Controlling a heating element

Thread Starter

jmajor817

Joined Apr 7, 2013
3
Hello I am currently working on a project using an Arduino Uno, 1500 W immersion heating element from mcmastercarr, and PT100 RTD temperature sensor. I am wanting to read in the temperature from the PT100 to the Arduino, and based on the reading control the heating element. I am looking to control the temperature of around three gallons of water for upwards of an hour with a +/- .5 F precision. I currently have a mechanical relay that can control the on/off of the heating element. However, I am not sure whether or not I be able to control the element enough with a simple on/off tactic. I am considering a couple of options:

1. First, I realize that the mechanical relay will need to be replaced with a Solid State Relay because of the amount of switches that will occur per use. But if I had a SSR, I am thinking of trying to tinker with an equation to program into the micro-controller that will be able to control the on/off to desired precision. Something like as the temp nears the desired temp, i.e. within 10 F of 150 F, cycle on 10 seconds and off one second, and them within 9 F a different cycle. The SSR shouldn't have any problem handling this amount of switches, or so I have read. Or does anyone have any experience using the Arduino PID library?

2. I am also considering using a TRIAC. Kind of like the way a light dimmer works, but with a longer period because the heating is a lot slower process. It is my understanding that a TRIAC can be used like a relay, only the output can be controlled based on the input into the TRIAC that can be controlled by the micro-controller. So maybe something like lower the output as it gets within X degrees of the desired. Then simply use the on/off without cycling but just at a lower percent.

Any comments, concerns, or improvements on these two approaches would be greatly appreciated. Also if anyone has a different/better idea as to do this I am definitely open. Thanks in advance to any responses.
 

#12

Joined Nov 30, 2010
18,224
That size heater can move the entire 3 gallons by 1/2F in less than 8.8 seconds. Stratification is part of the process, as is its enemy, convection. Have you considered movement of the water?
 

Thread Starter

jmajor817

Joined Apr 7, 2013
3
Sorry I forgot to say that the micro-controller also controls a pump that will be circulating the water in order to keep the water evenly heated. The process will need to heat to one temp, around 150, hold for an hour +/- .5 F, then after this bring it to a boil for another hour.
Both the pump and heating element will be plugged into a GFCI outlet that will be connected to a 120V 15A main. The relays, controlled on/off by the micro-controller based on the read in from the RTD, will control the pump and heating.
 
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