Calibration Back-Pressure and the Megalodon CCR

Calibration Back-Pressure and the Megalodon CCR
By Kevin Watts



Recently, several threads on RebreatherWorld.com have discussed the possibility that calibration procedures common to CCR's may induce errors in PO2 measurement during a dive. One possible error during calibration is the introduction of excess pressure (back-pressure) into the environment surrounding the oxygen sensors during calibration. Calibration in an environment where back-pressure is present causes rebreather electronics to associate an oxygen sensor millivolt output to a lower than actual partial pressure of oxygen. This potential association would cause the rebreather to run richer than the diver's requested setpoint during a dive and, if the error is critical, could induce a CNS oxygen toxicity convulsion with death by drowning a likely outcome.

Rebreather divers are correct to take the possibility of calibration error due to back-pressure very seriously in any rebreather calibration they perform.

The purpose of this article is to discuss back-pressure measurements I performed on Innerspace System's Megalodon CCR using the Meg's head-only calibration procedure. The Meg's head-only calibration is very efficient in the use of oxygen. The rebreather "head" (electronics and oxygen sensor carriage) is removed from the rebreather housing that holds the head and CO2 scrubber. The head is then sealed at both ends. The input end contains a hose into which a gas is flowed and the output end contains a plug with a similarly sized hose out of which gas flows. Inside the head the oxygen sensors are exposed to the flowing gas and, therefore, can be calibrated to the known gas passing over them. This method consumes very little gas.

However, does this calibration system produce enough back-pressure within the Meg's head to cause significant errors during calibration?

After reading diver's thoughts about the potential of calibration back-pressure on the Megalodon (and some claims of direct measurement of excessive back-pressure), I contacted Leon Scamahorn (ISC CEO) to see if ISC had directly tested for this problem. At the time, the ISC staff had performed many tests that went something like this:

1. Calibrate the Megalodon using the Megalodon's head-only calibration procedure.
2. Place the calibrated head in a pressure pot and compare at various pressures the PO2 readings provided by the calibrated handset with the actual PO2 known to exist within the pressure pot.

I was informed by ISC that these tests uniformly showed that back-pressure was not influencing calibration because the PO2 measurements after calibration were very accurate.

This testing methodology is likely very accurate. The purpose of calibration is to associate millivolt output of oxygen sensors to changes in the partial pressure of oxygen. If calibrated systems indicate the correct PO2 when subjected to various pressures, then the calibration procedure seems to be doing its job.

However, after thinking about this testing methodology I decided to remain a skeptic for the following reasons:

1. The testing procedure depended on the oxygen sensors for validation. This point may seem counter-intuitive. But whether or not back-pressure exists at the time of calibration is completely independent of the oxygen sensor. Excess pressure either is or is not present at the time the calibration button is pressed.
2. The testing procedure could not answer the question, "How much pressure exists in the Meg's head at the time of calibration."

Another test for back-pressure would be to directly measure the differential pressure at the time of calibration. That is, the pressure within the head during the flow of gas over the oxygen sensors can be compared directly to the pressure outside the head. Accurately measuring and recording the difference in pressure would directly answer whether or not back-pressure develops during the Megalodon's head-only calibration.


Predictions

The physics of head only calibration can be approximated as a series of connected tubes. The Meg's head geometry is complex with at least two chambers in the head wider than the inflow and outflow tubes. One of the wider chambers houses the oxygen sensors.

The width and length of each section of the head was measured (approximately). A software model of 7 tubes connected at 8 points was constructed (when connected to the Oxycheq analyzer, the model was 8 tubes and 9 connection points). The gas flow rate was assumed known and could be modeled to flow into the first connection point. The gas winds its way through the model with pressures and flow rates varying in each chamber. Calculations can then be made for this Cyber-Meg-head to predict what the pressure might be in each section.

The resulting predictions of the amount of back-pressure in the chamber holding the oxygen sensors appear below:


Note that the model does not predict significant back-pressure developing until a flow rate 5 times the maximum ISC recommended rate is used. At 10 liters per minute, the model would predict an error of approximately 2% (0.0198 ATM) if the outflow tube is connected to the analyzer. Also note the exceedingly low back-pressure predictions for flow rates less than 2 liters per minute. When unconnected to an analyzer, a flow rate of 1.7 liters per minute results in a predicted back-pressure of about 0.0005 ATM.

Now we have some predictions, on with the tests!


Testing Methodology

The instruments used to test for back-pressure are shown below:


Various restrictors are shown on the left with flow rates between 0.5-1.7 liters/min. The medical oxygen flow meter was used to test a 3.7 liters/min flow rate. I didn't have equipment to test higher flow rates, but then again we shouldn't be calibrating with those higher rates either. The Dwyer flow meter was used to verify flow rates of the restrictors (after installation of the end caps). The Oxycheq oxygen analyzer was used to test whether connecting the outflow tube to an analyzer induced additional back-pressure. On the far right, of course, is a Megalodon CCR head. For these tests it is irrelevant which version (Apecs or Shearwater) is tested since the geometry of the gas flow is identical.

