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Friday, December 27, 2013

CryoBUG: Running Load Tests

Having zeroed in on the refrigerant charge I would like to use, I thought it was time to create a heat load versus temperature chart. Since the Demo unit has a vacuum insulated cold head, it doesn't lend itself very easily to attaching a load resistor. So I pulled out of storage my original bread-board prototype and recharged it with the same refrigerant as I presently have in my Demo unit (HC/Argon blend).

Heat Load Test Set-Up

Load Resistor
For the load heater I used a 6 ohm 50 watt resistor suspended directly in the center of my 6 foot long 1/4" OD tubing evaporator coil. The evaporator was then wrapped in multiple layers of aluminum foil to help contain the heat emanating from the resistor within the interior space of the evaporator coil. The entire thing was then insulated with fiberglass.

My heat load power supply consisted of a 0-18 VDC 3 Amp adjustable unit with digital readouts for current and voltage. So basically this gave me a top range of nearly 55 watts.

Monitoring of all the temperature data was via a USB-TEMP module made by Measurement Computing, and running a DasyLab charting application on my laptop computer.

Test data was collected at zero applied load, and then at 5 watt increments ranging from 5 to 45 watts applied. Estimating the added static heat load at 10 watts from insulation losses, I would say that the total heat load with 45 watts applied, is in reality 55 watts (10 W static + 45 W applied). This is pretty much the absolute maximum heat load that CryoBUG is capable of handling with a -150 °C refrigerant charge. And to be conservative, I would rate it for 50 watts total, because it starts to lose it after that (see chart below).


CryoBUG Temperature versus Heat Load Chart

From some of my earlier refrigerant charge tests, I can see the possibilities for taking on more heat load, assuming that the ultimate temperature requirements were to be relaxed.


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