift apart under the applied E field.  If the carrier lifetime (tau) is
> long, as it is in PbS (milliseconds iirc), it may be much longer than
> the transit time t_T.
>
> The photoconductor then exhibits gain, so that the charge transferred
> per detection event is
>
> q = e tau/t_T,
>
> which in most PbS detectors is a big number (thousands).(2)  You'd think
> that this would be great--a poor man's photomultiplier.  In some
> respects it is, but it comes with a huge cost in speed and flakiness.
>
> (2) Perceptive readers may wonder how this can be--the carriers all
> recombine when they reach the metallic contacts, so even if
>
> tau = 500 t_T,
>
> how can that work?  The physics is that the electron and hole don't
> arrive at the contacts simultaneously, so that there's a net charge
> imbalance for a bit, leading to carriers being emitted by the contacts
> to maintain overall charge neutrality.
>
> Maxwell's demon is still on vacation, so this doesn't happen
> carrier-by-carrier, but rather stochastically.  Nevertheless, on average
> one detected photon increases the current for a time tau rather than
> T_t.  The stochastic nature of the recombination process doubles the
> shot noise in photoconductors compared with photodiodes, where the
> recombination is deterministic and only the generation is stochastic.
>

Thanks, Phil, for the in-depth explanation. Very much appreciated.

Cheers!

John S wrote:
===========
> I have a CDS photocell.

Illegal in any country that follows RoHS rules.

Insane crap IMO cos the amount of cadmium is so minuscule.

..... Phil

On 5/19/2021 7:10 PM, Phil Allison wrote:
>
> John S wrote:
> ===========
>> I have a CDS photocell.
>
> Illegal in any country that follows RoHS rules.
>
> Insane crap IMO cos the amount of cadmium is so minuscule.
>
>
> .... Phil
>

Agreed!

John

On 2021/05/19 5:10 p.m., Phil Allison wrote:
>
> John S wrote:
> ===========
>> I have a CDS photocell.
>
> Illegal in any country that follows RoHS rules.
>
> Insane crap IMO cos the amount of cadmium is so minuscule.
>
>
> .... Phil
>

Aren't there still Roman vintage aqueducts/pipes in Europe that are lead
lined?

http://www.romanaqueducts.info/technicalintro/lead1.htm

How does that tally with RoHS?

John :-#)#

John Robertson wrote:
==================
>
> Aren't there still Roman vintage aqueducts/pipes in Europe that are lead
> lined?

** U tell me - Mr Smartarse context shifter.

...... Phil

On 2021/05/19 6:03 p.m., Phil Allison wrote:
> John Robertson wrote:
> ==================
>>
>> Aren't there still Roman vintage aqueducts/pipes in Europe that are lead
>> lined?
>
>
> ** U tell me - Mr Smartarse context shifter.
>
>
>
> ..... Phil
>
>

Your point was minute bits of Cadmium are enough to get CDS cells banned
in Europe, so why aren't they digging up any and all lead pipes? And car
batteries...if you catch my drift.

Yeah, I shifted the context slightly, but felt it was on your modified
topic enough (you added RoHS) that it still fit...

I use 70 or so CDS cells in my strobe light Rifle Gallery - once we open
again later this summer when crowds aren't a big risk - so I was
following the discussion avidly.

John ;-#)#

On Wednesday, May 19, 2021 at 9:03:26 PM UTC-4, palli...@gmail.com wrote:
> John Robertson wrote:
> ==================
> >
> > Aren't there still Roman vintage aqueducts/pipes in Europe that are lead
> > lined?
> ** U tell me - Mr Smartarse context shifter.

No real profanity. I see Mr. Phil is having a good day.

--

Rick C.

