"differential temperature"

[vince-1961]

I am installing a two-stage Ranco Electronic Temperature Controller. The instructions have a phrase that I do not understand. It says to set the setpoint for heating at, say 76 degrees, with a "differential temperature" of, say 2 degrees, and to set the setpoint for cooling at, say 78 degrees with the differential temperature of, again say 2 degrees. I do not understand what the smaller number is supposed to represent.
What I want is to keep the temperature at 77, heater comes on at 76, chiller comes on at 78.
Does anyone know at what number I should set the "differential temperature" on this thing?
PS - 77 degrees fahrenheit is a good temp for a reef tank, isn't it? What's the ideal temp?

 

[statto]

Hello Vince,
Differential is also called hysterisis
What this means is the point ta which the heating/cooling will turn on
For example,
if you set your heating at 80 with a diff of 5 it will turn on at 75
so as you have said you want 77deg, with the heater comming on at 76 and the chiller comming on at 78
set the heating controls to 77 with a 1 deg diff
set the chiller to 77 with a 1 deg diff
Monitor your tank water for a few days as you may find that at those setting you may be heating and cooling at the same time but the tank maintains at 77
If this is the case try upping the chiller to 78 with a 1 deg diff and keep monitoring the tank temp, if it now settles out at 77deg with only the heater on OR the chiller on at anyone time you have cracked it
Statto

 

[vince-1961]

Thanks