Programming Heater for Waterbath

[robinsmith]

I'm preparing experiments on newly settled coral larvae and trying to setup a water-bath system in which I maintain the temperature of the internal 5 gallon tank at 32.0°C +-0.1°. My temperature probe is in the internal tank which has incoming turnover water of 27-29°C depending on the time of day. I have a 300watt heater in the 10 gallon water bath surrounding the internal tank. I currently have the following programming:

Fallback OFF
If Temp < 31.7 then ON
If Temp > 31.8 then OFF

Unfortunately, when the heater turns off it overshoots my desired temperature (32.0°C) by ~ 0.7°, resulting in a range of 31.6°-32.7°, which is unacceptable for the experiments I need to run. When I look at the output log, the heater turns on for approximately 12 minutes.

How do I program the heater to turn on for only 6 minutes, or a specified amount of time so as to not overshoot my desired temperature in the internal tank? Or can someone suggest another way to program to achieve 32.0° with as little differential as possible?

Any help would be greatly appreciated.

[RussM]

My guess is that the heat transfer from the outer tank to the inner tank is too slow... the walls of the inner tank may be essentially being an insulation barrier. Is that inner tank acrylic, glass, or some other material? Glass would be better than acrylic.

[robinsmith]

Hi Russ, the inner tank is simply a commercial grade food storage container (High-density polyethylene I think), but the walls are fairly thin. Regardless, I think I can get the differential down if I simply control the amount of time the heater is on, regardless of the temperature probe reading at that exact moment. Can you help me with a programming string for that?

[zombie]

The problem is you think your hysteresis is 0.1 degrees, but you are actually telling it to maintain between 31.6 and 31.9. The thermal separation compounds further and allows some overshoot.

My suggestion is change to F and use this code.

Fallback OFF
If Temp < 89.0 Then ON
If Temp > 89.0 Then OFF

This will vary Temp between 88.9 and 89.1 F with a potential for similar overshoot as your previous code. That would result in likely a 88.9 to 89.2 or 89.3F range which gets you in the 31.6 and 31.8 range which hits your margin of 31.7 +-0.1C

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[robinsmith]

The problem is you think your hysteresis is 0.1 degrees, but you are actually telling it to maintain between 31.6 and 31.9. The thermal separation compounds further and allows some overshoot.

My suggestion is change to F and use this code.

Fallback OFF
If Temp < 89.0 Then ON
If Temp > 89.0 Then OFF

This will vary Temp between 88.9 and 89.1 F with a potential for similar overshoot as your previous code. That would result in likely a 88.9 to 89.2 or 89.3F range which gets you in the 31.6 and 31.8 range which hits your margin of 31.7 +-0.1C

Sent from my SM-G986U using Tapatalk

Hi Zombie, thank you, I didn't think of the hysteresis that way, but makes sense. I'm going to give your suggestion a try. I'll keep you posted.

[robinsmith]

That worked, thank you! I've dialed in my incoming flow rate to compensate for ambient temperature change (we're pulling fresh seawater from the bay in front of the lab) and solar heating throughout the day, and have managed to get my temperature differential in my larval chambers to +-0.3°C, which is acceptable.

[robinsmith]

Hi Zombie, thank you, I didn't think of the hysteresis that way, but makes sense. I'm going to give your suggestion a try. I'll keep you posted.

Hi Zombie, I'm hoping you can help with another set of experiments. I'm pulling in seawater from in front of our lab, which obviously has a diurnal temperature variation. This splits into 2 header tanks. One supplies ambient water to a set of experimental tanks. The other is heated, and supplies heated seawater to a set of experimental tanks. The heaters are controlled by the energy bar. I have temperature probes in an ambient experimental tank, a heated experimental tank, and both header tanks. I want to mimic the natural diurnal variability in the heated experimental tanks, but +2 degrees C. Any programming you could suggest would be appreciated.

Thanks!