- Get 1,000 miles of free Supercharging
- Tesla referral code - https://ts.la/richard11209

John Robertson wrote:
> Phil Allison wrote:
> > John Robertson wrote:
> > ==================
> >>
> >> Aren't there still Roman vintage aqueducts/pipes in Europe that are lead
> >> lined?
> >
> > ** U tell me - Mr Smartarse context shifter.
> >
> Your point was minute bits of Cadmium are enough to get CDS cells banned
> in Europe, so why aren't they digging up any and all lead pipes? And car
> batteries...if you catch my drift.
>

** I am not liable for retrieving random pieces of your mental flotsam.

FYI:
The bans on Cd, Pb solder etc are not retrospective.
98% or so of car batteries are re-cycled in the EU.
Pb plumbing has been banned for decades.

What I called you is clear and precise.

..... Phil

On Wednesday, May 19, 2021 at 10:52:28 PM UTC-4, palli...@gmail.com wrote:
> John Robertson wrote:
> > Phil Allison wrote:
> > > John Robertson wrote:
> > > ==================
> > >>
> > >> Aren't there still Roman vintage aqueducts/pipes in Europe that are lead
> > >> lined?
> > >
> > > ** U tell me - Mr Smartarse context shifter.
> > >
> > Your point was minute bits of Cadmium are enough to get CDS cells banned
> > in Europe, so why aren't they digging up any and all lead pipes? And car
> > batteries...if you catch my drift.
> >
> ** I am not liable for retrieving random pieces of your mental flotsam.
>
> FYI:
> The bans on Cd, Pb solder etc are not retrospective.
> 98% or so of car batteries are re-cycled in the EU.
> Pb plumbing has been banned for decades.
>
> What I called you is clear and precise.

I really do laugh out loud sometimes when I read this guy's posts. Truly epic.

--

Rick C.

+ Get 1,000 miles of free Supercharging
+ Tesla referral code - https://ts.la/richard11209

On 5/19/2021 5:23 PM, whit3rd wrote:
> On Wednesday, May 19, 2021 at 2:31:52 PM UTC-7, John S wrote:
>> I have a CDS photocell. The data sheet specifies a minimum of 10k-100k
>> ohms at 10 lux. Is the most likely value the arithmetic mean or the
>> geometric mean at 10 lux? Or neither?
>
> Arithmetic, surely (the resistance is proportional to the illumination, not exponential).
> More important, though, is the COLOR of the illumination, since photon energy
> determines the interactions that free those conduction electrons. It's possible that
> the minimum for a given sample is much of that range, depending on the light color.
>
> There's an order of magnitude between the response to IR and blue, for CdS sensors.
>

10 lux @ 2856 °K produces a minimum of 10k and a maximum of 100k ohms.
In a batch of, say 1000 photocells, what would be the most likely
resistance value of a photocell chosen at random?

Data sheet provided by Digikey:

https://tinyurl.com/rcvj339b

On Wed, 19 May 2021 16:31:43 -0500, John S <Sophi.2@invalid.org>
wrote:

>I have a CDS photocell. The data sheet specifies a minimum of 10k-100k
>ohms at 10 lux. Is the most likely value the arithmetic mean or the
>geometric mean at 10 lux? Or neither?

Geometric.

The typical value would be about 30K with a 3:1 spread in either
direction.

But a *minimum* of 10-100K? That makes no sense.

--

John Larkin Highland Technology, Inc

The best designs are necessarily accidental.

On Thursday, May 20, 2021 at 12:19:53 PM UTC-4, jla...@highlandsniptechnology.com wrote:
> On Wed, 19 May 2021 16:31:43 -0500, John S <Sop...@invalid.org>
> wrote:
> >I have a CDS photocell. The data sheet specifies a minimum of 10k-100k
> >ohms at 10 lux. Is the most likely value the arithmetic mean or the
> >geometric mean at 10 lux? Or neither?
> Geometric.
>
> The typical value would be about 30K with a 3:1 spread in either
> direction.
>
> But a *minimum* of 10-100K? That makes no sense.

That's like saying, "Expect the shipment no later than 2 to 4 weeks from today". So if the shipment arrives in 1 week I should call back to complain?

When a company quotes delivery in days, such as 110 days, is that calendar or work days?

--

Rick C.

