Petri Computor 35 battery requirements

[Dr. Strangelove]

I just purchased a Petri Computor 35 in a good working condition. There were two A76/LR44 alkaline batteries in it, but originally it probably used two 1.35 volt mercury batteries. Does anyone know if the light meter requires the correct 2.7 volts or can I use the alkalines and still get correct exposure? The ASA setting method is the traditional 'variable hole' type, i.e. it lets more light to enter the CdS cell at higher ASA settings. To me that suggest that the light metering circuitry probably requires a constant voltage, but you can never be absolutely sure just by looking at the camera.

Of course I could test it with slide film, but if anyone knows it could potentially save a film. Thank you in advance!

 

[daniel~]

I used 2 LR44 in my Petri Computor to for a test shooting, when I bought it over a year ago, so I know it works this way. Not sure how accurate the exposure meter works, though, because I shoot only negative film.

 

[Dr. Strangelove]

I used 2 LR44 in my Petri Computor to for a test shooting, when I bought it over a year ago, so I know it works this way. Not sure how accurate the exposure meter works, though, because I shoot only negative film.

Thank you for the reply!

I know it works too, or in another words the shutter works, and exposure and f-stop seem to change according to light levels. I mostly shoot negative too with my fixed lens RFs, but I would still like to know, if I could trust the meter in the occasion I would like to shoot slide film. I suppose I could just use 1.4V zinc-air batteries, but I know I will not be using this camera all the time, and using the short-lived zinc-airs would be kind of wasteful... But thanks again for your answer.

 

[daniel~]

You're welcome. 🙂 Well what I'd do is run a mini test: shoot a dozen of pics, rewind the film but leaving leader outside, then use one of my own perfectly-working camera to finish the rest. It's a quick & economical way to get a roll finished and to find out how the new gear performs.

Actually, I nowadays tend to shoot like this with all of my cameras. In this way, I do waste a frame or two, but it's better than leaving all those cameras gathering dust on the shelf.

My Petri Computor seems to have sticky shutter blades as it doesn't always response when I press the button, and therefore I haven't used it again since. The few test pics from it looked good to me, anyway. Good luck to you!

 

[Dr. Strangelove]

You're welcome. 🙂 Well what I'd do is run a mini test: shoot a dozen of pics, rewind the film but leaving leader outside, then use one of my own perfectly-working camera to finish the rest. It's a quick & economical way to get a roll finished and to find out how the new gear performs.

I happened to have one old ISO 100 12exp. print film in my fridge, so I used it. The results: constant underexposure. I checked the combined voltage of the alkalines with a multimeter and it read 2.55V (2x1.275). Quite normal for half-depleted alkalines and it also confirms my initial theory that the light meter needs constant voltage.

The underexposure was fairly serious (more than 1 stop) and clearly visible also in the prints, so I find it possible that the meter might be calibrated even for 1.55V silver-oxide batteries. On the other hand the age of the camera speaks against that, since in the early 1970s silver-oxides were new technology and most meters were still calibrated for mercury batteries.

So, now I will probably activate a pair of zinc-airs and shoot another roll of negative film with them. Fortunately the Rayovac zinc-airs I use seem to last at least 6 months after activation. No need for expensive WeinCells, which won't last much more than a year after activation either. The Duracell zinc-airs usually deplete in 2-3 months due to more efficient -- in this case too efficient! -- air intakes. I will probably take some shots with silver-oxides as well to see the difference.

 

[Dr. Strangelove]

OK, I just got my second test roll back from the Petri Computor 35. The frames exposed with 1.4 V zinc-air batteries are still somewhat (about 1 stop) underexposed, so I am now fairly certain that the camera is calibrated for 1.55V silver-oxide batteries, as the frames exposed with them are much better. The voltage difference between new alkalines (nominal voltage 1.5 V) and silver-oxides is small enough that it could explain why daniel~ did not notice anything wrong with alkalines.

Of course it could also be that the light meter calibration is simply off a little. The only way to know for sure would be to shoot slide film with the silver-oxides, since less than one f-stop under or overexposure is not really visible in negatives. Maybe I will shoot a slide test roll sometime in the future, but probably not right away. For now I am content with the fact that the silver-oxides work beautifully with negative film.