His and Hers

[kmac]

I remember reading ( don't remember where) that selenium meters don't totally die but that the cell becomes coated and can be restored by cleaning with a rubber eraser. Never tried it.

 

Worst case scenario for an old selenium cell. This one was in my 1958 Mamiya Metra, it was encased in plastic for some reason. The cell didn't really go bad, it still had power, but the plastic did go bad and pretty much rendered the light meter useless.

 

 

Best case scenario for old selenium cell. This one is a Polaroid 625 cell, still like new and had full power after 50 years. I cut it to size and used it in the Metra.

The Polaroid 625 was made by Gossen, they have the same notable styling as Gossen's Pilot and Sixtino. The two halves of the body of the 625 were close fitting so I guess the cell was well protected from outside elements. That's not to say there aren't some dead ones out there but I have five 625s and all have good cells still producing power. The blue wire is not a normal soldered connection to the front of the cell and it should be left on the cell. In some meters, the connection at the rear of the cell is not soldered, but just a spring loaded metal to metal contact and may need cleaning if the meter stops working or becomes sluggish.

[kmac]

I once got a dead Norwood Director working. The contacts were gold(?) rings pressing against either side of the selenium disc - no solder. I opened it up and cleaned between the rings and disc. Probably the problem with a lot of these old meters.

 

Interesting John, gold contacts ? - makes sense if they are, because from my Googling, it appears the Norwood Director is a professional meter with a 3D incident hemispherical dome, not cheap in their day, and yet it had only metal to metal contacts - not soldered ones ! How come ? it was the very low power flowing through the contacts, metal to metal was quite adequate.

 

The lesson learned is that what appears to be "cheap" non-soldered design may in fact be no big disadvantage. It certainly means easy maintenance for the repair man, and provided the original amount of pressure is always applied to the cleaned contacts, there's no reason why the meter won't work properly. I counted the metal to metal contacts in my Bronica ETRSi with the metered finder and motor drive attached, and there is no less than 19 active metal to metal contacts - wow ! And that's not taking into account the batteries contacting each other, nor the the electrically conducting springs applying pressure to the batteries. The use of metal to metal contacts has gone viral since the early days, very low power made it possible. Equipment requiring high power needs soldered or clamped contacts, the exceptions being household switches, overload switches and the like, and while they are metal to metal, they are designed so the contacts separate quickly to prevent arcing

 

Another interesting fact is: that blue wire on the 625 selenium cell is precariously "welded", it peels off quite easily and ordinary solder won't fix it back onto the cell. I tried it on another 625 cell and it simply didn't work. So if the wire comes off, goodbye, unless a way is utilized to press it against the cell, as per your Norwood non-soldered method.

 

In conclusion, metal to metal contacts are not a big worry, and there are conveniences, a cell without soldered or "welded" wires is easier and less troublesome to remove from the meter, and it's replacement without having to solder wires back on is a dream for anyone working on the meter. Replacing the cell with a new one or a good secondhand one would also be a dream to fit - no need to solder wires.

[rodeo_joe1]

and the front is a transparently thin layer of gold.

Not gold, I suspect, but tin or indium-tin oxide.

ITO is the industry standard material for transparent and conductive coatings. Gold is rarely used for such a purpose, to the best of my knowledge, due to its cost and thin-film colouration.

So if the wire comes off, goodbye.....

Conductive epoxy is readily available these days. However, it's silver-loaded and not cheap.

Incidentally, there's a body of opinion out there that maintains selenium cells don't deteriorate with age..

Yeah, right.

Opinion not based on any research or fact!

Edited February 5, 2020 by rodeo_joe|1

[glen_h]

Not gold, I suspect, but tin or indium-tin oxide.

ITO is the industry standard material for transparent and conductive coatings. Gold is rarely used for such a purpose, to the best of my knowledge, due to its cost and thin-film colouration.

 

Conductive epoxy is readily available these days. However, it's silver-loaded and not cheap.

 

Yeah, right.

Opinion not based on any research or fact!

 

ITO is the favorite transparent conductor, which you most often (don't) see in grocery store freezer sections,

where it warms the glass enough not to condense on. You will see the wires that attach onto it to

run enough current though to keep it warm.

 

I don't know it in enough detail, but besides being a conductor, it is also part of what makes the

necessary p-n junctions. I suspect that the semiconductor physics wasn't at all understood at the

time, which makes it even more surprising that it works. I suspect that it isn't well understood

now, with not much work being done on selenium as an amorphous semiconductor.

 

At some point, it has to be different enough from iron in its electrical properties, and the more different,

the higher voltage you get out. But that is about as far as I know it. Some years ago, I almost got interested

in amorphous semiconductor physics, but then decided to do something else.

 

More recently, hydrogenated amorphous silicon has been used for solar cells, but I don't know

much about that, either.

[glen_h]

It seems that selenium solar cells go back to 1839, and Edmond Becquerel, father of the

discoverer of radioactivity.