- Get 1,000 miles of free Supercharging
- Tesla referral code - https://ts.la/richard11209

On 20.5.21 3.23, John Robertson wrote:
> On 2021/05/19 5:10 p.m., Phil Allison wrote:
>>
>>   John S wrote:
>> ===========
>>> I have a CDS photocell.
>>
>> Illegal in any country that follows RoHS  rules.
>>
>> Insane crap IMO cos the amount of cadmium is so minuscule.
>>
>>
>> ....  Phil
>>
>
> Aren't there still Roman vintage aqueducts/pipes in Europe that are lead
> lined?
>
> http://www.romanaqueducts.info/technicalintro/lead1.htm
>
> How does that tally with RoHS?
>
> John :-#)#

There are other old uses for lead. The Parisians are having a
problem of lead after the Notre Dame fire destroyed coloured
windows in the cathedral, and there is lead dust around the
Ile de Cite now.

--

-TV

On 2021-05-20 18:53, Tauno Voipio wrote:
> On 20.5.21 3.23, John Robertson wrote:
>> On 2021/05/19 5:10 p.m., Phil Allison wrote:
>>>
>>> John S wrote:
>>> ===========
>>>> I have a CDS photocell.
>>>
>>> Illegal in any country that follows RoHS rules.
>>>
>>> Insane crap IMO cos the amount of cadmium is so minuscule.
>>>
>>>
>>> .... Phil
>>>
>>
>> Aren't there still Roman vintage aqueducts/pipes in Europe that are lead lined?
>>
>> http://www.romanaqueducts.info/technicalintro/lead1.htm
>>
>> How does that tally with RoHS?
>>
>> John :-#)#
>
> There are other old uses for lead. The Parisians are having a
> problem of lead after the Notre Dame fire destroyed coloured
> windows in the cathedral, and there is lead dust around the
> Ile de Cite now.
>

Nothing to do with the windows. It was the lead cladding on the
roof. Many ancient building have lead-clad roofs. Even on my
own roof I have some lead sheet parts.

Jeroen Belleman

Jeroen Belleman wrote:
> On 2021-05-20 18:53, Tauno Voipio wrote:
>> On 20.5.21 3.23, John Robertson wrote:
>>> On 2021/05/19 5:10 p.m., Phil Allison wrote:
>>>>
>>>>   John S wrote:
>>>> ===========
>>>>> I have a CDS photocell.
>>>>
>>>> Illegal in any country that follows RoHS  rules.
>>>>
>>>> Insane crap IMO cos the amount of cadmium is so minuscule.
>>>>
>>>>
>>>> ....  Phil
>>>>
>>>
>>> Aren't there still Roman vintage aqueducts/pipes in Europe that are
>>> lead lined?
>>>
>>> http://www.romanaqueducts.info/technicalintro/lead1.htm
>>>
>>> How does that tally with RoHS?
>>>
>>> John :-#)#
>>
>> There are other old uses for lead. The Parisians are having a
>> problem of lead after the Notre Dame fire destroyed coloured
>> windows in the cathedral, and there is lead dust around the
>> Ile de Cite now.
>>
>
> Nothing to do with the windows. It was the lead cladding on the
> roof. Many ancient building have lead-clad roofs. Even on my
> own roof I have some lead sheet parts.
>
> Jeroen Belleman

Back in the '90s there were folks here who offered churches a brand new
lead roof in exchange for their old leaky one.

Sounds like Abanazar trolling for Aladdin's lamp, doesn't it?

The issue was that the lead in the roofs had been smelted a century of
so ago, so the accompanying radioactive isotopes (produced from decay of
uranium, thorium, and so on) had decayed, leaving valuable 'low-alpha
lead' for soldering VLSI chips.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510

http://electrooptical.net
http://hobbs-eo.com

On 5/20/2021 11:19 AM, jlarkin@highlandsniptechnology.com wrote:
> On Wed, 19 May 2021 16:31:43 -0500, John S <Sophi.2@invalid.org>
> wrote:
>
>> I have a CDS photocell. The data sheet specifies a minimum of 10k-100k
>> ohms at 10 lux. Is the most likely value the arithmetic mean or the
>> geometric mean at 10 lux? Or neither?
>
> Geometric.
>
> The typical value would be about 30K with a 3:1 spread in either
> direction.
>
> But a *minimum* of 10-100K? That makes no sense.
>

You are correct. It is an ambiguous comment. Let me fix it...
The data sheet specifies a minimum of 10k and a maximum of 100k ohms at
10 lux @ 2856°K illumination.

