Maybe i should clarify. I thought i explained it but maybe not. I use a dummy tub. One tub in every rack is used for the sensor. My melamine racks have back heat. My freedom breeders and vision racks do not. I have the probe suspended midway over the hot side. I have done it this way for years and never had a problem. I dont use substrate. I either use paper or pre cut liners in all the tubs. Now i understand for those that need every spot for an animal this could be difficult. If thats the case and you have a melamine rack you can use a router and route a channel in the roof of the top slot place the probe in and fill it with silicone so it stays in place and the channel is protected from moisture. For me i want to know that my thermostats are picking up the temps that the animals are feeling without worry of flucuation. Flexwatt can have deadspots in it. Ive seen it happen. What happens if your probe is on a section that has a dead spot occure? Your tubs will be out of whack. I am very anal about heating in my racks to the point every single rack has johnson controls thermostats with helix backups. I know that seems like overkill to some or too much money in electronics for one rack but im not trying to loose any animal regardless of value.