Do CdS meter cells age?

Discussion in 'B&W: Film, Paper, Chemistry' started by Paul Verizzo, Jun 14, 2008.

  1. Paul Verizzo

    Paul Verizzo Member

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    I know that selenium ones, the no batteries needed type, do. What about CdS?

    I build a Knight LG-275 light meter when I was a freshman in college, 1965, probably. That new CdS technology was all the rage! That particular meter got lost along the way, so I found another one on eBay for nostalgia purposes. And I still have the user manual!

    The two 1.35v PX625 mercury cells are of course, history. So the local electronics shop ordered me two of the 1.5v alkaline ones. In typical EV 15 outdoor bright light, the meter would swing past full scale. Man, that's only .3v or 11% more voltage. I'll bet that the meter would still be happy with 2.4V, the same amount off but less instead of more.

    Being incredibly clever, I figured that all I needed to do was adjust the potentiometer on the back of the meter for a full scale battery check reading, and it's me and Ansel, right?

    Wrong. However, I did get EV 15 reading with micro-fine twiddling with the pot (and lots of sweat, standing in that late PM Florida sun.)

    My Vivitar 45 match needle meter also used mercury cells. Now with alkalines, the meter swings only a bit more, nothing like the overread of the Knight. It's spot on if I use the left side of the target circle instead of the middle. My Knight now agrees with it under various lighting situations.

    My cheapo Tundra meter uses a silver oxide battery, so I do have a confirmation of EV 15 out in the bright light.

    Soooooo, it would appear to me that the CdS cell has lowered its internal resistance quite a bit for a give luminance level.

    All this also raised the ugly fact that the new batteries, of course, don't work right in my cameras. (The last time I was using them much I still had mercs in them.) They are off by a full stop, needing to be set at 200 for 100 film. What's really aggrevating, as any electrical geek knows, all any of these folks had to do was use a simple bridge circuit instead of the voltage reference of the battery, no problemo.
     
  2. bobwysiwyg

    bobwysiwyg Subscriber

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  3. epatsellis

    epatsellis Member

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    the easiest and cheapest way is hearing aid batteries, right voltage and similar discharge characteristics. Making them fit is an exercise in ingenuity, some use O-rings.


    erie
     
  4. Fred Aspen

    Fred Aspen Member

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    A six pack of the 675 hearing aid batteries ($6-7 at the pharmacy) and some #9 rubber O rings at Home Depot will put you back in business.

    Fred.
     
  5. Lee L

    Lee L Member

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    Some equipment uses an edge contact with the older mercury cells, so O-rings won't work there. For battery compartments like that, purchase a single Wein cell, which comes with a fitted metal washer. After you're done with the Wein cell, just keep transferring the Wein washer from one 675 hearing aid cell to another.

    Lee
     
  6. jun

    jun Member

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    Dear Paul Verizzo,

    Yes, Cds may age.
    Even the pots, meters are OK, the readings may shift by Cds aging effect.
    I have seen this with the Nikon F, F2 Photomic finders and other cameras that uses Cds cells.
    I don't exactly know why this happens, but one time, at the Nikon sevice center (in Japan) I have heard that this is caused by the decomposition of the CdS compound itself ..... however I cannot be sure this is true or not.

    From my experience, Selenium cell are far more reliable and less prone to aging compared to the Cds.
    I have seen lot of cameras that used Cds has shifted.
    But not for the Seleniums.
    I do have the CONTESSA 35 camera (made in 1953) that uses Seleniums, but the readings is still accurate (although this camera meter is prone to have intermittence connection problems to the meter ... mine was fixed for the intermittence connection problem only, the cell is original, 55 years old!).
    But the readings of the Nikon FTn Photomic that I used before had shfted more than 2 stops, when we still can obtain the mercury cell (FTn Photomic used mercury cells), making the meter useless.

    If the Cds age, you may can adjust the pots to agree the meter with some light level, but, if the light level changes (say if it gets darker), again the meter will disagree compared with the correct light meter readings.
    So most of the times, if the Cds aging is causing the problem, the only true cure of the problem is to replace the Cds and recalibrate.
    But you need the replacement Cds which may be difficult to find (both size and electrical characteristic must be the same).
    May be you could adjust the meter to be accurate within some specific brightness range, but may not to the whole brightness range without replacing the Cds if the Cds is bad, depending on the circuit of the metering system, and the how the Cds are aged.

    For example, Nikon sevice in Japan, cannot fix the Nikon F & F2 Photomic Finder any more (although they still do overhall for the Nikon F and F2 cameras) since no parts are available, but some third party repair shops DO repair the Photomic Finder by using a special order Cds that can be fitted to the Photomic finder, replace it, and recalibrate it.