Thanks for your input.

On Thursday, May 20, 2021 at 1:31:35 PM UTC-4, Jeroen Belleman wrote:
> On 2021-05-20 18:53, Tauno Voipio wrote:
> > On 20.5.21 3.23, John Robertson wrote:
> >> On 2021/05/19 5:10 p.m., Phil Allison wrote:
> >>>
> >>> John S wrote:
> >>> ===========
> >>>> I have a CDS photocell.
> >>>
> >>> Illegal in any country that follows RoHS rules.
> >>>
> >>> Insane crap IMO cos the amount of cadmium is so minuscule.
> >>>
> >>>
> >>> .... Phil
> >>>
> >>
> >> Aren't there still Roman vintage aqueducts/pipes in Europe that are lead lined?
> >>
> >> http://www.romanaqueducts.info/technicalintro/lead1.htm
> >>
> >> How does that tally with RoHS?
> >>
> >> John :-#)#
> >
> > There are other old uses for lead. The Parisians are having a
> > problem of lead after the Notre Dame fire destroyed coloured
> > windows in the cathedral, and there is lead dust around the
> > Ile de Cite now.
> >
> Nothing to do with the windows. It was the lead cladding on the
> roof. Many ancient building have lead-clad roofs. Even on my
> own roof I have some lead sheet parts.

Has everything to do with the windows too. All the stained glass was leaded glass, and they were also very big on using copious amounts of lead caulking to seal everything, even into early 20th century- even potable water cisterns!
Dunno what it's called in Europe, but metal roofs in America were a tin-lead alloy on steel called terne. And that stuff was still being installed, mostly in restorations, as recently as 30 years ago when it was banned. America is full of standing seam terne roofs still, mainly because it lasts forever.
https://en.wikipedia.org/wiki/Terne

>
> Jeroen Belleman

On Wednesday, May 19, 2021 at 7:17:39 PM UTC-4, Phil Hobbs wrote:
> John S wrote:
>
> > I have a CDS photocell. The data sheet specifies a minimum of 10k-100k
> > ohms at 10 lux. Is the most likely value the arithmetic mean or the
> > geometric mean at 10 lux? Or neither?
> Hard to say. CdS (and especially CdSSe) is a truly horrible material,
> whose conductivity can vary 5:1 depending on its previous history. Its
> main virtue is a gigantic photoconductive gain.(1) Long ago, it was
> considered superior in camera light meters because its spectral
> sensitivity curve is sharply peaked in the blue, making it a good match
> for orthochromatic B&W film.
>
> Unlike photodiodes, photoconductors are unipolar--there's no PN
> junction, just a chunk of doped amorphous or polycrystalline
> semiconductor that functions as a very sensitive light-dependent
> resistor (LDR).
>
> Cheers
>
> Phil Hobbs
>
> (1) When light hits a photoconductor, it produces carrier pairs that
> drift apart under the applied E field. If the carrier lifetime (tau) is
> long, as it is in PbS (milliseconds iirc), it may be much longer than
> the transit time t_T.
>
> The photoconductor then exhibits gain, so that the charge transferred
> per detection event is
>
> q = e tau/t_T,
>
> which in most PbS detectors is a big number (thousands).(2) You'd think
> that this would be great--a poor man's photomultiplier. In some
> respects it is, but it comes with a huge cost in speed and flakiness.
>
> (2) Perceptive readers may wonder how this can be--the carriers all
> recombine when they reach the metallic contacts, so even if
>
> tau = 500 t_T,
>
> how can that work? The physics is that the electron and hole don't
> arrive at the contacts simultaneously, so that there's a net charge
> imbalance for a bit, leading to carriers being emitted by the contacts
> to maintain overall charge neutrality.
>
> Maxwell's demon is still on vacation, so this doesn't happen
> carrier-by-carrier, but rather stochastically. Nevertheless, on average
> one detected photon increases the current for a time tau rather than
> T_t. The stochastic nature of the recombination process doubles the
> shot noise in photoconductors compared with photodiodes, where the
> recombination is deterministic and only the generation is stochastic.

