Accurate PO2 Calibration

Accurate PO2 Calibration
By Joe Radomski

©2005 Joseph Radomski
No parts of this text may be used without Express written permission.


This article will demonstrate a procedure to accurately calibrate and verify many CCR controllers. The procedure used here can be extended for use at altitude and to use oxygen percentages other than 100% even though the controller may be designed to only accept 100% oxygen for calibration at Sea Level.

Similar procedures have been shown by others without necessary precautions. The procedure outlined in this article is for a HammerHead equipped CCR but will work on any similar design. Most people do not realize that complete flushes are difficult to attain and similar methods as detailed here fail to address the implication of leaving an oxygen analyzer attached.

A few custom plugs had to be machined to plug off the inlet tube and outlet on the top of the canister. An OPV is fitted to insure during a High PO2 verification or forgetting to open the needle valve doesn’t result in dangerous internal pressures.


The first part made was a restrictor designed to attach to inspiration quick disconnects, other than this the only change is that the restrictor has been made for very low flow rates (about .3 lpm) which is necessary to insure safety, to allow better diffusion, and longer oxygen exposure times for the cells without wasting lots of oxygen.


The second part is a plug for the inlet tube with an o-ring seal and overpressure valve for protection. The locking ring is used to secure the plug in place. This is a standard part available from APD.


The third component made was a plug to fit the top of the canister with AP locking ring, It is fitted with an o-ring to make a gas tight seal, a 1% accurate pressure gauge (0- 3 psi), and a t piece to connect a needle valve and oxygen analyzer.


The needle valve is connected after the pressure gauge and will be used to set the desired backpressure for calibration at altitude, alternate oxygen percentages, or High PO2 verification.


The picture above shows all the components assembled and ready for calibration.

It is important that the solenoid fire several seconds to flush any air out of it. If the solenoid fires after the canister has been flush and air remains the calibration will be in error. Turn the oxygen on and use a set-point of 0.7ata. This will start firing the solenoid and filling oxygen into the canister. It is better not to leave the set-point here, after a few seconds lower it to 0.4 ata.

Observe the PO2 analyzer and when the readings stabilize at the analyzed oxygen percentage, the system is ready for calibration. The time it take varies based on flow rate of the restrictor and if the canister is full of sorb.



These pictures above are showing the oxygen percentage seen by the analyzer and the pressure gauge showing this analyzer has added almost .2 psi of back pressure even at an extremely low flow rate. The higher the flow rate the greater the back pressure will be. I compared several analyzers with off the shelf restrictors and most fell in the range of .5 to 1.0 PSI of back pressure.

The next two photos show the measured PO2 before a new calibration. The handsets were last calibrated over 2 months prior to these photos. The first picture shows the readings with the Oxygen analyzer attached, followed by a picture immediately after removing the analyzer.


It can be seen that the PO2 readings drop slightly (1%-2%), greater flow rates will demonstrate a greater change. As altitude increases small pressure changes during calibration induce a larger overall error.

These pictures also demonstrate the stability of the HammerHead’s calibration. There is only a 3%-4% error per sensor, even after an extended period between calibrations.


The HammerHead displays PO2 in the proper units of ATAs unlike many other CCR controllers out there which chose to ignore the standard and indicate PO2 in bar. The nominal reading for sea Level should be 1.00 ata, unlike a controller indicating in Bar of 1.013. The calibrated wrist units are displayed on the right.


Controllers like the HammerHead assume Sea level and an Oxygen percentage of 100% to set the internal references. The advantage of using an accurate pressure gauge with the ability to create a predetermined back pressure is that the unit can still be calibrated at altitude or another Oxygen percentage as long as the combination of internal pressure multiplied by the oxygen percentage results in a PO2 of 1.00 ata. The following table shows various Oxygen percentages and altitudes and the required back pressures to accomplish this task.


Once the handsets are calibrated it is possible to check the cell’s output for a higher PO2 by simply adding a specific pressure to the pressure required for 1.00 ata. My normal set-point is 1.2 atas, this requires a back pressure of .2 * 14.696 = 2.94 PSI, a1.3 would require .3 * 14.696 = 4.40 psi. A 3.0 psi gauge works out quite nicely since 3.0/14.696 = .204, which results in a PO2 of 1.204.

Below is the test system pressurized to 3.0 PSI and the wrist unit displays the expected PO2! I now know these cells are linear from 1.0 up to my expected set-point and current limiting at this point is unlikely.





©2005 Joseph Radomski
No parts of this text may be used without Express written permission.


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