 

Solar cell - Wikipedia

 

and 1883 for the gold film cells. But that was way before any understanding of

semiconductor physics and band structure.

[kmac]

Conductive epoxy is readily available these days. However, it's silver-loaded and not cheap.

 

That's good to know, I'll check it out

 

Yeah, right.

Opinion not based on any research or fact!

 

Re: "selenium cells don't deteriorate with age".. They deteriorate from exposure to light, just as rooftop solar cells get weaker over their 25year life. If selenium cells and solar cells are not exposed to light, moisture, extreme heat or anything else detrimental, they'll last a very long time. In other words if they were wrapped up and placed in cool dry storage there's no reason why they shouldn't last forever.

 

Today what we are seeing is 50 and 60yr old selenium meters still working, not all of them, but the meters that are still going and reasonable accurate are obviously the ones that haven't had much exposure to light, and when not in use, were probably left in dark environments like drawers or camera bags. Used ones like those are the ones to buy if only a cheap light meter is required by a beginner, because they are likely to still be fairly accurate in low light.

[rodeo_joe1]

They deteriorate from exposure to light,

Not my experience.

I have a couple of old Weston iiis that initially had low output after being obviously unused for some considerable time. Exposure to bright light actually revived one of them to the point of usefulness, while the other increased its output, but was still woefully under-reading.

 

OTOH, I've come across dozens of Weston iv and v models that have totally dead or unusable selenium cells, and not because they've been over-exposed to light. They just have lousy cells. Weston obviously changed the construction of the cells between the iii and iv model. Such that you can easily find model twos and threes in perfect working order, while fours and fives are almost invariably dead. This has absolutely nothing to do with how much light they've been exposed to.

[kmac]

Not my experience.

I have a couple of old Weston iiis that initially had low output after being obviously unused for some considerable time. Exposure to bright light actually revived one of them to the point of usefulness, while the other increased its output, but was still woefully under-reading.

 

There's other components in the meters that need waking up when they've been laying around for long periods, resisters and the motor (the needle armature), the pointed bearings on the armature would have been resting all that time. The resisters need "activating" before they start working again properly. Selenium cells are very low voltage, about 0.25v, selenium meters usually need a few seconds of "waking-up" every time they're used, whether they're new or old.

 

If a selenium meter reads low but appears to be good in every other respect, then there's every reason to suspect it's been used quite a lot in which case the cell itself will have deteriorated by all the exposure to light, but it would take a long time for that to happen.

 

I have an example of a 40-50yr old unused meter (not a single mark on it and came in a brand new looking case). I compared a reading with a digital camera that still pumps out well exposed images. It proves, to me at least, that selenium cells don't deteriorate if stored in the dark

 

Firstly I took a reading of the torch light. The reading was a little less than 1/250sec at f8 at 400ASA ... now go to the reading below from the digital camera photographing the same torch.

 

 

 

Secondly, I took a shot with the digital camera, the reading is almost identical. Had I more accurately aligned the green pointer on the Pallas meter, it would be even closer to the digital camera reading of 1/200sec

 

 

Not bad for a selenium cell as old as that. (I'm not sure of the real age of the meter, perhaps someone might know.)

[rodeo_joe1]

The resisters need "activating" before they start working again properly.

You've obviously never had a Weston III, IV or V apart. There are no resistors in there, or even resisters.

 

The meter mechanism also needs no 'waking up' unless the needle obviously sticks.

 

The only other part of the circuit that affects operation is the spring-ring contact around the selenium cell. This can, admittedly, become slightly resistive, but rubbing the ring against the cell is all that's needed to get good contact again.

 

In addition, I usually test a bad cell by probing it directly with a DMM for voltage and current. So I can say with some certainty that it's the selenium cells that die or become weak in Weston IVs and Vs - nothing else. In fact the now-defunct company 'Megatron' made a good living for a few years by replacing the duff cells in Weston meters.

[kmac]

You've obviously never had a Weston III, IV or V apart. There are no resistors in there, or even resisters.

 

OK, but some selenium meters do have resisters, as examples: Sekonic Auto Lumi 86 and a Russian meter I have here

 

I have a Weston V, I inherited, I've never pulled it apart. It only zeros properly if the meter is vertical, in the horizontal, the needle moves away from zero. I don't use any meter with that sort of inconsistency. It also seems to be 1 to 1 1/2 stops out, overexposing. I know for sure this Weston has done few miles but the cell is still active, possibly the rest of the meter needs looking at, but it's hardly worth my time.

 

Other meters to be suspicious of are meters with buttons on the side for changing to "Low" and "High" light conditions. I have two cds Sixtars here, one inherited, one bought on ebay. The inherited one is a total wreck and the ebay meter looks mint and works but gives inconsistent readings. Cds meters have given me nothing but headaches over the years, I simply don't bother with them anymore and instead turned to quality selenium meters in good condition. They are much more reliable than cds meters IMO