Good question, but is this unit in a heated room? Doesn't really have anything to do with the sensor but a heated rack in an unheated room heats completly different. I would hate to give advice for one and dismiss the other.
I had an closed rack, in my heated room. I am unfamiliar with AP racks so bear with me.
In my situation I it took quite a bit of time to adjust the controller, a single Ranco.
Basically I set it at the max temp range I wanted the temp to be placed the probe about half way up the side of the rack next to the heat element of the rack.
Afte a few hours I took a temp reading of the tub directly above the element. This took awhile to get the temp correct, basically to compinsate for the differences in temp of the element vs the floor of the tub I moved the sensor closer or further away from the element, while still taped to the side wall.
I kept the sensor on the top rack but noticed the temps further down the rack were only a few fractions of a degree cooler. At the far bottom I think it was only 1 degree off of a 10 slot rack.
Basically that was it I started with the max temp so that when I finished installing the back of the rack, made of reflectix. I could dial it down to get the same temps as well as a gradient towards the front.
Over all I was satisfied but it was a lot of work. I currently have an open rack. I used the same technique to set it up but I don't use a ranco anymore I use dimmers.
To directly answer your questions;
A. taped to the heat rope?
No this will give you false temps in your tubs unless you adjust the temp higher to achieve proper heat within the tubs. I don't do this as I like to look at my thermostats when I used them and see the actual temp without using my temp gun. I now use my temp gun for everything and use dimmers instead of thermostats.
B. taped to the bottom of one of the tubs (inside or outside?)
Again No, it will be really difficult to remove teh tub with the probe taped to it or in it. You will get similar readins if you just tape the probe partly up the side wall.
>>C. Just taped against the back wall of th rack (inside)?
I would perfer to tpe it to the side wall, because if the tub slides out width wise the probe on the back will allow the max heat to be at the back of the rack. Which is fine but the area directly above the element will be slightly warmer not a problem if you adjust. With the same rack setup but the probe taped to the side wall it will be closer to the zone of the element making the reading that much more accurate.
Hope that helped
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Jeremy
"I am become death, the destroyer of worlds" July 16, 1945 Robert Oppenheimer
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