The interior placement can be used safely if a few things are met first. The big one in any set up is the probe must be secure. 100% secure outside this is easy there are a ton of materials that will work to do this. Inside it harder no tape should be used. Hot melt glue securing the probe and a good amount of the cord is effective. IT MUST NOT MOVE!! The second thing is a failsafe. in this type of set up especially a fail safe should be used. This is a second T-stat or a T-stat that has over and under set points that will kill the power if the probe moves primary fails ect.

Hydrofarm T-stats reading cannot be trusted.

They are speced at 3ºC up or down (that is 6ºF over the set point or under it) add the 4 degree swing they have and you have a monster window. (16ºF) You must disreguard hydrofarm temp and use a known accurate thermometer to set it. This also is a rub as inexpensive digital ones generally have 4ºF ranges and most (all maybe safe) IR guns about the same. Personally I'd either buy at least 3 the same and 5 or more would be better test them against each other and eliminate the odd ones. generally this will be close to accurate. the other way is to buy a traceable thermometer that has been individually calibrated when it was made. They come in all kinds of ranges from 4º to 0.04º. I would suggest that 1 degree up or down is fine. Sadly the cheapest I know of is more than most of the T-stats on the market by a fair bit. The helix and the herpstat, ecozone, herpkeeper and VE all have a one degree error (+/- 1ºF) or better and the lower priced ones are actually quite cheap in the realm of accurate thermometers.

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