Install External Sensor on Resul CL-280

I have been wondering for a while whether low temperature help the plants a lot or not. Many peoples in forum seems to agree that low temperature improve the plants grow. So I decided to add Resun CL-280 chiller to my 3ft tank (October 11th), to see whether it is great improvement or just marginal. The average air temperature is around 30 degree C, target water temperature is around 25 degree C. Tank size is 200 liters, minus rocks, substrate, wood and plant mass, probably left about 150 liters. The setup is "Tank -> Eheim 2217 -> Chiller -> CO2 reactor -> Tank"

Resul CL-280 spec:
- Power: 280W (1/10 hp)

- Rec Pump: > 900 l/h

- Max tank size: 300L (8 degree below air temp) and 200L (15 degree below air temp)

It took about hour pluss to chill from 28 degree C to 25 degree C. After that, chiller will go to stand by mode and turn on every 5 minutes for about 2 minutes. It is stop when the temperature at 25 degree C and on when temperature is rise to about 27 degree C. This is not what I expected, water temperature should not fluctuate than much for short period of time. It was tested at night with light off, the only heat generator are filter pump and chiller it self. Furthermore the thermometer inside the tank show constand 25 degree C. To confirm this, I bought couple of digital thermometer and place in different area of the tank. To my surprise all of them show around 25.7 degree C when the chiller start turn on.

Did some research on the net and found the following:

If the water flowrate is too slow, the thermostat will read false reading and cause the chiller to start and stop regularly in short period of time. The reason is because water does not leave the heat exchager fast enough before reaching targeted temperature threshold. As a result chiller with shutting down the compressor. When the warm water from the tank gradually replaces the water inside the chamber, the chiller will start the compressor again.

There are few ways to fix my start/stop problem:

1. Change to more powerful filter or use independent pump.

I don't really like the idea, I felt that my fishes are already struggling with current 1000l/h filter. Other reason is money, new filter is not cheap. Also, I do not like extra pipe.

2. Add external thermostat

I should be able to get external thermocoupler, hook it up and put the sensor inside the tank. That should give better accuracy.

3. Use external temperature controller

Currently doing DIY temperature controller, will post once it ready.

I decided on second solution, adding external thermostat. I open up the chiller, wirring is prety staright forward, you can easily find which connector is for sensor.

The sensor is located inside small box with thin silver insulator around it, wonder if the insulator really effective. If not, the heat from the radiator might effecting the sensor reading. The tank water just enter and exit the box, no compartment to direct the water flow. It is just a simple box with coil for heat exchanger.

The sensor is normal resistive thermocoupler, inside the plastic chasing. Plastic is an insulator, I wonder is the sensor response effected because of this.

I went to Sim Lim (Electronic component shopping center) to get external thermostat and what do you know, it is not as expensive as local fish shop quote me. The fish shop quote me for S$150 but it is only S$15 at Sim Lim (Acez Instrument Pte Ltd, #B1-38). Actually, it was spare part from Fox temperature controller. By the way, Fox temperature controller is only S$65 (There goes my Temperature Controller project, total cost of DIY might not be much different than from the shop). I also manage to get the same connector use on the chiller for only S$0.80. There is slight drawback, the resistance of new sensor is slightly lower than original sensor, measuring at same temperature. It was about 600 ohm lower. Thinking of adding fix resistor to compensate but on second though, my chiller temperature control display is only 2 digit. 0.3 or 0.4 degree will not make any much different.

This is what the external sensor look a like, it made from metal or maybe encapsulate inside metal. The cable is quite long. Either way response should be faster compare to the original plastic thermostat.

After adding the external sensor, the Chiller kick in every 2 and a half hours, prety good. There is still slight problem though, by default the chiller will turn on the compressor when the temperature reading 2 degree above the set value. I wonder if this will stress the fauna? I guess DIY temperature controller were I can set the maximum and minimum threshold still good idea.

I measurement chiller power ussage using device similar to Kill-a-Watt over 24 hours. The chiller used 1.17kWh over 24 hours, with the rate of 0.3045 per kWh, 1 day equal to S$0.356, 1 month = S$10.688. Additional ~S$10 per month isn't that bad, I can leave with it.