Thanks for the information.
For all of you Homebrewers out there...
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Richeod
Guest
Here is another good one that looks very nice.
http://www.instrumentation2000.com/catalog/pressure/01/0201a_ashd1005ps.html
0-10,000 PSI .5% Full Scale and only 2.5" all for $195.
For another $45 upgrade to .25% Full Scale.
It has a ton of standard features also.
http://www.instrumentation2000.com/catalog/pressure/01/0201a_ashd1005ps.html
0-10,000 PSI .5% Full Scale and only 2.5" all for $195.
For another $45 upgrade to .25% Full Scale.
It has a ton of standard features also.
I use two digital gauges from Cole Parmer (el cheapo $114 job mentioned above) on the fill whip I take into the field. Frankly in several years of mixing with analog gauges, they are an improvement. The one thing that annoys me is the reading jumps around while filling at a constant flow... 1345 psi - 1356 psi - 1346 psi and so on. Both guages do it and more or less out of sync. It's pretty, but annoying.
As for general accuarcy, I think they're fine since the analysers we use to measure gas ratios give results that are somewhat dubious and far from scientific.
DD
As for general accuarcy, I think they're fine since the analysers we use to measure gas ratios give results that are somewhat dubious and far from scientific.
DD
What is wrong with the analog guages. All the gas I have blended has been on an analog guage, and I am usually no more than a percent off. I have not had the luxury or the money to purchase a digital guage, so I use an incremented analog guage that ranges from 0-4000 PSI. I will admit the guage is a little big (about 5 inches diamteter) but it is for blending and not diving. I know I will go to digital in the future, but for now, I see nothing wrong with my analog.
ADAM
ADAM
Stone
Contributor
I continuous blend, so I don't need a great gauge, but I do transfill my emergency O2 tanks. I like the analog gauge because it is easy to see the rate at which the needle is moving (to keep my fill rate slow).
As for fill accuracy, continuous blending with an O2 sensor on the compressor output is dead on (within the accuracy of the analyzer). The only variability is that the FO2 starts to creep up a little after the tank reaches about 2000 psi. I don't bother to adjust for this, so if I'm shooting for 34%, I know I'll get 34.1%. The calculations tell me this, not the analyzer. The analyzer will move +/- .1% if you look at it funny.
Not sure which statement you need explained... if it's about the accuracy of the digital gauges being fine for me... I am prepared to work within the parameters of their inefficiencies when mixing in the field. I use standard mixes and a set procedure when filling and come out with consistant results. (Worth noting perhaps that I also use a fudged interpretation of Van de Waals equation to compensate for real gases behaviors. But find my results acceptable.)
If you are refering instead to the statement about oxygen analysers being "somewhat dubious and far from scientific," the degree of inaccuracy from an O2 analyser is well documented and associated with so many variables -- such as humidity and temperature -- that the readings given are approximations and as such could not in any way be thought of as a scientific measurement... to be so, one would have to point to a baseline set in a generally repeatable and practical situation... and blending scuba gas simply does not happen that way. It's more art and alchemy than science.
Doppler
M.Sc.
(tehehe)Yep, thats what I thought. You dont have a real understanding of gas analysis methods. The real source of error is the sample analysis protocol and not the analyzer. With a proper analysis protocol the influence of environmental variables will be eliminated or minimized to an inconsequential amount.
Specifically, humidity is only an issue if the initial calibration is done to ambient air and the sample tested is a compressed gas. The use of a calibration cylinder of gas will provide a standard with essentially the same moisture content. This is an easy practice to implement. An alternative is the use of table of correction factors or the use of an equation to determine the relative humidity and hence the change of volume as a result.
Any competent blender will tell you that you should not analyze a hot mix. The proper procedure is to make sure the gas is at ambient temperature. This is standard laboratory practice and can also be easily incorporated into garage blending by using patience and planning ahead and only analyzing a mix after it has cooled. The use of temperature compensated sensors and/or analyzers also help.
You forgot the one of the most important point: Pressure. Galvanic oxygen cells respond to the partial pressure of oxygen. The use of a flow meter to provide repeatable flows (i.e. pressure) is very important. Again another easily implemented practice.
Next selecting the appropriate calibration gas and calibration points (this is your baseline) is important as well. You want to bracket the target gas composition with the calibration points. Also after analyzing a mix you should check for instrument drift by analyzing the calibration gas and noting if it is in agreement with the initial value(s).
Just following these simple steps will greatly reduce the sources of error and provide a much more reliable and repeatable gas analysis. You can get the typical oxy-hacker oxygen analyzer to provide an accuracy of better than +/- 1% by following standard gas analysis methods.
There are a number of other steps that can be included as well which each reduce measurement error. The use of 2 & 3 point calibrations, a series of three consecutive readings of the same calibration gas, and use of the mean of the three values for the calibration point, a Constant Temperature Bath that can maintain the temperature of the cylinders at a set level, Using polarographic sensors which provide a more accurate reading etc. etc
It all depends on what you want to do, be lazy and sloppy and chalk it up to an Art or do it in a competent scientific manner.
omar
Specifically, humidity is only an issue if the initial calibration is done to ambient air and the sample tested is a compressed gas. The use of a calibration cylinder of gas will provide a standard with essentially the same moisture content. This is an easy practice to implement. An alternative is the use of table of correction factors or the use of an equation to determine the relative humidity and hence the change of volume as a result.
Any competent blender will tell you that you should not analyze a hot mix. The proper procedure is to make sure the gas is at ambient temperature. This is standard laboratory practice and can also be easily incorporated into garage blending by using patience and planning ahead and only analyzing a mix after it has cooled. The use of temperature compensated sensors and/or analyzers also help.
You forgot the one of the most important point: Pressure. Galvanic oxygen cells respond to the partial pressure of oxygen. The use of a flow meter to provide repeatable flows (i.e. pressure) is very important. Again another easily implemented practice.
Next selecting the appropriate calibration gas and calibration points (this is your baseline) is important as well. You want to bracket the target gas composition with the calibration points. Also after analyzing a mix you should check for instrument drift by analyzing the calibration gas and noting if it is in agreement with the initial value(s).
Just following these simple steps will greatly reduce the sources of error and provide a much more reliable and repeatable gas analysis. You can get the typical oxy-hacker oxygen analyzer to provide an accuracy of better than +/- 1% by following standard gas analysis methods.
There are a number of other steps that can be included as well which each reduce measurement error. The use of 2 & 3 point calibrations, a series of three consecutive readings of the same calibration gas, and use of the mean of the three values for the calibration point, a Constant Temperature Bath that can maintain the temperature of the cylinders at a set level, Using polarographic sensors which provide a more accurate reading etc. etc
It all depends on what you want to do, be lazy and sloppy and chalk it up to an Art or do it in a competent scientific manner.
omar
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