Best location is directly on the heat pad. If you stacked the two cages, sandwich the sensor between the two cages directly under the heat pad for the upper cage. Your thermostat will need to be set so the heat pad is hot enough to get the cage temps you want, typically some number of degrees warmer. Do you have a temp gun? I use mine to spot check the cages and adjust the thermostat as needed.
BTW, I have 12 boaphile cages (6-421D, 3-421Townhouse and 3-421condo), 3 boaphile hatchling racks, and lots of homemade cages/racks. In total I have 5 helix, 1 double ranco from boaphile and one other don't remember mfg thermostat and I like the Helix the best. Looking to get a few more for additional thermostat needs and future expansion.


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Thanks,
Dave "Rainbows-R-Us"

0.1 Wife (WC)
0.2 kids (CBB)
4.12 Brazilian Rainbow Boa
2.1 Hypomelanistic BRB
0.1 Het for Hypomelanistic BRB
0.1 BCI "Elvira" normal from 1989
1.0 BCI albino / het-anery
0.1 BCI Hypo / het-albino
0.1 BCI Anery / het-albino
0.1 BCI Hypo (possible super)
1.0 BCI albino het stripe
1.0 BCI salmon hypo
0.1 BCI ghost
lots.lots.lots feeder mice and rats 