Molex Connectors

[LFMarm]

I am considering building a cell-checker pressure pot as well. Many thanks for sharing your experiences in this thread.

How do you account from the fact that the vessel initially has air and the fraction of O2 changes as you increase the pressure?
I did some quick calculations below to simulate going from 0 to 38 psi measured by the gauge by adding O2 to a vessel of 1.26 L of volume initially filled with air at an ambient pressure of 820 mbar. The fraction of O2 goes from 21% (air at the beginning) to 81% when reaching 38 psi by adding O2. The pO2 in the vessel is calculated based on absolute pressure and fraction of O2 in the vessel.

 

[LFMarm]

Did some more investigating and happy to share the findings.

Method 1 (most commonly used): linearity check in pure O2

1. Take a reading in air at ambient pressure (this is the point at the bottom-left of the chart, used to extrapolate the expected readings), pO2 ~ 0.17-.21 depending on altitude, temperature, humidity.
2. Flush the vessel completely with O2 at ambient pressure (exhaust valve open). pO2 ~ 0.8-1 depending on altitude
3. Start measuring mV as you increase the pressure in the vessel until you get to about pO2 = 2.0.

This gives a chart similar to the one below (data is randomly generated). You can get to pO2 = 2.0 with a vessel pressure of just 17 psi.

Method 2 (alternative): linearity check in enriched air

1. Take a reading in air at ambient pressure (this is the point at the bottom-left of the chart, used to extrapolate the expected readings), pO2 ~ 0.17-.21 depending on altitude, temperature, humidity.
2. Start filling the vessel with O2 without flushing (exhaust valve closed).
3. Measure mV as you increase the pressure in the vessel until you get to about pO2 = 2.0.

This gives a chart similar to the one below (data is randomly generated). As you can see there are more measurements around pO2=1.0 compared to the previous method. This second method requires getting to a higher final pressure of about 27 psi in order to reach pO2 = 2.0.

Once I build the vessel for testing, I will check which one of the 2 methods is easiest and gives the best results.

 

[LFMarm]

Just wanted to provide an update. I have been regularly testing sensors with the vessel and it really helps to identify sensors that should be replaced, especially if pushing them beyond the recommended life (I found that just looking at overall mV reading for a certain pO2 overtime is not a reliable indicator as you may lose linearity well before the mV at pO2=1 goes below the recommended range).

As an example, I am still comfortable using the sensor on the left while the one on the right is showing current limitation and would not use for a dive.

In addition to extending life of sensors, I found this very helpful to test sensors that may be going bad before their end of life (e.g., because of flooding).

 

[rjack321]

Start with air, calibrate your millivolt output to your barometric pressure and humidity in air.
record mV
Flush your checker with O2
Start your linearity check at 100% O2

Stop using cells beyond their recommended life. A pressure pot is not an excuse or method to deviate from manufacturer guidelines.

 

[inquis]

Diving at altitude, the highest I can dry test is a PO2 of 0.8 (pretty far from my setpoint of 1.2), so the initial descent is my "pressure pot". I briefly stop at the depth that should give a PO2 of 1.6 to verify that linearity to within ±0.05 bar (3%) and note any cell that might be a bit slower to respond.

 

[LFMarm]

Stop using cells beyond their recommended life. A pressure pot is not an excuse or method to deviate from manufacturer guidelines.

I experienced the opposite with 3 cells over past 2 years. Cells becoming non-linear at higher pO2 before their expiration date.

 

[rjack321]

I experienced the opposite with 3 cells over past 2 years. Cells becoming non-linear at higher pO2 before their expiration date.

They do that too. Just saying manufacturer expiry, typically one year from opening with a second not to be used past date should be a hard stop. This is regardless if they still appear linear.

Revo people are particularly bad about keeping 2 of their 5 cells past 1 year