Crazy temp issues with new rack - tub reading 5-6 degrees HOTTER than tstat

[pretends2bnormal]

I recently set up a DIY rack using the Home Depot shelving unit method, with a few slight alterations (bottom shelf left normal height for storage instead of for more another layer of tubs and using plastic sheet instead of peg board to prevent any moisture issues). I went ahead and got a Herpstat 2 and 6" flexwatt heat tape from The Bean Farm and installed per the usual directions. Since I'm using it for smaller than the 41 qt tubs, I put it along the back on the 3 foot side for now so I can use 28 qt tubs, 2 per layer. Probe is taped on the wire to put it right in the center of the heat tape on each of the 2 "active" shelves (3rd shelf is for future expansion and has no heating). Since I have 2 tubs per shelf, that puts it right between the tubs, rather than under one of them.

I'm seeing some really bizarre numbers come out of it, though. I'm trying to keep the 2 shelves at different temperatures, since I already have a corn snake who is going into the rack and my first ball python that I am intending to get (as soon as I get things working sans snake). I put the BP shelf on top due to physics and heat rising so that the lower corn snake temps shouldn't be as affected due to proximity.
Since I will most likely be getting a baby, I set up a 12 qt tub (same height as the 28 qt, and while I don't see it on the Sterilite site, that was the brand it was from Home Depot. Seems like a 16 but a bit shorter?) to use initially while the baby settles until it grows a bit, then move into a 28 qt. The probe is hot glued down in the little lowered channel around the button edge of the tub on the hot side, which I then slide right over the middle of the flexwatt.

Initially, I set the Herpstat to 94 F on the top shelf, so that it would make it through to the tub. At that point, I hadn't put any substrate in as I was testing out temps. Inside the tub where the probe was glued (glue up to the black plastic casing part of the digital zoomed thermometer/hygrometer, black) was reading 99 F... I thought that was fishy and used the temp gun I have to check and targeting the plastic of the probe I was getting 96-99 F there as well over a few checks in 30 minutes time. The Herpstat probe was showing 93.8 and <50% power. This sort of pattern is continuing, and since I have tried using coco husk bedding (which may have been only 90% dry and had to remove due to instant 99% humidity, oops) and am now using paper towels to keep the humidity down to a reasonable level with a small bowl of water only. With the Herpstat set to 88 F, I'm seeing temps from the thermometer at 91.8 F and mirrored by 91-92 F with the temp gun and 62-67% humidity.

This same thing is happening with the corn snake tub by the same 5-6 degree amount, although it is less concerning since the temperatures don't need to be quite as accurate and instead I've set it a bit lower to keep the readings from the thermometer/temp gun below 85. (Mr. Noodle has shown no issues before with fluctuations between 80-85 on the hot side, so as long as it's between those he should be happy)

The flexwatt itself shows a 1-3 degree variation across the strip from the Herpstat probe, which without tubs and the A/C running an airstream nearby doesn't seem too odd to me since there were no tubs in place.


Since I just sort of vented all of that without much organization, I'll summarize the main points..
1. Herpstat probe taped by the wire at the center of the flexwatt at a point between 2 tubs
2. Thermometer probe hot glued to the lowest point of the tub over the flexwatt
3. Herpstat consistently reading 5-6 degree COOLER than both the thermometer probe in the tub AND the temp gun

Only speculation I have that may impact on the BP shelf is that the 12 qt tubs don't cover the entirety of the tape, but the part that sticks out is at the ends. (tubs centered as close as possible to the thermostat probe at the center of the shelf)
-- wouldn't take into account similar issues with the corn snake shelf with 2 28 qt tubs...
OR that not having the back/sides covered and an A/C vent that blows nearby when the A/C kicks in is skewing the air-exposed thermostat probe. Not too likely, as the probe is closest to the wall and the unit is back against the wall on the heat side, so max 5-6 inches from a wall with a vent overhead that points into the middle of the room from about a foot away from the wall on the ceiling.

Anyone have any thought why this might be happening? I don't think my thermometer AND temp gun are both inaccurate to that extent, and the 96 F reading did feel mildly warm to me (who always has people complain about my cold hands) which makes that temp seem correct as well. Are the odds good for 2 probes being bad with a Herpstat 2 but being consistent in that way?

I'm about to give up and just let my rack be weird with the hot side setting at 87-88 F on the thermostat for a BP and getting 91-92 F in the tubs... I can try to get pics later tonight if anyone thinks it would help, or my explanations were too confusing.

Once again sorry for the massive amount of text!! I've lurked on here for months and went through every post about this type of rack I could find before I started and hadn't seen anyone with a similar problem...
[side note - despite being in Texas, my biggest issue so far has been putting enough holes in my corn snakes tub to keep humidity BELOW 60%... y'know.. like 58%. His previous tub kind of looks like swiss cheese with all of the small holes in it]

[Randall L Turner Jr]

Are the probes covered with anything? When I attach probes in racks I usually put a piece of insulation over the top of it.

[pretends2bnormal]

The thermostat probes are uncovered, although there are tons sitting on either side of them that should block most air flow. I can certainly try insulation, although everywhere I've read says to not put any tape directly on the probe so I assumed I shouldn't cover it with anything directly touching it.

What kind of insulation would you suggest, Randall? Would something like a toilet paper tube cut down the length set over it work, you think? Definitely don't want to do anything that would be a fire hazard...

[Randall L Turner Jr]

I usually use a little piece of reflectix insulation (bubble wrap insulation) placed over the probe. With the probe uncovered it will lose heat into the surrounding air fairly quickly so will cycle the thermostat back on, at the same time the tubs will have retained some of the heat from the heat tape and thus when the heat tape comes back on it will already be warmer than the location the probe is at, so will eventually get warmer than the probe set temp.

[Starscream]

If your heat tape and tubs arent allowing any air flow, it happens. This is just heat buildup, and the simplest to do is to lower the set temperature on the thermostat. The temperature the Herpstat is reading isn't necessarily important -- the temperatures in the tubs are. You adjust the thermostat to get those temps correct.

But, if you're wondering why the herpstat is reading lower, it's probably because its not situated beneath a tub. The heat is disipating into the air, giving the herpstat probe a slightly lower reading. Again, that's fine. You can just lower the desired temperature on the herpstat until the tub temps are where you want them.