sensor load and life expectancy

York

Morning guys,
I know I am always pondering these rather hypothetical questions, but I was wondering if one could extend the life-expectancy of a cell by removing the load when not in use.

I found the teledyne Q&A website Teledyne Analytical Instruments | FAQ, and it says specifically:
>Tell me about the life expectancy for an R-17 or R-22 sensor.
>
>The life expectancy of an R-17 or R-22 oxygen sensor is a function of the
>integral of the amount of current that flows through the sensor. The sensor
>is designed to nominally last three years at 20.9% O2 at room temperature.
> ...


The load resistor would close the circuit and hence allow current to flow continously, therefore removing the load when the cell is not in use might extend its life. Now I blieve the temp-compensation circuit might still allow for some current to flow, although I'm not sure how the temp-compensation circuit actually works. Internal ion-diffusion inside the cell also lets the cell age, but this could be a lot less than an external load.
This may not be a factor when diving a lot, but when only diving once in a while, could it make a difference?



Joerg

jradomski

Open up a cell.. On a properly working cell there is ALWAYS current flow.. a cell is a current source NOT a voltage source.. The reason you can read a voltage is that you are reading a value across a resister..

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nigelh

Basically no.

The circuit looks roughly like this

Add/remove a 10K load across P1-P3 and you will make less than 1% difference to the current consumed.

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nigelh

jradomski

Thanks Nigel.. You saved me the trouble of making up a schematic for the guy..

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York

Hrm, "the guy" now certainly understands that he underestimated the influence of the temp-compensation. Immediate question now is: why is the 10K load needed at all if it only attributes to less than 1% to total current?

May I ask where above schematics is coming from?

Thanks!!!!
Joerg

jradomski

sorry Joerg...

Look at Nigel's web page he has a breakdown of the cells.. I have done the same thing fro my own uses.. The resister values vary depending on the type of cell.. Higher output cells just use a different main resistor to change the voltage range..

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York

Hi Joe,
I finally got my lazy ass around actually analysing the board as you suggested, and found the circuit similar to what Nigel posted. It has one more resistor. This from a AP Cell (S/N 658917, board has S/N B77102 REV.0).
So the effective internal load is basically more or less the same as Nigel suggested.

Why the heck is the 10K externally then needed?

thanks guys

Joerg

Attached Images

cell.jpg (8.2 KB, 188 views)

jradomski

The 4th resistor surrised me.. All that I have opened had 2 plus a thermistor.. The "external" resitor is meant to keep the temp compensation ckt within predcted range.. From my tests there was very little variance between the measured voltage with and wiithout the load over the typical rage of temps experienced by Rebreather divers at the cells..

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York

The circuit is by concept slightly different from what Nigel quotes. There is no in-line resistor (probably to deliver the maximum potential to the connector).

When you say 3 plus thermistor, do you mean including the 10K load?

They could have included the 10K into the the internal circuit, so there would be no requirement for an additional resistor externally - historical reasons possibly? Earlier sensors needed it, so now they stick to it?

Edit: Just thought about one more possibility: without the external load, the electronics would be measuring the potential, but without current flow (nA or less). This would make measurement very susceptible to interference from magnetic fields if long sensor leads are used. By adding the requirement for an external load (and planning for it in their internal circuit) it would ensure at least some current flows, hence provide more stabil reading - maybe?

J

aknelson

Why the heck is the 10K externally then needed?
Joerg[/quote]

Good question........
The internal current load by the approximatly 100 Ohm internal cell circuit is much greater than the current load added by the 10k external load.
The cells internal load actually causes the voltage you are measuring at the pins. Since a current flows between the electrodes.

I have seen different industrial designs for analog input circuits. Dependig what you want to measure a Current or a Voltage; A load resistor is in place mainly to protect the analog input. A lot of times a capacitor and a zehner diode is added too to limit the input voltage. Resistors in series and capacitors in parallel result in a signal delay or a low pass filter. The 10k load could be part of the recommended analog input circuit. Unless you know the complete analog input circuit it is hard to say that the 10K makes a difference. This brings us to the question how rugged the circuit is for electromagnetic compability if the circuit was ever evaluated for EMC.

Regards,
Andreas