I used one in a project where it was activated by a one of those laser pointer gadgets in average room ambient light. IOW the ambient light resistance was trimmed out and the laser reliably pushed it deeper into low resistance, and from quite a distance away too.

>
>
>
> --
> Dr Philip C D Hobbs
> Principal Consultant
> ElectroOptical Innovations LLC / Hobbs ElectroOptics
> Optics, Electro-optics, Photonics, Analog Electronics
> Briarcliff Manor NY 10510
>
> http://electrooptical.net
> http://hobbs-eo.com

On 2021-05-20 20:58, Phil Hobbs wrote:
> Jeroen Belleman wrote:
>> On 2021-05-20 18:53, Tauno Voipio wrote:
>>> On 20.5.21 3.23, John Robertson wrote:
>>>> On 2021/05/19 5:10 p.m., Phil Allison wrote:
>>>>>
>>>>> John S wrote: ===========
>>>>>> I have a CDS photocell.
>>>>>
>>>>> Illegal in any country that follows RoHS rules.
>>>>>
>>>>> Insane crap IMO cos the amount of cadmium is so minuscule.
>>>>>
>>>>>
>>>>> .... Phil
>>>>>
>>>>
>>>> Aren't there still Roman vintage aqueducts/pipes in Europe that
>>>> are lead lined?
>>>>
>>>> http://www.romanaqueducts.info/technicalintro/lead1.htm
>>>>
>>>> How does that tally with RoHS?
>>>>
>>>> John :-#)#
>>>
>>> There are other old uses for lead. The Parisians are having a
>>> problem of lead after the Notre Dame fire destroyed coloured
>>> windows in the cathedral, and there is lead dust around the Ile
>>> de Cite now.
>>>
>>
>> Nothing to do with the windows. It was the lead cladding on the
>> roof. Many ancient building have lead-clad roofs. Even on my own
>> roof I have some lead sheet parts.
>>
>> Jeroen Belleman
>
> Back in the '90s there were folks here who offered churches a brand
> new lead roof in exchange for their old leaky one.
>
> Sounds like Abanazar trolling for Aladdin's lamp, doesn't it?
>
> The issue was that the lead in the roofs had been smelted a century
> of so ago, so the accompanying radioactive isotopes (produced from
> decay of uranium, thorium, and so on) had decayed, leaving valuable
> 'low-alpha lead' for soldering VLSI chips.
>
> Cheers
>
> Phil Hobbs
>

I'm not sure that would work for lead on exposed rooftops.

I heard it mentioned about lead cargo recovered from Roman
ships that sank in the time they still had an empire. It
would have been protected from cosmic radiation and nuclear
test fall-out by several hundred meters of water, in addition
to having had lots of time for its own radioactivity to have
decayed.

I did not verify, but I believe it was also used in some of
CERN's detectors because of its low background radiation.

