Quote: (Originally Posted by
gtzavelas)
That’s an interesting concept!
Given the fact that the O2 consumption is really small (for a 3 Lt tank we could be talking about 0,3 bat / minute – that’s around 0,15% of full scale ) can you really detect the pressure drop with enough accuracy?
What’s the accuracy of the pressure sensor?
Do you compensate for the temperature?
Does it work in practice? very interestuing to see...
Cheap digital sensors are 12 bit resolution and monotonic within 1 digit (11 available bits). That is 1/2048. This is much less than the thermal noise (see below). On the O.R. units we use a 24 bit Sigma Delta ADC (Sigma Delta is always perfectly monotonic, and also linear to 1 bit), with about 3uV of noise, so have over 18 bits of data from the electronics.
The accuracy limit is the thermal effects. These are much bigger, even just within the sensor itself (thermal compensation of the sensor is not perfect), then the tank volume changes with temperature too: we look at just the rate of change of temperature in monitoring O2 consumption, not actual temperature.
There is a 2litre O2 tank on the O.R. units. This should drop by 0.4 bar per minute minimum and 15 bar per minute maximum. The rate of change of temperature then puts a margin on this.
Alex