Sensor Hack

Well, after being so happy with my proof that the server room A/C unit wasn't cycling too often, I realized that my sampling rate was low enough (10 minutes) that if it was cycling really often, I wouldn't necessarily be able to measure it.  After some old-fashioned empirical analysis (i.e. watching the unit for about 30 minutes), I decided that something needed to be done.

The real problem is that the A/C unit is being too aggressive at keeping the set temperature.  A single degree of difference will cause it to kick in.  With such a small space and such an oversized unit, it doesn't take long to cool the room, or for it to heat itself back up a degree.  Thus, I needed a way to introduce a multi-degree differential, but without modifying the electronics in the unit.  The condenser talks some serial protocol to the compressor unit, so trying to be smart and switching the signal line isn't an option.  It causes a communication error and forces the whole thing to be restarted.  Thus, I resorted to a physical solution.

 The temperature sensor in the condenser unit was mounted inside next to the coil, which has the main fan constantly circulating air over it.  This makes it very sensitive to the room temperature.  To solve this, I pulled the sensor (which has a nice long cable, thank you $manufacturer) out of the unit.  I put it inside a small length of thick poly tubing, and stuffed some fiberglass insulation in either end of it.  The result is a nice long cycle period and a very distinctive sawtooth shape to my temperature plot.

 

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Category(s): Hardware

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