Helium analyzers

How does a He analyzer work?

I understand how an Oxygen analyzer works, but He is an inert gas.

Would someone explain it please.

TwoBit

TwoBitTxn once bubbled...
How does a He analyzer work?

I understand how an Oxygen analyzer works, but He is an inert gas.

Would someone explain it please.

Click to expand...

It compares the thermal conductivity of the sample gas to that of a known reference. It uses heated resistance thermometers (this is similar to the mass flow sensors in cars). See http://www.atomox.com/he_tech.html

Helium is much more conductive than nitrogen and oxygen.

The measurement technique is a pair of Thermal Conductivity Cells in a Wheatstone Bridge circuit.

To determine gas composition by thermal conductivity each gas must have a significantly different thermal conductivity to be able to differentiate the amounts. Because the thermal conductivities of Nitrogen and Oxygen (N2 and O2) are very similar they can be considered to behave as a single gas. Hence, a mix of oxygen, nitrogen and helium can be considered a binary mix for thermal conductivity measurements. The thermal conductivities of Air, N2 and O2 are: 26.2, 26.0, 26.3, milliwatts per meter Kelvin, and the thermal conductivity of Helium is 156.7 mW/mK. (Values for 300 degK/27 degC)

The technique is not suitable for measuring extremely low levels of gas (say, <1% by volume resolution), and it has difficulties when the gas to be measured has a thermal conductivity similar to the background gas. Thus measurement of oxygen in air is not feasible because oxygen has essentially the same thermal conductivity as air. However, for certain gas mixtures it is an extraordinarily useful method of measurement.

In addition, the conductivity of a gas mixture is always less than a weighted average based on the number of moles (or molecules) of each component in the mixture. This appears to be related to the different effect that molecular weight has on thermal conductivity and viscosity. Light gases are usually better conductors than are heavy gases, whereas heavy gases are often (but not always) more viscous than are light gases. There also seems to be some correlation between molar heat capacity and thermal conductivity.

Because of this theoretical models turn out to be poor predictors of the behavior or response of thermal conductivity analyzer to a gas mix. This is not unexpected. The science regarding thermal conductivity of gases is still some times referred to as black magic because there are inadequate explanations for the actual behavior of gases.

omar

I understand how it works now.

Omar,

If I understand you correctly, at low levels thermal conductivity meters can have accuracy problems.

At helium levels trimix divers are using are thermal conductivity meters accurate or is there another way to meaure the He in a cylinder.

TwoBit

The low levels I was referring to are less than 1% or ppm (parts per million) range. A trimix of only 1% helium would in effect be the same as air.

I know that the ATOMOX HeLIUM ANALYZERS http://www.atomox.com
are accurate +/- 1% (actually better).

There are other techniques to determine helium contents such as the speed of sound or the optical wavelength of the gas fractions.

omar

An interesting distinction is that, while an O2 analyzer is actually reacting the amount of O2 in the sample, a He analyzer is just surmising what the He content is based on the assumption that the other gases are nitrogen and 02. That is to say, if you inadvertently feed some other mixture of gases to it, an He analyzer will dutifully attempt to report its %He even if there is no He in it at all. This shouldn't really be a problem in real life, unless you are keeping bottles of hydrogen or maybe neon in the mix room.

BTW, the He analzyer game is heating up.
http://www.oxycheq.com
has a nifty new pocket sized He analyzer. These things are quite simple mechanically once you get the electronics sorted out; it will be interesting to see what effect competition has on the price.

LOL Vance…. (heating up ;-)

But, it seems with the introduction of the oxycheq the price has gone up. The oxycheq MSRP is more than the Atomox distributed by Dive Rite and you have to buy the smart charger on top of that.

As far as operation of the oxycheq you need to screw around opening and closing the flow cap to capture a gas sample every time you want a reading, it takes a minute to get each reading, you need to turn it on and off between readings to save the battery, you can not use the smart charger when the unit is on, and no mention is made if running a high oxygen mix through the sensor/flow cap is safe. So as far as using it for continuous blending it is very limited.

