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York · 30th September 2007, 15:59

Quote: (Originally Posted by jradomski)

The 4th resistor surrised me.. All that I have opened had 3 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..

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

30th September 2007, 15:59	#9 (permalink)
York probubbly not Current Rebreather/s: Megalodon Other Rebreather/s: Join Date: Mar 2005 Location: Bristol, UK Posts: 118	Re: sensor load and life expectancy Quote: (Originally Posted by jradomski) The 4th resistor surrised me.. All that I have opened had 3 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.. 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 Last edited by York : 30th September 2007 at 16:06.
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