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Picking up on Kenargos excellent comments for much the same reason, here is the temp control portion of my program:
If Temp < 78.8 Then HTR ON
If Temp > 78.9 Then HTR OFF
If Temp > 79.1 Then FAN ON
If Temp < 79.0 Then FAN OFF
If Temp > 81.0 Then MH1 OFF
Max Change 060 M Then MH1 OFF
If Temp > 82.0 Then MH2 OFF
Max Change 060 M Then MH2 OFF
If Temp > 82.5 Then ACT OFF
Max Change 060 M Then ACT OFF
If Temp < 77.5 Then ALM ON
If Temp < 77.5 Then HTR OFF
If Temp > 82.5 Then ALM ON
I just added the the last three lines the other day. Came home early on Friday and found the tank lights all off and the temp reading 115. Called Neptune and they confirmed that temp probe is bad. (It was a sudden failure; temp was reading steady steady steady on the data log, then all of a sudden it went haywire.) Had it failed "low" and stayed low, the heater would have come on and cooked the tank. By adding the last three lines (second to last one in particular), a failed temp probe should not cause tank to boil. (Note that I only have cooling fans on open top tank, which are not able to "freeze out" the whole tank. If you use a chiller, you should have an analogous statement (a la Kenargo's) to shut off the chiller (and lights, to be on the safe side) in the event temp exceeds some crazy number, e.g., 87, that would not be reached in ordinary real life situation.)
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