The primary instrument used to test for the presence of back-pressure (shown just to the right of the Oxycheq analyzer) is a Dwyer 477 Series differential pressure digital manometer. Two poles are provided by the instrument to compare the pressure of two gases. When one pole is left exposed, the exposed pole measures the atmospheric gas, or ambient pressure. When the other pole is connected directly to the pressure within the Megalodon's head, the instrument measures the difference between ambient pressure and the head pressure at calibration. This procedure provides a direct test for the presence of back-pressure. The manometer is accurate to within +-0.5%.

To get an idea of how sensitive the manometer can be consider the pictures below:



The first shows an increase in pressure of 4.5 millibars (above ambient pressure) when a finger is placed on one pole. Evidently squishing the air inside the pole causes about 4.5 millibars (0.0044 ATM) of "back-pressure". The second photo shows that placing the pole cap on the left pole causes about 78 millibars (0.077 ATM) of differential pressure. As reference, standard atmospheric pressure is 1013.25 millibar.

During the test I used the ISC calibration kit with one alteration. An extra hole was drilled into the outflow end cap as shown below.


An additional tube was inserted into this hole. The longer tube shown in the picture was sized so that, when placed inside the head, it was near the oxygen sensors. The other end of that tube was connected to the positive pole of the manometer. In this way the differential pressure next to the sensors at any point in the testing process can be measured.

Below is a picture of the assembled test equipment.


The IP hose connected to the restrictor is shown at top left. This hose feeds into the top of the head (not viewable in picture). The two hoses exiting the head allow gas to flow out of the head during calibration. The manometer is connected to one and, in the picture shown above, the other is connected to the Oxycheq analyzer. The rig is ready for testing!


Back-Pressure Measurements

• Test #1 - Flow rate = 0.5 liters/min

The manometer recorded no differential pressure at any point in the experiment whether or not the Oxycheq analyzer was connected to the outflow hose. The evidence indicates that restrictors producing 0.5 liters/min flow rate produce no measurable back-pressure and connecting to an Oxycheq analyzer is not a factor.

• Test #2 - Flow rate = 1.3 liters/min

The maximum differential pressure observed at a flow rate of 1.3 liters/min was 0.1 millibar (0.0000987 ATM) if the outflow tube was not connected to the Oxycheq analyzer. When the outflow tube was connected to the analyzer, the maximum back-pressure observed was 0.3 millibar (0.000296 ATM).

Although measurable, this amount of pressure is very small. If you're worried about this during calibration, I've got a list of a 1000 other calibration issues to keep you up at night!

• Test #3 - Flow rate = 1.7 liters/min

The maximum differential pressure observed at a flow rate of 1.7 liters/min was 0.4 millibar (0.0003948 ATM) when disconnected from the Oxycheq analyzer. When connected to the analyzer the maximum back-pressure observed was 0.6 millibar (0.000592 ATM).

• Test #4 - Flow rate = 3.7 liters/min

This flow rate was tested just to see what a much larger than recommended rate would do to calibration. The maximum pressure observed at a flow rate of 3.7 liters/min was 0.9 millibar (0.000888 ATM) when disconnected from the Oxycheq analyzer and 2.1 millibar (0.0021 ATM) when connected. A flow rate this high is not recommended by ISC, but you can see how robust a properly executed head-only calibration method is to differences in flow rate.

The table below summarizes the measurements performed:

Summary

We now have three evidences that back-pressure should not be a problem in the Meg's head-only calibration procedure assuming hoses are unobstructed (e.g. not squished) and a restrictor with a known flow rate under 2 liters/min is used.

First, ISC has performed numerous calibrations with verification in a pressure pot. Second, a software model simulating the physical characteristics of the Meg-head was constructed and that model does not predict back-pressure anywhere close to the amount necessary to influence calibration until flow rates are significantly over ISC's recommended rates. Finally, direct measurements were performed that also failed to observe any significant back-pressure at flow rates of 0.5, 1.3, 1.7, and 3.7 liters per minute whether or not the outflow tube was connected to the Oxycheq analyzer.

The chart below shows both predicted and observed back-pressure for the Meg's head-only calibration procedure.


All tests were performed with hoses that had an inside diameter of about 1/4" (black hoses) or 0.17" (tan colored). Both end caps had 1/4" holes. The combination of hose wall width and end cap hole width produced restrictions in the inflow and outflow hoses of about 0.17". Under these test conditions, not even a flow rate of 3.7 liters per minute induced enough back pressure in the Meg head to cause a calibration problem. Connection of an outflow hose to an Oxycheq analyzer was not observed, or predicted, to be detrimental to the process, although small increases in pressure were observable. I guess at ISC’s recommended flow rates divers must decide whether they want 0.000395 or 0.000592 ATM's of back-pressure. If you want the former, don't connect the outflow hose to the Oxycheq analyzer.

These tests address only the amount of back-pressure that developed during my testing of the Meg's head-only calibration procedure. Nothing in this article should be interpreted as implying that divers should in any way deviate from ISC’s recommended calibration procedure (e.g. know your flow rate is under 2 liters/min). Proper calibration is a multi-step process and should be thoughtfully executed and validated each time. Thinking about how you handle your life support system is your responsibility, but your life is worth the extra effort.


For what it's worth, I have no financial interest in ISC or Oxycheq or any project sponsored by either company. This project originated because I, as a Megalodon diver, wanted answers for myself to the questions raised by other divers concerning back-pressure during the Meg's head-only calibration procedure. I am not an engineer, nor an expert in any of the physical sciences disciplines.


Discuss this article HERE