Confused about thermostats and rheostats.

[Tim C.]

I've been lurking on these boards for months now, learning what I can, but I'm confused about what, exactly, thermostats and rheostats do.

Here's my situation:

I have an Exo-Terra heat mat (UTH), on my 20 gallon long, covering about 1/3 of the bottom. I'm using a screen top, as well as repti-carpet. The temps on the carpet itself are only reaching about 80-85 degrees, which I understand is not quite hot enough for LGs. So I'm looking at thermostats and rheostats.

From what I understand, thermostats do not influence the temperature, they merely keep whatever part of the tank the probe is in from getting too hot (and turn them on when it's too cold). Rheostats, I think, act like dimmers and can turn the power of the UTH down - but can they also increase the power beyond the basic heating strength of the UTH, thus providing more heat than I'm currently getting?

I'm up for recommendations. My only other thought is to get a heat lamp for the top of the tank, and provide some auxiliary heat that way.

-Tim

 

[RampantReptiles]

Sounds like you understand the concept.

Im pretty sure that a rheostat cannot do that, I think the way it works is 100% on high of the rheostat would be equivalent to plugging it into the wall so you can only turn stuff down and if it could I dont think I would recommend doing that either as it was not meant to work that way. Sounds like a fire hazard to me

Are you using an appropriate terrarium thermometer to measure the temps? You would want the thermometer laying right on the carpet where the hot spot is.

A UTH should not have any problems getting hot enough when used with reptile carpet. Maybe there is something wrong with it, did you ask the pet store you got it from?

Only other option would be to get a heat lamp with a low watt bulb, probably a 25w(maybe 50w?) basking bulb or something.

Good luck!

 

[TheWolfmanTom]

The way I understand and use them is like this.
Thermostat controls temps.
Rheostat controls lighting ie dimming

 

[CapCitySteve]

Ok here's the answer. Yes a rheostat will increase the temperature of your heat mat. And for your setup I would reccomend one since they are only about $25. I use a rheostat hooked up to some heat tape to control the temperature for my roach bins and crickets. I can get the surface temperature up to around 110 using it, which would be much less without it like your experiencing (note: I keep my roaches around 90 perfect temp for LGs) The trick here is that you have to use a seperate thermometer in order to measure the surface temperature over your heating pad. This temperature will fluctuate during colder periods of the day however because a rheostat merely controls the amount of electric current running through your heat pad. Get the Zoo Med Rheo Stat, I promise it will work.

A thermostat is simply a fancier version. It has a rheostat like operation that automatically adjusts the electric current running through your heat element in order to keep the temperature constant, no matter what the ambient temperature is. I use a thermostat for my rack systems. Please note that on initially setting up a thermostat you should also use an external thermometer to confirm the temperature setting matches actual temperature. Sometimes it takes some adjusting of your probe to get an accurate reading.

I hope that clears things up,

Steve

 

[Shera]

I don't think using a rheostat is going to allow you to get it any warmer, I think they can only turn it down. I use 2 layers of paper towel and put the probe between the 2 layers. If I just lay the probe on top I find I don't get an accurate reading because the probe is getting the air temp from above too. I account for the fact that my readings may appear slightly high. If I just use one layer of paper towel and lay the probe on top it still appears too low. I think the probes are meant to be inserted in substrate or immersed in water. It can be hard to get an accurate temp just laying it down, and I always find that when I do that, the probe has very little actual contact with the ground. Maybe you could try taping it down with masking tape or something?

 

[sammer021486]

Sorry, but this is going to be a long post explaining the differences between rheostats, ON/OFF thermostats, and proportional thermostats.

A rheostat in the most basic terms, is a gate. When wide open, the electricity flows through at maximum capacity. When you dim the rheostat (begin closing the gate) the flow of electricity is reduced and in turn decreases the temperature of the UTH. A rheostat is not affected by environmental temperatures, the gate and flow stay the same whether or not the UTH gets cooler or warmer, it has to be manually adjusted.

A rheostat that is set to full does not allow the UTH to become any warmer, than if you had just plugged the mat straight into the wall. It is only effective at reducing the temperature of the mat.

There are two types of thermostats, ON/OFF and proportional.

