Sensors: which gas shall I have in the loop when not in use?

[Agro]

Anyway, does anybody know which gas is in the origial cell packaging? I would expect nitrogen.

 

[RedSix]

Page 19f of, in your opinion incompetent, Kevin Gurrs "Oxygen sensors and their use within Rebreathers":

Storage and Sensor Life.
The production date of a sensor is normally printed on the sensors and can be expressed using two common methods;

1. Three (or four) numbers where the first (or first two) indicate the last number of the production year, and two other numbers that show the
month a. (example: 1205: 5th month of 2012, or May 2012)
2. A letter followed by a number(s), where the letter indicates the month, and the number the last number of the year a. (example: A12 : January 2012

Oxygen sensors do not come from the manufacturer stored in bags filled with inert gas or a vacuum. They do come in bags that once sealed allow the oxygen level in the bag to be depleted by the sensor thus slowly reducing the sensors output until all the oxygen is exhausted.
If an oxygen sensor is staved of oxygen it will almost stop providing an output as the chemical reaction is inhibited (but this can take a long time).
When they are removed from the bag they need time to activate and start responding to oxygen. Although they will give what may seem to be a
reasonable output quite quickly, it is best to allow 24 hours of exposure to air for them to become fully responsive. Two hours (exposure to air) should be
the minimum time before a sensor is used in a rebreather. If the output is still problematical then the sensor should be exposed to 100% oxygen for 2-3
minutes.
The storage temperature specification of the sensor is important. If they are stored in a hot and dry heat then evaporation of the electrolyte can result. Sensors are best stored within the rebreather after any condensed moisture has been removed.

Sensors are often supplied in a membrane shipping bag. A new sensor’s life expectancy is 142 weeks in air or 30 weeks in 100% oxygen10. After 1 year in a bag this falls to approximately 28.5 weeks equivalent to a 5% reduction. Therefore all sensors have a ‘shelf life’ when sealed in their shipping bags. In most cases this is two years. If the sensor has not been used within this period it must be discarded. Shelf life is a function of storage temperature. Temperatures outside of the manufacturer’s specification will reduce the life. If sensors are stored in 100% airtight bags for a long period, deactivation of the cathode may result and the sensor may not recover or may need 100% oxygen to recover. As a general rule after 6 months exposed to 100% Oxygen the output of the sensor can be expected to reduce by 5mV from a staring point of 50mV in 100% Oxygen. In a good sensor design, once the sensor is showing a loss of substantially more than 10% of its starting voltage say 13%-15%, then it should be changed. In the above example where a sensor started at 50mv in Oxygen then it should be changed when the same exposure reaches 45mv. Historical evidence of sensor use in rebreathers indicates that a maximum change-out period of 18 months11 should be adhered to.

Even though it might sound like a good idea to store the cells in inert gas, it isn't.
It also answers your question, which gas is in the bag when the cells are sold.

 

[Agro]

Page 19f of, in your opinion incompetent, Kevin Gurrs "Oxygen sensors and their use within Rebreathers":



Even though it might sound like a good idea to store the cells in inert gas, it isn't.
It also answers your question, which gas is in the bag when the cells are sold.

Thank you for that. I had such terrible experiences with Kevin Gurr, so I do not believe one of his words anymore. What he sais sounds resonable but out of Kevins mouth it can be true or lying or partially true, partially lying.

Is there any cell manufacturer who sais anything about this topic?

Anyway who is the producer of our 22-D? I might ask him.

 

[Wibble]

I had sensor problems which most divers have. As the unit has 4 sensors that's not a major problem.

Most people expect to replace their sensors every year.

That is just the cost of running a rebreather.

Also, I hope you know that new sensors need a couple of days to fully "wake up" when opened for the first time.

 

[Agro]

I had cell failure on brand new cells and on cells of 2-14 month old.

Most rb divers tell me the cells last for at least 18 month, that was my experience on 20 years of AP Inspiration divinging as well. In the Liberty the first cells survived for 14 month, then I had to exchange them all. One cell even died after 2 weeks, one cell died after 3 month.
The second serie survived for 15 month, during this I had to exchange one cell after 3 month.

The interessting thing was: after 14/15 month the cells passed the 3.5 bar pO2 test, nearly perfectly linear reading. But the Liberty kicked them out during dive, obviously the rebreather found something which was not detected by the pressure test, perhaps slow reaction?

Anyway I will not safe money on cells. If I have to change I change.

 

[Agro]

Sensors should not be tested above 2.0 atm O2. That's what Kevin Gurr is telling us.

Divesoft testes sensors up to 3.5 atm O2. When I sensor comes to his end I often saw a perfect reading from zero to 2.5 pO2 but poor reading between 2.5 and 3.5 pO2.

I am on Divesoft's side. I think Kevin Gurr is wrong.

 

[evandroairton]

Being watching this discussion for some time.
It really seens that some training and basic knowledge about the machine itselsf is missing, but, anyway, i will not point fingers to none specifically because i don't know who are the student and instructor involved.
I've been teached tô "dissect" both my inspo's and my Sidewinder in order to know How the machine works, and how it would try to "silent kill me" If i am not aware of it, and take good care of it betwen dives, days, and etc.
In fact Kevin Gurr wrote a lot of good things, for some period his computers (from the VR2 to the VRx) were second to none, although a bit dificult to use, and created ways of integrate them to the classic inspo Head in ways tô have realtime Deco, and the sentinel, afaik, is still going relativelly strong within a niche of users, with support from Avon, and for now VMB.
Although i have only seen the liberty for myself once, some basic principles are common to every CCR, so i would recommend this book, for those who want to know more about sensors.

And by the way, i do dry the heads after every dive, rinse the loop dayly, desinfect constantly, check linearity as learned on my training, have issues with condensation recently on one dive for user mistake, and store my cells in Air.


Kind regards to all

 

[Akimbo]

Anyway, does anybody know which gas is in the origial cell packaging? I would expect nitrogen.

I was at the Teledyne factory that made the cells in the late 1970s and they used Nitrogen, I can't say what the procedure is now or if other cell manufacturers follow the same procedure.

 

[evandroairton]

I was at the Teledyne factory that made the cells in the late 1970s and they used Nitrogen, I can't say what the procedure is now or if other cell manufacturers follow the same procedure.

Dragger does the same with it's medical range

 

[Agro]

Great, I ordered this book, thank you