    Regards,
     
  7. Nicholas Lindan

    Nicholas Lindan Advertiser Advertiser

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    CdS and selenium are not intrinsically shorter lived than other solid state devices.

    It’s often the packaging that gives the problem. Both can corrode if moisture gets to them: CdS is a salt and Selenium in the presence of light has a voltage across it and voltage + water = corrosion. It was apocryphal knowledge that keeping selenium cells in the dark extended their life -- the thinking at the time was they ‘wore out’ like a battery. In reality it was corrosion caused by the generated voltage that caused early failure.

    Cameras use plastic dipped CdS cells rather than hermetic cells because of size and cost. The epoxy gives way with time and the cell fails.

    Selenium cells are also dipped in plastic and again will fail if the moisture barrier is breached.

    Some selenium and CdS cells are not coated at all and have a poor lifetime in humid environments.

    Some selenium meters use springs to make contact with the cell - like battery contacts these develop corrosion with time. Removing the cell and cleaning the contacts with a pencil eraser, applying a drop of contact cleaner and reassembling the meter can sometimes get things working again.
     
  8. Paul Verizzo

    Paul Verizzo Member

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    One thread becomes two :smile:

    As to the substitution of zinc-air cells for mercury via o-rings (clever!) or the Wein adapter, I've read that the life is pretty short, especially in dry climates. the electrolyte dries out before the cell is otherwise used up. Not an issue for hearing aids, but for infrequent or lightly used equipment, could be an issue. Replacing and removing the sealing tape after shooting sessions is not practical, generally. Nevertheless, a good low cost option for some situations.

    My father's old Weston selenium meter is way off, haven't checked it in years, but way off. I've read elsewhere about selenium aging issues, so that is where my question started from.

    It appears that based on the information above (thanks!) it's all a crap shoot. The CdS cell in my Knight is hermetically sealed in a can with a glass top. Of course, the cell's original characteristics are unknown, so replacement is impossible.

    My re-metering seems to be working fine EV10-15. I've not tested lower yet. I do understand the question of non-linearity of the CdS cell making a meter adjustment potentially invalid, but so far it seems linear. I might just try the zinc-air trick to give me a reference again.

    Thanks, all, again.
     
  9. Paul Verizzo

    Paul Verizzo Member

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    Final Solution?

    Anyway, I realized that my Knight meter has a lot of space inside. I can just put in a simple voltage regulator for 2.7v total. A bit over $2 plus a few resistor or pots.

    Some years ago I built a custom bracket with NiCads in the handle and appropriate voltage regulators to reduce the nominal to 6V for the flash. Worked great. Still a nice unit althout I've not used it for 15 years.

    Time moves on......
     
  10. nworth

    nworth Subscriber

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    CdS cells have lots of problems. They shouldn't age, but if poorly made they may appear to do so. Contamination, leaks, and poor preparation of the CdS cause problems like that. The response of a well built CdS cell should be linear, with a scale factor depending on its construction. Bad ones may be nonlinear. CdS cells all have a funny color response, with a strong peak in the yellow green part of the spectrum. For some of them, this peak is quite narrow. That can cause weird readings depending on the color of the illumination and of the scene. I have an old Gossen LunaPro meter that still seems to work, and the meter on my Olympus 35SP is still OK, but I would prefer a selenium cell anytime.
     
  11. gainer

    gainer Subscriber

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    CdS is usually a photoresistor. Ideally, it should have infinite resistance at zero illumination. Any corrosion or remaining solder flux, etc., may form a path around the cell that varies with age. Selenium is usually used in photoelectric mode. It's output can be sufficient to drive a meter directly as it does in my Leica meter. This leads to other problems because the meter must be a rugged, non-linear microammeter. The selenium cell can also be amplified by any of several (hundred?) integrated circuit operational amplifiers which can be made to have logarithmic output for linear input and in turn drive a more rugged linear meter or whatever circuits an automatic camera may present to it. Don't forget phototransistors. By the way, we are talking about analog computers here. Even d*****l cameras use them.
     
  12. thuggins

    thuggins Member

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    First and foremost, you CANNOT replace a mercury cell with and alkaline cell. The voltage of alkaline cells changes constantly as they discharge. The reason mercury cells were used was because they had a linear voltage ouput thoughout their life, then died almost immediately. The zinc-air cells can be used as a replacement, or get an MR-9 adapter and a silver oxide cell from C.R.I.S.

    CdS cells do age, which causes both accuracy and linearity problems. I have had much better luck with selenium meters.