A thermostat MUST be used in any rack system
The customer is responsible for proper installation of the thermostat probes in the rack. The thermostat probe must be on the heat tape itself for best results and highest safety.
We cannot stress this enough. DO NOT place your thermostat probe inside one of the tubs or off of the heat tape in a belly heat setup. The most foolproof, consistent method is to place and secure the thermostat probe directly on the heat tape itself. You need to make sure it is in constant contact with the heat tape at all times. Secure your probe to one of the black bars directly on the heat tape. It is a very good idea to check your thermostat probe periodically as part of your regular maintenance to make sure it is securely in place as well as the temperatures in your tubs to ensure all is working properly and as expected. There is no substitute for paying attention where animals are concerned.
Here is some helpful information if you are having issues with the probe placement.
Some temperature variance between the shelves in the rack is normal. The number one purpose of the thermostat, and the sole reason we (and other manufacturers as well) instruct customers to install the thermostat probe directly to the heat tape, is to ensure that the heat tape does not overheat. Heat tape, if plugged into the wall without any thermostat or regulation will quickly overheat. Its capacity to heat up quickly and efficiently over a long area is the primary reason heat tape is used for this application. The temperature measured on the surface of the heat tape is going to be higher than the felt temperature inside the tubs. This is due to the insulation factor of the air-gap above the heat tape, ambient temperature of the room, airflow around the rack, insulation factor of the tub material and the amount of substrate present in the tub, etc.
For our personal racks, we tend to run one thermostat positioned on the middle shelf in the rack. This placement allows the thermostat to read the closest thing to an average temperature for the rack. We try to put the thermostat probe as close to the middle of the stack/rack as possible. This means for a single 3 tier rack, we’d put it on the middle shelf. Same for a single 6 tier rack. However, if we were to set up a stack with two 3 tier or two 6 tier racks, then we would put the thermostat probe closer to the top of the bottom rack in the stack and closer to the bottom of the top rack in the stack.
Heat rises, so you would expect everything below the thermostat probe to be hotter and everything above it to be colder. We have probe placement holes on each shelf so that you can select the best shelf for thermostat placement, in your environment.
Whichever shelf the probe is placed on should have the most consistent heat-tape surface temperature with respect to the measured value on the thermostat. However, the thermostat measured temperature and setpoint are NOT what we pay attention to when we adjust the rack temperature. Instead we adjust the thermostat set point, up or down as the case may be, until the measured temperature on the surface on the inside of the tubs directly over the heat tape is about where we want the warm spot to be.
As an example, the thermostat we currently have in our 3-stack is set to 90f on the middle shelf. The measured temperature on the surface of the heat tape at the probe is between 88f and 92f due to the +/-2f histeresis we have our thermostat configured for. The inside tub temperature of the bottom shelf measures at 83.1f to 85.5f, the middle shelf measures at 83.3f to 84.9f and the top shelf measures 82.4f to 84.4f. The shelf temperatures are all about +/-2f in range due to the histeresis. Temps are also all within about 1f of each other shelf-to-shelf. You'll notice the top shelf is colder than the middle, and the middle is colder than the bottom. If we move the thermostat probe to the top or bottom shelf, the difference between the top and bottom shelf temperatures will be greater than what we have now.
Since we can see the temperature in the tubs is approximately 5f lower than our thermostat setpoint, we need to increase the setpoint by 5f in order to achieve the 90f intended temperature within the tubs. That 5f difference between the setpoint and measured temperature in the tubs is specific to the setup we have here in the office. We have an ambient temperature slightly below 70f, we have no sunlight, no direct vents/airflow near or around the rack, and have limited substrate in the tubs.
Just to reiterate, the thermostat's main purpose is not to regulate the basking or warm spot inside the tubs; that is its secondary purpose. Its main purpose is to regulate the heat tape to cap the maximum temperature that the heat tape/rack will be allowed to reach. Tweaking the setpoint allows you to dial in the internal tub temps, but must be done iteratively and somewhat specifically to the environment your rack is in.