Jeroen Belleman

torsdag den 20. maj 2021 kl. 22.55.55 UTC+2 skrev Jeroen Belleman:
> On 2021-05-20 20:58, Phil Hobbs wrote:
> > Jeroen Belleman wrote:
> >> On 2021-05-20 18:53, Tauno Voipio wrote:
> >>> On 20.5.21 3.23, John Robertson wrote:
> >>>> On 2021/05/19 5:10 p.m., Phil Allison wrote:
> >>>>>
> >>>>> John S wrote: ===========
> >>>>>> I have a CDS photocell.
> >>>>>
> >>>>> Illegal in any country that follows RoHS rules.
> >>>>>
> >>>>> Insane crap IMO cos the amount of cadmium is so minuscule.
> >>>>>
> >>>>>
> >>>>> .... Phil
> >>>>>
> >>>>
> >>>> Aren't there still Roman vintage aqueducts/pipes in Europe that
> >>>> are lead lined?
> >>>>
> >>>> http://www.romanaqueducts.info/technicalintro/lead1.htm
> >>>>
> >>>> How does that tally with RoHS?
> >>>>
> >>>> John :-#)#
> >>>
> >>> There are other old uses for lead. The Parisians are having a
> >>> problem of lead after the Notre Dame fire destroyed coloured
> >>> windows in the cathedral, and there is lead dust around the Ile
> >>> de Cite now.
> >>>
> >>
> >> Nothing to do with the windows. It was the lead cladding on the
> >> roof. Many ancient building have lead-clad roofs. Even on my own
> >> roof I have some lead sheet parts.
> >>
> >> Jeroen Belleman
> >
> > Back in the '90s there were folks here who offered churches a brand
> > new lead roof in exchange for their old leaky one.
> >
> > Sounds like Abanazar trolling for Aladdin's lamp, doesn't it?
> >
> > The issue was that the lead in the roofs had been smelted a century
> > of so ago, so the accompanying radioactive isotopes (produced from
> > decay of uranium, thorium, and so on) had decayed, leaving valuable
> > 'low-alpha lead' for soldering VLSI chips.
> >
> > Cheers
> >
> > Phil Hobbs
> >
> I'm not sure that would work for lead on exposed rooftops.

the outside can be can be washed, the contamination left in from
the initial melting process has to decay
> I heard it mentioned about lead cargo recovered from Roman
> ships that sank in the time they still had an empire. It
> would have been protected from cosmic radiation and nuclear
> test fall-out by several hundred meters of water, in addition
> to having had lots of time for its own radioactivity to have
> decayed.
>
> I did not verify, but I believe it was also used in some of
> CERN's detectors because of its low background radiation.

I think that was mostly for low background steel,
steel is recycled again and again so once it has be contaminated
with radioactivity in the air from test fallout or radioactive materials
used in the process there is no way back

torsdag den 20. maj 2021 kl. 22.34.30 UTC+2 skrev Fred Bloggs:
> On Thursday, May 20, 2021 at 1:31:35 PM UTC-4, Jeroen Belleman wrote:
> > On 2021-05-20 18:53, Tauno Voipio wrote:
> > > On 20.5.21 3.23, John Robertson wrote:
> > >> On 2021/05/19 5:10 p.m., Phil Allison wrote:
> > >>>
> > >>> John S wrote:
> > >>> ===========
> > >>>> I have a CDS photocell.
> > >>>
> > >>> Illegal in any country that follows RoHS rules.
> > >>>
> > >>> Insane crap IMO cos the amount of cadmium is so minuscule.
> > >>>
> > >>>
> > >>> .... Phil
> > >>>
> > >>
> > >> Aren't there still Roman vintage aqueducts/pipes in Europe that are lead lined?
> > >>
> > >> http://www.romanaqueducts.info/technicalintro/lead1.htm
> > >>
> > >> How does that tally with RoHS?
> > >>
> > >> John :-#)#
> > >
> > > There are other old uses for lead. The Parisians are having a
> > > problem of lead after the Notre Dame fire destroyed coloured
> > > windows in the cathedral, and there is lead dust around the
> > > Ile de Cite now.
> > >
> > Nothing to do with the windows. It was the lead cladding on the
> > roof. Many ancient building have lead-clad roofs. Even on my
> > own roof I have some lead sheet parts.
> Has everything to do with the windows too. All the stained glass was leaded glass, and they were also very big on using copious amounts of lead caulking to seal everything, even into early 20th century- even potable water cisterns!
> Dunno what it's called in Europe, but metal roofs in America were a tin-lead alloy on steel called terne. And that stuff was still being installed, mostly in restorations, as recently as 30 years ago when it was banned. America is full of standing seam terne roofs still, mainly because it lasts forever.
> https://en.wikipedia.org/wiki/Terne

afaict tell it is the cheaper easier mass produced alternative to pure lead.. Making steel sheet takes a lot more tools and machines than lead sheet

https://youtu.be/pDG4LaUMAbE