This product is not intended for uneducated users. It is provided for experienced rebreather and mixed gas users ONLY. If you have not been properly trained by an internationally recognized technical certification agency and/or don't have a firm grasp of electronics then DO NOT USE THIS STUFF. God forbid you actually use it underwater, everyone knows water and electronics don’t mix.
This design, if used as part of a life support system, could indirectly kill you, and it probably has faults. The designer does not warrant that it won't get you killed, or that it will produce safe, reliable, results. This dive product is experimental, and if you choose to use it than you do so at your own risk. Diving in general is fraught with risk and playing with things you don’t take the time to understand and verify adds significantly more risk.
In putting together an O2 rebreather and a mixed gas rebreather I’ve developed a few O2 display boards using Datel Digital Panel Meters (DPM). The features incorporated into the displays are, small board layout for fitting into a 1-1/4” ID clear tube or other housings, epoxy encapsulated DPM, back lit DPM, calibration pots are easily accessible, DPM has a low battery alarm annunciator and low power consumption , there is a space for a sensor load resistor if required, and they can be used with O2 sensors outputting < 22mV in air.
The principle of operation for all the display units are the same. The output voltage from a galvanic oxygen sensor is feed across a load resistor, if required, and to the input of a DPM. The displayed value is adjusted to read correctly in a calibration gas and pressure with an external gain adjustment. A Teledyne K-1d oxygen sensor outputs ~10.5 mV in air and ~50.0 mV in 100% O2 at 1 ATA so by applying a displayed gain of two we have a display that reads 210 and 1000 respectively. Using the display units setup for use in rebreathers this would read 0.210 and 1.000 to represent partial pressure of O2 . On the display unit designed for use in rebreathers or as a mix analyzer the decimal point is moveable to represent Fraction of O2 or partial pressure of O2 .
Typically an O2 sensor is linear, meaning if you get 10.5 mV in air at 1 ATA (0 fsw) and 50 mV in air at 4.78 ATA (125 fsw) you should get 80 mV in air at 7.66 ATA (220 fsw). Which would read on our DPM (with a display gain of two and the decimal point in the PO2 position) as 0.210, 1.000, and 1.600 PO2 respectively. O2 sensors read Partial Pressure of Oxygen and if you don’t know what that is I’d suggest you start by reading Rich Pyle’s “Diving Physics and ‘Fizzyology’”
http://www.bishop.hawaii.org/bishop/treks/palautz97/phys.html#Partial Pressures .
DPM - The DPM I use is the Datel DMS-20LCD-0-9B, it’s a 0-200mV input, 9-14VDC, back lit display, Figure 1. This meter has an internal precision reference voltage which is feed across an internal voltage dropping resistor and a potentiometer to pin 10 (analog common) with the pot wiper output on pin 8 (reference voltage). Typically the reference voltage out (pin 8) would be feed directly back into the unit at pin 7 (reference voltage in). You would then use the pot adjustment on the back of the DPM to calibrate the display. I’m not going to get into how this DPM works except to say that if you want a displayed gain of 1 so that a 199.9mV input voltage would read 1999 on the display you need 100mV at pin 7 (reference voltage in). But if you want a displayed gain of two ,like we do for an O2 sensor, then you would put ~50mV on pin 7 (reference voltage in). So in order to achieve an adequate display gain and take advantage of the internal precision reference voltage a 25 turn 100k ohm pot is placed across pin 10(analog common) and pin 8(reference out) with the pot wiper connected to pin 7 (reference in), Figure 2.