When I continuous blend it is for a number of hours. A limited battery life and not being able to hook it up to the main power supply is a detriment. During start up I need continuous, instantaneous readings of the mix so I can get it dialed in without wasting a lot of gas and the last thing I need is to have to worry about capturing a gas sample for a reading. Atomox recommends the sample flow rate be 2L/min but has indicated that the accuracy of the unit will still be within +/- 1% for flow rates of 1 to 12 L/min. So all I do is turn on my unit, plug in the gas and that's it.

The oxycheq is smaller and more portable which would be a benefit if you were mixing on a boat.

omar

omar once bubbled...
The oxycheq is smaller and more portable which would be a benefit if you were mixing on a boat.

Click to expand...

Well I don't think the Atomox is that big a package to take on the boat at all... I mean if your boat is big enough to be tech diving off of then that little pelican box is nothing... alas, I do not have a battery pack for mine... just a lowly adapter that plugs into the wall.

But really... who needs the He analyzer for boat mixes anyway.... just use PP blending and analyze your O2.

The Atomox is the ticket for Continuous Blending.

I have the atomox and the OMS O2 analyzer, they both fit ncely into the same pelican cased the He analyzer came in. and btw if you take the cover off the He analyzer you can put batteries in ther. I ahven't done that yet but i understand the batteries willl last quite a while

i find the He analyzer most usefull for air top ups for your second dive on the same tanks

omar once bubbled...
LOL Vance…. (heating up ;-)

But, it seems with the introduction of the oxycheq the price has gone up. The oxycheq MSRP is more than the Atomox distributed by Dive Rite and you have to buy the smart charger on top of that.

As far as operation of the oxycheq you need to screw around opening and closing the flow cap to capture a gas sample every time you want a reading, it takes a minute to get each reading, you need to turn it on and off between readings to save the battery, you can not use the smart charger when the unit is on, and no mention is made if running a high oxygen mix through the sensor/flow cap is safe. So as far as using it for continuous blending it is very limited.

When I continuous blend it is for a number of hours. A limited battery life and not being able to hook it up to the main power supply is a detriment. During start up I need continuous, instantaneous readings of the mix so I can get it dialed in without wasting a lot of gas and the last thing I need is to have to worry about capturing a gas sample for a reading. Atomox recommends the sample flow rate be 2L/min but has indicated that the accuracy of the unit will still be within +/- 1% for flow rates of 1 to 12 L/min. So all I do is turn on my unit, plug in the gas and that's it.

The oxycheq is smaller and more portable which would be a benefit if you were mixing on a boat.

omar

Click to expand...

Nope..

If my partner sees this he'll probably be pissed (he has a good friend that works for DR) but we now have any Oxycheq one in the store.

I have used both.. I now have the Oxycheq analyzer (actually I was the First customer to get one)..
The Oxycheck analyzer does need to be charged. It lasts a few hours on the charge but the charger comes with it..
the test time is very fast..all you have to do is flow gas through for a few seconds(at low flow rates 5 seconds is more than enough).. if you have a high flow rate its basically instantaneous(the flushing).., push in the sampler and wait.. on average you will get a stable reading in under 15 seconds.. I find mixes up to 50% average well under 10 seconds(its within 1 % in under 3 seconds).. the atomox takes much longer and uses much more gas for most uses.. plus you have to let the unit go through a long heatup (4 minutes if I remember correctly) before you can take a reading.. IF you like a unit like that oxycheq will also be selling a unit with built in O2 made for him by teledyne and will have all the same drawbacks as the atomox..

I have tested it(oxycheq) on mixes from 0% helium (which the oxycheck reported 0.1) to (100% helium which it reported as 99.8) the atomox can't test for less than 4% or over 90% (actually I have never seen it read 90% on mixes that should be 90%) I did a 90-10 heliox were my o2 analyzer said .10 but the atomox said 89% (still witihin it error range) but clearly not as accurate as Oxycheq's.
If you aren't traveling with a He analyzer weight wount be an issue.. I travel with lots of gear and every pound I save (it would be several) is welcomed.

Even if you are continuous blending trimix there is no reason to have to constantly monitor He content.. O2 can be constantly monitored (as a precaution) if it changes you know the He is changing.. with good flow regulators there should be no drifting once you get your mix set.. the better regs are compensated for drop in tank pressure.


I will do some detailed timed testing that will probably be shown in AdvancedDiver magazine.