It sounds like there is a fairly substantial restriction in your Nitrox Stick (I think Eisa bought the Tri-Hunter). Typically this is seen with a cascading orifice, or other mixer design that mixes by reducing the cross sectional area to cause turbulent flow. Be careful, this can ultimately reduce the lifespan of an air compressor. While she would be able to measure the pressure drop with a manometer, you can also do it mathematically based on the reading she has from her oxygen analyzer.
14.7psi*(19.9%/20.9%) = 14.0 psi (pressure at intake) 14.0-14.7 = -.7 psi pressure drop due to the restriction in the continuous gas blender. (.7 psi/14.7 psi)*(34 feet fresh water)=1.6 feet of fresh water (manometer reading).
And strictly speaking, the air going into your air compressor does not contain 20.9% oxygen. Any humidity in the air will dilute the remaining gases. This matters because your air compressor through the course of pressure drying the air, will remove most of the moisture and concentrate the remaining gases. The net result is that if you are using an oxygen analyzer, you should calibrate that oxygen analyzer to some valve less than 20.9, but not the actual percentage of oxygen in the air, to get the correct mix at the outlet of your air compressor.
In short, if you cal to 20.9, and have a high level of humidity (say 5% of total airborne gases), you will not add enough oxygen and the mixture that comes out of your air compressor will be low. If you calibrate your oxygen analyzer to the actual concentration of oxygen in the humid air, immediately after the Nitrox stik you would have the desired oxygen percentage in your mixture, but your air compressor will remove the moisture and concentrate the remaining gases, and you will end up adding too much oxygen and your mixture will come out rich.
Any apparatus (mixer) at the intake of your air compressor will cause some level of restriction and an associated pressure drop. The idea is to minimize this pressure drop. You should calibrate your oxygen analyzer to the desired value when the air compressor is running, to account for this pressure drop.
Below is a link to a spreadsheet that recommends the correct calibration setting based on your humidity levels, and the desired nitrox percentage.
For people who live in places with moderate levels of humidity, your input oxygen analyzer should be calibrated to 20.7 to 20.8. South Florida in the summertime, will need to calibrate the analyzer to a much lower reading. One touch calibration oxygen analyzers are not recommend, as they only calibrate to 20.9%
http://www.rubberduckiedesigns.com/Documents/Nitrox Controller Downloads/Humidity Spreadsheet.xls
There are several companies that make inexpensive temp/pressure/humidity meters, you can find them on eBay.
http://shop.ebay.com/items/__temp-h...idityQQ_ckwZpressure?_trksid=p3286.m104.l1199