I see the same questions about thermostat probes over and over again so I thought I'd put the common answers to this here for all new people to look at.
Before saying what to do let's say what not to do.
DO NOT place a probe loosely...anywhere.
I used to just bury it under the substrate over the heat element. It quite frequently got moved around by the snake as it would bury it self to get closer to the heat and move the probe around.
DO NOT tape the probe inside an occupied tank/tub. A snake can over time undo the tape and the probe.
While not exactly to do with the probe it is important to not trust the read out on the thermostat. Depending on where you put the probe, the thermostat reading may be several degrees higher than what the reality in the tub/tank is. Always use a temp gun to double check. Accurite probes can work but tend to get moved around just like a thermostat probe.
Here are the 2 common viable options for UTH.
1) Using electrical or foil tape, attach the probe directly to the UTH element underneath the tank/tub. This is the recommended way for tanks. If using a rack, and there's space, you can place the probe in between the tubs on the heat tape. With this method, the thermostat will likely have to be set in excess of 100 degrees to transfer the correct heat through the tank/tub bottom to get the desired temps. This is where a good temp gun is needed to check the temps inside the tank/tub.
2) This applies only to racks (unless you have multiple tanks). In a rack that doesn't have enough space to attach a probe directly to the element but has an unused tub, the probe can be taped down to the tub over the UTH area. My hatching rack is this way and I drilled a hole through the back of the rack and tub to feed the probe through. The advantage of this is that the thermostat reading is going to reflect more of an accurate reading for the tub temperature. But a temp gun should always be used to double-check. The only problem is now you have a tub that you can't use.
So what about heat lamps?
These are simple actually. This is assuming using a screen top. I actually don't know of any other scenario for a lamp other than sitting on a screen top.
Simply tape or otherwise adhere the probe to the part of the screen that will be under the heat lamp. Like having a probe on heat tape, you may have to set the thermostat higher than the desired temp as the heat disappates over serveral inches of air to the bottom of the enlosure. And again, use and temp gun to get the real temps in side the enclosure.
If anyone has anything else to add please do.
Thanks.