Is anyone still using a Selenium light meter?

[chuck_foreman1]

On 2/5/2023 at 5:04 PM, Dustin McAmera said:

..except that if the selenium cell dies, ... 

Help me out... A lot of myths about Selenium cells... If this is an element... it can't die...right?  I read somewhere, someone (flacky sources eh? ) opined that it was not that the cells died but the contacts or method of transferring the properties??? to the ..well ...match needle ...or the galvonmeter etc info. 
So is it the cells that die?? that they lose their properties over time?? Is this due to fragilty??
Enquiring minds need to know!

 

[Dustin McAmera]

Selenium is an element, yes, but the selenium cell is an assembly, which can indeed die.

I only half-understand this. The resistance of selenium decreases when it is illuminated. Somehow (don't ask me!) this means that if you have a layer of selenium illuminated on one side, it generates a small voltage between the dark and light sides. So a selenium cell is made as a robust metal plate as the backing and one electrode, on which the thin layer of selenium is (somehow) applied. On top of that is a thin, transparent conductive layer, and electrodes are attached on top of that, at the edge. The transparent conductive layer is needed for the voltage from the whole area to reach the upper electrodes.

I found this page by Ian Partridge, who offers to replace the cell in some Weston meters:

https://ian-partridge.com/cells.html

He says the cells fail when lacquer applied to protect that top layer fails; dries up and cracks, letting oxygen get at the selenium.

Edited February 7, 2023 by Dustin McAmera

[rodeo_joe1]

On 2/6/2023 at 8:48 PM, David Lindquist said:

As far as what is available new with a selenium cell, I'm guessing that's what the Sekonic L398 uses. Does anyone know? Sekonic calls it an "Amophous Photo Sensor" and comments that it doesn't require a battery.

It's a Selenium cell. The Sekonic ones seem to be a bit more robust and long-lived than Weston's later cells, but in my experience the calibration of Sekonic's L398 'Studio' meters is a bit lax. I have 2 of them, and both are a lot more optimistic of light levels than any other meter I own. Make that 3 samples, if you include a Lux meter built in the same body shell. The Luxmeter has a linearity issue and can't be calibrated accurately at high and low readings on the same scale simultaneously. That doesn't inspire me with confidence in Sekonic's ability to make an accurate moving-coil meter at all. 

Which reminds me of an easy check for a dying cell in a Weston meter: Simply find a light level that puts the needle at 10* on the High scale. Then, without moving the aim of the meter, open the baffle and see if you get close to the full-scale 10* reading on the Low scale. If the cell is failing it won't read 10, and may not be able to reach a full scale deflection at all unless exposed to ridiculously bright light. 

*That's 50 on earlier Westons.

[rodeo_joe1]

13 hours ago, chuck_foreman1 said:

Help me out... A lot of myths about Selenium cells... If this is an element... it can't die...right?

Wrong! There are 'dead' Selenium cells to be found everywhere.

The cell properties depend on the crystal structure of the Selenium. Llike carbon, it can exist in different forms, but unlike carbon those Selenium allotropes aren't stable and can change over time or with temperature, or as Dustin has said, through atmospheric contamination or oxidation.

Edited February 8, 2023 by rodeo_joe1

[John Seaman]

In a lot I just bought from a local auction was a Weston Master 5, with invercone and instruction manuals, all new looking. It just about responds if I open the low light baffle and point it directly at the sun.

[kmac]

I've been experimenting with solar cells from garden lights to replace selenium cells in light meters. So far I'm getting adequate voltage, but haven't yet discovered how to reduce their size to fit in a meter and still retain the two connections. I'm still working on it.

[rodeo_joe1]

18 hours ago, kmac said:

I've been experimenting with solar cells from garden lights to replace selenium cells in light meters. So far I'm getting adequate voltage, but haven't yet discovered how to reduce their size to fit in a meter and still retain the two connections. I'm still working on it.

I did manage to get a 1" square silicon garden solar cell into a Weston Master V case. I was fiddling with it trying to find a suitable piece of blue filter gel to get the sensitivity right and combat the overly red response... when the delicate moving-coil mechanism got stuck. Probably picked up some magnetic dust or swarf. Gave up and binned the whole lot after that. 

There are limits to how much time and effort you can put into reviving old stuff. 

Like, who's going to pay the £179 asking price to have a Weston Master fully serviced?

When there are much better and more modern meters that can be got used for less money. So, they need a new battery now and then - big deal! 

[rodeo_joe1]

P. S. I think a better approach might be to start with a modern light-sensing module and find room for a small battery. 

This tiny thing (the veroboard's there to make it big enough to handle) works off between 2.3 and 5.5 volts, outputs between 5 and 50 uA over a near 5 decade illumination range, and has a near human-eye spectral response curve. 

It's an Osram SFH-5711 and costs peanuts. There are other similar devices out there too. 

One of those fitted in a Weston V case would make an excellent meter and be a real novelty. 

Afterthought: The 3.7ish volts of a rechargeable lithium cell would be a suitable power source, and the meter could then be plugged into a USB cable for recharging. 

Edited February 9, 2023 by rodeo_joe1