measuring cell voltage with a panel meter

4ster

I'm building a cell checker and have a question about the test circuit design. I know just enough about circuit design to get myself into trouble. (I got through the electrical part of my college physics class with a lot of help from friends - that was a long time ago.) Luckily this is pretty basic stuff.

When a small panel meter is used for measuring PO2, the designs I have seen put a 10k load on the cell. Do you do the same when measuring cell voltage? Why?

I have spent a few hours looking for previous posts here about cell testers and have not found a "This is how I did it" post. Can anyone point me in a general direction?

Thanks in advance.

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Steve
"My problem is that I have trouble reconciling my gross habits with my net income." Errol Flynn

Stephane

Hi Steve,

Because it is required by the cell!
Even if you are using a microntroler with an analog/digital converter, you have to use a 10k load resistor.

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Stéphane
Hammer-rEvo powered!

nigelh

Because that's what it says in the manufacturers spec for the Teledyne's.

Actually I'm quite sure they mean 'not less than 10K'.

Actually this only makes sense if you are using a nice high impedance electronic panel meter. If you are using a moving coil 'real' one you'll be pushed to get the total load up to 10K even with a very sensitive one.

You're only expecting 60mV or so in 100% so choose your panel meter carefully. A lot of the electronic ones are 2V minimum full scale unless you delve into the works.

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nigelh

lizardland

Or just make one, the design on Mark Munro's site works as well as an analyser as it does as a PO2 meter. It's just a jumper which selects the decimal position.

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WD8CDH

You have to be carefull about the load. Check the mfg. specs. For example, the Teledyne K-1D is internally loaded and should be directly read with a high impedance (greater than 100K ohms) meter. The R10D should see a 6.0K ohms load. The R22D requires 10K.

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Ron Schroeder
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Dutchy

A cell is basically a poor quality battery. In an 'ideal' situation the battery reaches the theoretical posible (galvanic potential). This 'ideal' situation is all the oxygen present in abundance and NO load.
However a cell does its work for us by grace of lacking the oxygen needed to reach the full potential while there is a load.
Without a load (not even electrons leaking it would still reach the max (galvanic) potential.
In practise there will be a (leak) current and however small this means you'll have a reacting using up oxygen. When you deprive the cell of oxygen the voltage will drop hence an indication.
Just a leak current only isn't a good idea. It is hard to control and sensitive to noise. By adding a load the current increases (the cell is used up faster) winning response time. There are also other factors that come into play such as temperature and form factors and not to forget the metal(s) that are sacrifised (oxidated).

By choosing the right circuitry the effect of temperature may be (somewhat?) compensated and a good compromise between life span and response speed may be obtained. Your manufacturer allready did that and asks you to use what he deems best.

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4ster

Thanks everyone, all of the above posts have been helpful. Now I have to take this box of bits I have assembled and make a working cell tester.

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Steve
"My problem is that I have trouble reconciling my gross habits with my net income." Errol Flynn