Nitrox stick O2 sensor placement

[Diver-Drex]

I have an MCH 16, which I’m estimating an average cfm of 10cfm over the operating pressure during filling. This would give me 39lpm of O2 flow for EAN 32, and 67lpm for EAN 40.

I’m going to use a typical O2 welding reg and a 10-100lpm flow meter. 2” PVC for the mixing chamber. My compressor intake is 1”.

My question is placement of the O2 sensor. If I tap into the 2” the sensor would be seeing 5cfm flow. Is this going to cause an issue with readings because of high flow rate? If I use a 2x1 inch tee to hold the sensor out of the main flow, would this ‘correct’ the high flow rate impact or possibly not give enough flow? Or tee off a 1” section just prior to the intake hose?

Thanks,
DD

 

[rjack321]

The flow rate issue comes into play when the gas is flowing "against" the sensor as it creates a relatively pressurized membrane. If there is a T in intake line with the cell off to the side it's not an issue. Also you calibrate with the analyzer with the compressor running and no O2 flowing. So any minor pressure differentials skewing the sensor up or down are accounted for under air (20.9%) conditions anyway.

 

[tbone1004]

doesn't particularly matter so long as it is after the mixing chambers. The closer to the intake head the better. It is very important to note that depending on what you use for the mixing baffles that you are not creating too much negative pressure in the intake. This matters for two reasons, first is it can cause the pump to work harder and less efficiently which is not good for it, but also if you calibrate the meter with the compressor off you'll end up with a skewed reading on the sensor since partial pressure is directly related to ambient pressure. As @rjack321 said if you calibrate with the compressor running and O2 off then you'll be fine but this can throw the final mix off depending on how large the filtration system is so you do want to be very quick with the calibration.

I personally would use a reducing T to go from the 2" to the 1" and put the sensor there

 

[Diver-Drex]

Thanks. I highly respect input from the both of you.

 

[tmassey]

It is very important to note that depending on what you use for the mixing baffles that you are not creating too much negative pressure in the intake.

Any suggestions on what we should be targeting for back pressure?

I just rebuilt my trimix stick. For my 6 CFM compressor I’m using approximately 6 feet of 2 inch PVC with the following layout: 02 injection, then approximately 24 inches of bio balls, then O2 sensor, then helium injection, then approximately 24 inches of bio balls, then O2 sensor. Approximately 4 inches between each injection spot, bio balls, sensor, etc. which uses up the extra 2 feet of length.

Measuring at the final O2 sensor spot, a magnahelic measures approximately 4 1/2 inches (water) of vacuum. Unfortunately that doesn’t include about 8 feet of 1 inch vinyl tubing that goes between my trimix stick and the compressor itself.

I’m not using any kind of tee for the O2 sensors: I simply drilled a hole in the PVC and stuck the O2 sampling nipple into the flow. I have paid attention to the difference in calibration from still air and with the compressor running. Probably varies about 0.3% between the compressor running and not running.

I’ve looked up compressor specifications for supply piping length and number of elbows and I’m well, well under that length. But I’m assuming they’re not packing their tubing full of bio balls, though. So I’m just trying to get a general consensus on what is a reasonable amount of pressure drop on the intake.

From a blending standpoint, my setup is 100% fine: I start my compressor, let everything stabilize for maybe 15 seconds, then hit the calibrate button on my O2 sensors. They calibrate, and stay nice and stable. But from a loading my compressor standpoint I have no idea. That’s actually the part I’m most worried about. That’s primarily what’s motivating my question, but I’d take any information anyone could supply.