An ON/OFF thermostat uses a sensor to read the temperature and it will cut the flow of electricity to the UTH based on the reading it senses from the UTH. When the UTH reaches the temperature the thermostat has been set to, the thermostat will cut all power to the UTH. The thermostat will have a temperature set in its memory to tell it when the UTH has gotten cool enough to turn the flow of electricity back on to the UTH.

A proportional thermostat is a smart rheostat. Instead of just sitting at one setting, like the rheostat does. A proportional thermostat uses a sensor to read the temperature from the UTH and a computer in the thermostat will adjust the flow of the electricity to the UTH to help the UTH maintain the proper temperature.

If choosing between a rheostat or a thermostat, it comes down to how stable is the ambient temperature in the room? If the room where the reptile is being kept varies a lot then a thermostat is the best option. If the temperatures are very stable then a simple lamp dimmer will work as a rheostat and you will have to make the necessary adjustments.

In your case you say that the UTH is not hot enough and you would like to supplement the heat with a lamp. In that case it is better to use a rheostat.

Never use a thermostat on a lamp, the constant ON/OFF of the lamp will cause the element to weaken and you will constantly be replacing bulbs.

In the cases where a lamp and UTH are being used I think that a proportional thermostat would be best. Both heating devices can be plugged into the proportional thermostat and regulated, without fear of blowing bulbs all the time, because the proportional thermostat does not shut the power off, unless it gets way to hot.

If you are absolutely sure that your UTH is unable to reach the required temperatures and you decide to use a bulb, then it is alright to leave the UTH unregulated and add a rheostat to the lamp to adjust it so that you may achieve the correct temperature in your tank.

I am always leery about leaving UTHs unregulated because when I first got my gecko, he was always climbing as high as he could in his tank. Yet the thermometer said that his UTH was only at 85F, but it always felt extremely warm to me. I went out and purchased a new thermometer and placed it on the UTH and it was reading 115F all over the mat, just not in one place.

Here is a picture that I took yesterday of an Exo-terra 4"x5" mat, unregulated, it is not a true representation of a tank setup and the Styrofoam, probably increased the temperature a little, maybe even having the tape over the probe increased it. But the thermometer is decently accurate and if anyone would like me to test it this set up under an exo-terra 12" cube, to get a better representation, with the probe inside the tank on top of paper towel and a chuck of slate tile, I can.

 

[Tim C.]

Thank you all for the very well-thought-out responses. I'll add what I know so far:

My LG (currently nameless because it's of unknown sex), has been keeping in it's cool hide all day today, and was under it's warm hide all day yesterday, when I got it from the local hobby pet shop. (I say "hobby pet shop" because they're not a chain store, and breed their own animals, as well as "import" from local breeders.) I take the fact that it's been keeping in the cool hide all day, that the warm side is working somewhat as intended.

I had a thermometer, which I suspected as being faulty after reading a few posts on here. I tested it against our ambient, in-house temperature, and lo-and-behold, it's faulty. So now I'm off to get another thermometer tomorrow, and check more precisely the temp on the repti-carpet.

Thanks again for all of the intelligent responses. It's helped fix the jumble in my mind a lot.

-Tim

 

[Tony C]

Samuel explained it perfectly. A thermostat/reheostat will not allow a heating element to run hotter than intended, so if your tank is too cool you will need a secondary heat source. You didn't mention what type of thermometer you were have, unless I missed it, but if you have an analog dial thermometer they are usually not very precise, you are better off using a digital thermometer with a probe as shown in Sam's post.

 

[Tim C.]

You're right, I didn't mention what kind of thermo I had. It's some generic kind of indoor/outdoor thermometer with a probe on it. I tried both the probe and the indoor uses, and they both came up around 85 degrees. My house thermostat is set to run at 70 degrees; I let the probe and indoor part sit inside the house, and it was reading lower, like around 65. This leads me to believe that the UTH is actually sitting at 90 degrees - or thereabouts.

Either way, I'm getting a new thermometer tomorrow, and will be testing that against my old one. Hopefully - and it's a big hopefully - I'm just over-reacting and the UTH is heating up everything just fine. I'll post with the results, and get more input from there.

Thanks again everyone.

-Tim

 

[Tim C.]

Got a new thermo, and it's reading a steady 85 degrees resting above the carpet, and 90 degrees when sitting directly on the carpet. Everything seems to be in order now. Thanks again for all of the help.

-Tim