pmover:
Let me add a little twist that I've done in the past to read a dewpoint down to -90 oF:
If you're in Alaska (North Slope?) at present during the winter, I know that you must have a thorough safety coverage for fires within your facilities. The pain in the backside (or trade-off) in the "simple" and inexpensive dew cup method that I recommended is the ability to have a ready and convenient supply of small, Dry Ice pieces to cool down your acetone. When you don't have ready access to Dry Ice (as is often the case for operations outside the industrial gas production field) I have resorted to using a CO2 cylinder and expanding the liquid content by inverting the cylinder. The adiabatic free expansion of the H.P. liquid CO2 will give you a mixture of Dry Ice "snow" and flash vapor - both at -109 oF. I used to use a common filter paper to catch the Dry Ice snow and use it as the coolant for the acetone.
Now, here's a more practical twist to the method:
I know for a fact that you must have a bountiful supply of CO2 fire extinguishers in your living and operating facilities. Now, bear in mind, that I'm not advocating you resort to using your CO2 fire extinguishers as a Dry Ice source without discretion. But we know that your facilities must be ever-ready for the unexpected interior fire - especially since residual cigarette smokers still survive and persist in smoking in bed. So, what I'd do is contact the person (or persons) in charge of maintaining and servicing the CO2 fire extinguishers and explain your need to them. I'm sure they have spare extinguishers that they always keep charged and ready to go at any instant and they would be glad to cooperate by furnishing you with a spare one that they also keep charged. I prefer a CO2 extinguisher because it is smaller and easier to handle and it comes complete with a syphon tube in the cylinder valve that allows for adiabatic expansion without having to turn the cylinder upside down. It also comes with the customary "horn" that greatly reduces the velocity of the flashing vapors. You can also use something like a pillow case to capture the "snow" product. If you resort to this method, you'll find it convenient and soon learn by experience just how much liquid CO2 to expand to run your dew point cup tester effectively and with relative accuracy. I've done this countless times with repeated success. The worst inaccuracy that I remember registering using this method was around 10 oF. In other words, I registered -80 oF while an on-line dewpoint meter read -70 oF. And there was doubt about the accuracy of the automatic on-line meter because it hadn't been calibrated in a long time. I know from experience that I can easily obtain an accuracy within 5 oF using the dewpoint cup method - if I use diligent care and patience in visually observing the cup's surface.
I hope this is of some help to you.
(By the way, your product air will probably be saturated prior to being dryed - even if you use external, cold atmospheric air. The reason is that you are compressing and cooling in the air compressor and if you have a 2-stage compressor producing approx. 100-150 psig, you may not get any moisture condensed after the 1st stage intercooler but you probably will get moisture condensation after the 2nd stage aftercooler - depending on the dewpoint of the outside air)
Since you are so close to the North Pole, have a Merry Christmas with Santa and all the Elves.
Art Montemayor
Spring, TX
