Post
by Emmanuel Bigler » Mon Aug 31, 2009 11:47 am Etc/GMT-1+01:00
Hello from France
I'm not familiar with the Pentax Spotmeter V, but I'm familiar with the question of replacing 1.35V mercury cells with something actually available.
My preference would go for something that does not require to re-calibrate the meter.
First is to determine exactly the type of mercury cell and voltage required for the Spotmeter V.
For example the classical Gossen Lunasix CdS meter neded two 1.35V mercury cells and will work perfectly without re-calibration by using two silver oxide cells inside an adaptor sold by Gossen, the adaptor has the same outer dimensions as two mercury cells and subtracts the required amount of voltage to best fit the original requirements.
Among classical light metering instruments of the sixties and seventies, based on a CdS photo-resistor and supplied by mercury cells, we have two categories :
- type #1 : those who demand 1.35V or a combinaison of N times 1.35V in order to be accurate.
In this category, we find many classical 35 mm cameras where the meter display is a needle. In those meters, using a 1.5 or 1.55V battery instead of 1.35V may generate a metering error as big as two f-stops ! Moreover alkaline cells will see their output voltage vary during celle life, the error will be variable & almost unpredictable !
- type #2 : those who have an internal electronic architecture accepting without re-calibration to work with a higher voltage, i.e. 1.5V for alkaline cells or 1.55V for silver-oxide cells. In this category of meters we find the built-in meter of he Rollei 35 TE and SE.
So if the Spotmeter V demands 1.35V, the best solution is to use something that supplies 1.35V, except if the internal circuit is substantially smarter than the good old CdS ones of the sixties.
Possible replacement cells :
- either zinc-air replacement cells, they were specifically designed to replace mercury cells ; however they are often expensive and they do not last very long once activated (they need an activation by tearing off a label that allows air to enter the cell and start the electro-chemical reaction. Their shell-life prior to activation is reported to be excellent, but once activated they have a ridiculously short like if compared to mercury cells).
Among the cheapest zinc-air cells one can find those designed for hearing aids.
- somethings that takes 1.5 or 1.55V on input and delivers close to 1.35V on output.
The simplest electronic component performing this function is a kind of a Shottky diode that reduces by a certain amount the original voltage. The diode does not regulate the output voltage hence you need on input to use a battery with a very stable voltage if your device is type #1. Hence the choice of silver-oxide celles, alkaline cells see their voltage dropping too fast during cell life.
The easiest is to buy a commercialy available adaptor, fitted to silver oxide cells on input.
You can get some advice from specialised battery suppliers like the small battery company in the UK (I'm not affiliated, etc... I'm just a satisfied Continental customer !)
