All controllers (whether thermostats, humidistat’s or any other type of stat) are designed a certain way and are supposed to be installed at certain locations to operate properly.
The psychometrics of the air above the thermostat in David’s picture is not the same as the air below the thermostat, therefore it is not sensing the proper condition of the air to perform as designed.
Not that it makes that big a difference because you’re not trying to control the humidity to tight specifications in a residential situation. But when the humidistat overruns its set point and causes mold to grow in the house…
Gary, you have a thermostat that was “designed” to a different specification. I don’t understand the point! Are we comparing apples and oranges ?