Oxycheq El Cheapo II

[chrisc]

Built the El Cheapo, I purchased the upgrade 10 turn pot and flow resistrictor with cap. After leaving the o2 sensor (Teledyne R-17) out all day, I can zero (20.9) the sensor. When getting a reading from regular air in a tank I get a reading of .5 higher (21.4), when taking a reading of a tank with 31% I get 32.4%, I checked it with my neighbors analyzer and his reads 31.2%. When reading a tank that was filled at the shop 33.4%, I read 34.4% neighbors reads 33.6%.

Any ideas on what is possibly wrong? I checked all connectons so far.

Thanks-

 

[miesemer]

I have no experience with the OxyCheq kit but I'd assume calibration of its sensor would be no different than the sensors I'm using.

Were both analyzers calibrated at the same time in the same location? If not, you may find heat and humidity will affect open air calibration. For temps 70 - 80° and RH in the 70 - 80% range, I calibrate at 20.3 in open air or I calibrate at 20.8 with dry air from a room temperature (68°) air cylinder.

 

[DA Aquamaster]

Pressure makes a difference and the slight increase in pressure inside the sensor chamber causes a higher than normal reading.

I have found that if I calibrate on a tank of air and set it to 20.9, I get an open air reading of 20.6. Consequently, if I am doing only an open air calibration, I will set it to 20.6, not 20.9 and the results are then consistently accurate. Sensors and humidity may vary so I'd suggest calibrating on an air tank, then seeing where the reading settles a few minutes later after you remove the sensor chamber to expose the sensor to open air. That approach has the advantage of adjusting for humidity and if you track it under different humidty percentages, over time you'll get a feel for exactly where to calibrate it - but to be honest, adjusting for humidity is more accuracy than you need and it is lost in the noise of sensor accuracy anyway. If you need to be that spot on, you should be calibrating off a known source through the restrictor/sensor cap.

I also use the optional sensor saver cap to prolong sensor life (4 years and counting on the original sensor) but I have noted it takes a few minutes for the sensor to "wake up" so don't calibrate immediately if you are using the storage cap.

Beyond that sensor accuracy is normally +/-.5% so some disagreement with the neighbor's sensor could result if both sensors are on opposite ends of the acceptable range of accuracy.

 

[Cave Bum]

Built the El Cheapo, I purchased the upgrade 10 turn pot and flow resistrictor with cap. After leaving the o2 sensor (Teledyne R-17) out all day, I can zero (20.9) the sensor. When getting a reading from regular air in a tank I get a reading of .5 higher (21.4), when taking a reading of a tank with 31% I get 32.4%, I checked it with my neighbors analyzer and his reads 31.2%. When reading a tank that was filled at the shop 33.4%, I read 34.4% neighbors reads 33.6%.

Any ideas on what is possibly wrong? I checked all connectons so far.

Thanks-

Ambient air contains humidity which suppesses both the nitrogen and the oxygen percentages. So if you have 79.1% Other & 20.9% O2 and cram in some humidity on top of that, the percentages go to like 20.4% and 78.7% with the remaining 0.9% being mosture.

If that is indeed the problem you're seeing, calibrate on pure air (out of the compressor, not out of the air bank).

The other issue might be pressure. The galvanic O2 sensor is pressure sensitive, the higher the pressure the higher the O2 reading. Is the pressure during calibration the same as the pressure during tank sampling?

Have you tried it side-by-side with another analyzer that uses unpressurized ambient air to calibrate?

 

[Scuba Duck]

The output of the oxygen sensor is a voltage that is porportional to the partial pressure of oxygen, it does not respond to % of oxygen directly. Through the process of calibrating the analyzer, you tell the analyzer that a given sensor output corresponds to a particular % of oxygen. Anything that affects the partial presure of the oxygen will affect the readout of the meter, this includes pressure swings. Were you to leave the analyzer on all day, you could watch the reading change with the normal pressure swings of a given day, this is especially noticable when there is an incoming storm. The sensor also responds to changes in temperature (although Teledyne claims it is temperature compensated) . As the sensor heats up, the chemical reaction speeds up, and the output voltage goes up for no change in oxygen partial pressure. Additionally, if you put pressure against the sensor, the reading will go up, as the partial pressure goes up, for no increase in the % of oxygen.

To to best use your analyzer, you should calibrate the analyzer on a known standard. Then use this calibrated analyzer to measure the Nitrox content of your Cylinder. Compressed Scuba air is a fairly good standard. Through the course of pressure drying the air, little moisture/humidity is left that will through off your reading.

You can download this spreadsheet to see how a given temperature and %relative humidity affect the oxygen % of the air you breath. If you were so inclined, and equipped with a temp/relative humidity meter, you could determine the oxygen content of air, and use this value to calibrate your oxygen analyzer to.

http://rubberduckiedesigns.com/Documents/Nitrox Controller Downloads/Humidity Spreadsheet.xls

 

[DA Aquamaster]

The other issue might be pressure. The galvanic O2 sensor is pressure sensitive, the higher the pressure the higher the O2 reading. Is the pressure during calibration the same as the pressure during tank sampling?

If I read the OP's post correctly and correctly interpret what he has, he has the upgrade to the "expedition" sampling chamber that scews on the sensor. (Same thing I have on both the ones I own.)

In that case, there is a slight increase in pressure in the sampling chamber compared to an open air calibration. In my experience, for testing a tank pre-dive, open air calibration with a 20.6 calibration rather than 20.9 works fine. For gas mixing, my preference is to calibrate to 20.9% off a tank of air.

 

[dannobee]

Calibrate the el cheapo off of a tank of 100% oxygen. That will compensate for the PPO difference.

 

[Cave Bum]

Calibrate the el cheapo off of a tank of 100% oxygen. That will compensate for the PPO difference.

I'd suggest not for 2 reasons

1. It wears out the sensor much faster
2. The greater the offset, the greater the error. Calibrate with a known gas source as close to the PPO2 your testing as you can find.

That's my 2 cents.

BTW, thanks, DA, I wasn't aware that Patrick was selling an add-on chamber that would fit the home-built kit. Sounds like that may indeed be the issue, wave it around to calibrate a ambient pressure, sample tank at slightly higher pressure...

 

[dannobee]

You're really splitting hairs here, especially in light of the fact that the accuracy of the analyzer is suspect.

The little amount of time on 100% would be minuscule in the life of the sensor and would verify a problem with the expedition cap trapping too much pressure, which is likely the case.

If the output goes up when going from free air to a scuba tank of air, the problem is the pressure exposed to the sensor.

 

[chrisc]

Ok, thanks for everyones help! A few things that I screwed up on...

1- Battery was weak.
2- Left the sensor out overnight to ambient air to wake sensor.
3- I was calibrating the unit inside 76F with 20% rh, then going into the garage that's 94F and 85%rh and not reclibrating. :bonk:
4- Calibrated off a known Air (20.9) tanks, when removing the sensor cap reading dropped to 20.1%. Plugging info into Excel Spreadsheet list above caculation came to be 20.13%
5- Tried my tanks with shop % listed, the El Cheapo differance was .2% -.3%.
6- Neighbor keeps his OMS unit in the garage with his dive equipment, this already at the ambient temp & rh when calibrated.

Thanks for everyone's input & help!!