-45 Chiller Head

[AMSYSTEMS]

We are trying to make a cooler for a sensor head to get down to appx -45 Deg C. We have an SC18CL (R404A) with matched condenser, an AKV10-1 (I gather people don't seem to like these) running on a 4 second cycle at 15% full load and an EVR 2-22 on a timer to give a bypass flow to the compressor every couple of minutes to prevent overheating where cooling is not required on the sensor. The Evaporator is our problem. We currently have a brass block appx 46mm square with the AKV mounted on the inlet(the inlet tube to the evaporator is larger than the outlet) and 3/8" tube on the return. The block is a single machined chamber 12.5mm dia and 33mm deep. With the unit running we can achieve an on load temperature of -35 to -38 deg C (with the block etc fully insulated) but are unable to drop the extra 5 to 10 deg C that we would like. As you may have gathered, we are not refrigeration engineers! so can anyone give us any pointers as to what we can do to improve cooling? either by modifying/redesigning the block or by some "Cool" pipe trickery!!