High and dry – the Dielectric/SPX Dehydrator

This little monster showed up at my doorstep and I decided to see if I could get it running again. It’s an air dehydrator, which compresses air, separates the moisture out and ejects that, then uses it to pressurize an RF transmission line to avoid moisture intrusion.



The instructions mentioned a humidity sensor inside that has to be replaced every couple years of operation. I removed it and took a look, it showed no signs of wear or contamination. Upon powering the unit up I got a humidity alarm and the unit would just vent air via a “humidity bypass” burp valve inside which prevents it from firing wet air up the feedline should something go wrong internally.



After it ran on my bench for a while, the humidity alarm cleared and it pressurized its tank and was ready for business. The humidity sensor goes in that brass block with the 115V warning on it; the two cylinder blocks behind it contain the moisture absorbing material. The timer to its right controls some pneumatic valves which drop the pressure in the dessicant cylinders to zero every 30 seconds while the pump is running, firing air (and WATER!) out a waste hose in the back.image


On the right is the Gast pump that compresses air up to 40 PSI to run the works. On the left is all the control logic. The other unit I’ve worked with before is an Andrew DryLine, which just has one simple programmable logic controller board in it. This one, well, looking at it makes me think that the people who designed it chose to build their job security into the control logic – what even IS all of that?! 😉 The four pole contactor at the bottom doesn’t appear to actually be what controls the pump turning on and off. I… don’t even know. It works, that’s the important part. The humidity bypass valve is under that threaded hex nut/nipple on the left, and the time delay relay brick below it appears to be responsible for firing off the Excess Run alarm should a pump run cycle last too long (usually, this would signify an air leak!)image



The pump is cycled on and off by a simple Square D Pumptrol air pressure switch… off at 40 PSI, on at 20 PSI.image

An amusing note I found on the Gast pump. I’ve seen similar pumps used for a lot of laboratory applications including as low vacuum pumps, for things such as providing suction in dental offices (that slobber extraction hose they hang in your mouth)… Well, once the pump’s been used like THAT, they don’t want it back! Can you really blame Gast? 😉

imageThe Gast pumps are field rebuildable with a seal/gasket kit to keep the unit in service. On this unit, loosening four screws flanking the gray upper front cover is all you need to do to be able to reach in there and pop open the air inlet filter on the pump for cleaning, check the tank pressure, and get to most of the control logic. This is a sharp contrast to the Andrew unit I’ve worked on where you have to remove the whole thing from the rack and pop eight screws from the top cover to get to the filter, which the datasheet claims is behind the front access panel (NOPE!!!).


6 thoughts on “High and dry – the Dielectric/SPX Dehydrator”

  1. I have the same unit! The Solenoid went on mine, replaced it, but I’m having a hard time finding replacement humidity sensors. It’s just running and running in the humidity mode. I found a lot of 5 “slightly used” sensors on ebay for $100 but they were all a little too used up. If I put a 10k resistor across where the sensors probes connect it’ll get out of humidity mode. but that’s where I’m stuck. I may try to give a sensor a Isopropyl bath to see if that uncruds it.

    1. I have 4 of these sensors that appear to be BRAND NEW. BUT …. I can’t be 100% sure and you mentioned the ones on eBay appear to be “a little too used up”. My question is, how do I know if mine are for sure new, used, or perhaps a little too used up? I’m sure curiosity also has you wondering why I wouldn’t know about mine as well. Well, I aquire various items from estates, lots, ECT. These came to me in a clear plastic bag with a small tear in the top of the bag like someone ripped it open and perhaps grabbed some previously (I’m not sure how many come in a bag) there’s 4 clear plastic tubes with white lids approx. The size of a 35mm film canister but slightly taller and slightly skinner. Inside are these blue sensors with white labels and two prongs.

      1. I… wish I knew. I’m not really sure, my best guess is that it’d be a matter of checking electrical resistance but I don’t know what it’s supposed to read.

  2. hi Tom,
    I’m completely similar with this dryer and its operation. do you have specification of humidity sensor that used in this dryer. also I need to buy spare part for this equipment in Emirate.

    best reward

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