Louis,
The differential works of the set temperature. The temperature must drop one full degree. For example, if your set temp is 100 degrees than the temp will drop to 98 degrees which is actually one full degree. If a thermostat has a differential of zero than with the same settings the thermostat would turn off at 100 and back on at 99 degrees. When you see that 2 degree drop it is not actually 2 whole degrees. The 98 degrees displayed is very close to being 99 degrees. The differences between the Johnson and the Ranco may be how fast the probes process temperature change. I hope I explained this well enough, I know that it can be confusing sometimes.
Thanks,
Wil Combs
Captive Bred Creations

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