Thermostat rant

[Eric Alan]

Ok I am new to owning a herpstat. Actually got it on a deal with four snakes, so it was used when I got it but works great. Eventually I will buy another one when I build a hatchling rack, but that's down the road. If I understand correctly if the herpstat were to fail it will do so in an off position? Also, what if the probe becomes unattached to the heat tape. Will the herpstat one shut off or run a lot of power to the tape?

All Herpstats are programmed to fail in the off position if the sensor goes bad. Most (4 of 6) Herpstats also have a built-in safety relay to cause them to fail in the off position in the event of a mechanical failure.

If the probe becomes detached from the heat tape, it will sense the lower temperature of the open air and increase power to compensate (probably wide open). That would be a user error - do everything you can to ensure this doesn't happen.

[Bcycling]

How do I know if the herpstat I have will fail on off position if it does fail?

[Eric Alan]

How do I know if the herpstat I have will fail on off position if it does fail?

What kind of Herpstat is it?

[Jabberwocky Dragons]

Maybe I'm over thinking this or just too dense to understand the reasoning.

I use Herpstat 2's. After reading the thread Eric provided, it seems that you are just gambling on which unit will fail first, or conversely, which one you have MORE confidence in. It seems that if your secondary fails it may shut everything down. Thus, where's the back up to the backup?

Unless someone can dumb it down to me I see no reason for a back up to a Herpstat but maybe for other brands.

Sure, here's it simplified. Your secondary (on/off) thermostat should always be on. If your primary fails in the "on" position, your secondary will turn power off to both. Gambling or confidence doesn't factor in here.

If the secondary fails in the "on" position, it doesn't matter because it's not providing heat to anything. It will simply allow your primary to continue functioning as normal. Your secondary should always be on anyways. Herpstat is reliable but should still be backed up. Losing a clutch of eggs or a rack of snakes in not a fun experience, especially when a fool-proof method of prevention is so easy and relatively inexpensive.

There's a much greater risk when a thermostat fails in the on position than the off position. Herpstats are designed to fail in the off position. Cheaper stats typically fail in the on position. If the secondary fails in the typical manner here, the worst it would be doing would be supplying power to the primary Herpstat.

This kind of backup is designed to work if the primary were to fail in the on position. Since this is so unlikely with a quality stat like the Herpstats, I personally don't see much reason to back a Herpstat up. Would it hurt? No. Is it overkill? Possibly.

From Spyder Robotics: http://www.spyderrobotics.com/home/s...edocument.html.

It's been a long day so I may not be reading correctly. What I'm taking away from your link though is that Spyder Robotics says that their Herpstats are inherently designed to fail in the "on" position (per the Triac component). They have taken steps to help prevent this in the higher end models but the default failure is still the on position regardless. They also give this prominent warning:

Also keep in mind that even with the highest quality components ALL electronics have the possibility of failure.

It then goes on to recommend backing Herpstats up with a secondary on/off.

[Eric Alan]

It's been a long day so I may not be reading correctly. What I'm taking away from your link though is that Spyder Robotics says that their Herpstats are inherently designed to fail in the "on" position (per the Triac component). They have taken steps to help prevent this in the higher end models but the default failure is still the on position regardless.

The Triac component itself is what could (not will) fail in the on position. It sounds to me like it could also fail in the off position - a much better alternative. In addition to the upgraded Triac, they have taken a few other steps to promote safety, such as the sensor error coding and relay I mentioned earlier which would lead to failure in the off position.

[Ufoo9k]

This is really interesting talk, but I find the second Tstat to be somewhat useless especially if it is of less quality then the primary one ? You talk that if that one fails it will fail in on, how can you be sure it as failed or not ? If it fails on "on" then if for some really bad luck the primary Tstat also fails you will still get the same problem ? Would it not be simpler to make sure the Tstat goes off if it does break ?

[Eric Alan]

This is really interesting talk, but I find the second Tstat to be somewhat useless especially if it is of less quality then the primary one ?

It's less quality because its purpose is not to control the temperature continually - that's what the primary does. It's only purpose is to prevent severe injury in the event of a failure (in the on position) of the primary thermostat.

If it fails on "on" then if for some really bad luck the primary Tstat also fails you will still get the same problem ?

Exactly. It takes TWO failures for this to happen though, which is significantly less likely.

Would it not be simpler to make sure the Tstat goes off if it does break ?

Simple in theory, yes. In practice? Not so much... If it were, it would have been done already.

[Reinz]

Thank you Eric and JD for the explanation.

I was under the impression that when the back up shut off or broke that no more power was supplied to the primary unit. Now knowing that this is not the case, I have a better feeling about back ups.

[Bcycling]

I have a herpstat 1.

[Ufoo9k]

Well then! We'll have to make it happen! I have 2 Tstat (1 rack and 1 for quarantine) and hope they are the off kind... Is there a way to know or it's a wish for the best situation ? I took the first ones I could find which are made for baby plants, there is not much info on them except that they control temperatures :\