Calibration all over the place!

[geoff w]

I've used Analox nitrox analyzers for years in shops and dive boats, and finally got my own, an O2EII. This past weekend (send time I've used it), I calibrated with an air tank to 20.9, walked over to my tank to see what the dive boat was pumping in with their membrane, and before I measured my tank, looked down and the display on my O2EII and it was off by a few percent. Is this normal? It wasn't particularly hot or humid, just typical So Cal conditions. I recalibrated against the same tank, and when I got back to my tank the display read one percent different in the opposite direction. Do new sensors take a few times to settle in? Am I doing something wrong? Or is there something wonky with my O2EII?

It was bought new as a sealed unit, and I've kept the O2 sensor cover on except when on the dive boat; it lives in a little Pelican case when not in use

 

[fire_diver]

Lots of possibilities here. There is a pressure connection to the readings, so it may drop from the air tank blasting over it, to standing in open air. Another possibility is that either the battery is low or the O2 cell is expired. Yes, it's possible to get an old cell in a "new" unit if it's been on a shelf for a long time. These also have a life of about a year after opening, depending on use and conditions.

HAve you tried calibrating it on the tank, moving it to still air then going back to the air tank to see if it goes back to the 20.9 reading?

 

[Patti Analox]

Hi Geoff,

Patti here with Analox Sensor Technology. The unknown is actually what the O2 percentage is in your tank of air being used to calibrate the O2EII. I would recommend that you "fresh air" calibrate the O2EII (take off the dome flow restrictor and dial in 20.9%), then take a reading of both your Nitrox tank AND your air tank. Do not re-calibrate the EII between readings. How old is your EII? I like reading about the excellent care you are giving it, however, please be aware the O2 sensor should be changed after three years regardless of whether it has been in use or storage. Sealing the analyzer up in a Pelican box with the sensor saver does slow the depletion of the electrochemical, but it does not stop it.

 

[Brandon]

It'll take some time for the sensor to wake up after you pull the cap off - Analox says to give it at least a couple hours. Until then, the readings are going to be wonky.

Also - pull the flow restrictor cap off and just calibrate in ambient air. No promises that the other cylinders on the dive boat are 20.9%, but the air around you almost certainly is.

 

[geoff w]

Thanks all for the quick responses

Lots of possibilities here. There is a pressure connection to the readings, so it may drop from the air tank blasting over it, to standing in open air. Another possibility is that either the battery is low or the O2 cell is expired. Yes, it's possible to get an old cell in a "new" unit if it's been on a shelf for a long time. These also have a life of about a year after opening, depending on use and conditions.

HAve you tried calibrating it on the tank, moving it to still air then going back to the air tank to see if it goes back to the 20.9 reading?

It's about a month old. It looks like it was ordered by the dive shop for me, and date stamped on the container says April 2014. No low battery warning indication

Hi Geoff,

Patti here with Analox Sensor Technology. The unknown is actually what the O2 percentage is in your tank of air being used to calibrate the O2EII. I would recommend that you "fresh air" calibrate the O2EII (take off the dome flow restrictor and dial in 20.9%), then take a reading of both your Nitrox tank AND your air tank. Do not re-calibrate the EII between readings. How old is your EII? I like reading about the excellent care you are giving it, however, please be aware the O2 sensor should be changed after three years regardless of whether it has been in use or storage. Sealing the analyzer up in a Pelican box with the sensor saver does slow the depletion of the electrochemical, but it does not stop it.

I tried calibrating on a few different tanks and air. I'll try to fresh air calibrate this weekend. I don't remember reading it takes a couple of hours for the sensor to wake up; if so, it didn't have anything like that from when I pulled the cap off

It'll take some time for the sensor to wake up after you pull the cap off - Analox says to give it at least a couple hours. Until then, the readings are going to be wonky.

Also - pull the flow restrictor cap off and just calibrate in ambient air. No promises that the other cylinders on the dive boat are 20.9%, but the air around you almost certainly is.

I've got a fresh tank with just air and two leftover tanks from last weekend with different flavors of nitrox. I'll try letting the O2EII wake up, calibrate it with air, check the tank with air, and then see what I get from the other 2 tanks

 

[geoff w]

OK, here's what I tried the other day. Took analyzer out, took off sensor saver cover, turned it on, left off the dome port. In ambient air, it read around 22.7 for a while. Put the dome port on and calibrated it with a freshly filled tank to 20.9 (first fill on the tank). Let it sit for a minute or two and it stayed at 20.9, then analyzed two other tanks that had nitrox fills before, one that had been topped off with air a couple of times. The results roughly matched what I calculated the nitrox percentage to be (my math was only to ballpark). When I pulled the analyzer off the last tank, it slowly drifted back down to around 20.9 and seemed to stay stable instead of bouncing all over the place.

This is the first time I used the analyzer on land instead of on a dive boat, and it worked like all the O2EII analyzers I've used before. Is it typical for the O2EII to vary a bit more on a dive boat? Or are there any tricks to use for analyzing nitrox tanks on a dive boat I should be aware of to get better readings?

 

[Patti Analox]

Hi Geoff,

Good to read the O2EII is behaving as expected. The short answer is the O2EII should not behave differently on a dive boat vs land. It is made for use on a dive boat and is the preferred analyzer for many liveaboard operators. Only suggestions I have to be sure not to leave it in the sun as temperature will effect O2 readings. Pop open the pelican box when you get aboard so the temperature inside can acclimatize to the outside temperature ( I leave my storage case at home: the EII is tough enough to go it alone!)