This is a guide to how thermostats should be used, to ensure they function properly and that your Dragons are protected from overheating etc.
Because of the temperatures involved in a Dragon cage, some of the principles behind the use of thermostats are a little different to using them for species which require somewhat cooler cages, such as snakes and geckos.
Once the basic principles are understood, much more elaborate - but safe - lighting setups can be used to really recreate a slice of nature in your living room!
A thermostat ("stat") is used to control the temperature of a cage. It works by adjusting the power to your heating device to keep the temperature by your probe (or the box itself if you have a probeless one) to the temperature you set the thermostat to.
They really are that simple in principle - you set the temperature you want, and a correctly set up thermostat will do all the work to keep the cage how you want it. The problem is that there are several different types of thermostat, and several different ways to heat a cage...
This turns the power
off when the desired temperature is hit, then turn the power
back on when the temperature falls a certain amount below
what it's set to (the "on threshold" value). Depending on
the stat, this can be a few degrees. If you set the stat to
85f, and it had a 3f sensitivity, the temperature would go:
85f -> 82f -> 85f and so on. On-off stats are commonly used
to regulate central heating in rooms in your house. The
reptile-branded ones aren't terribly accurate, but they do
the job just fine for a non-light-emitting heat source
that's intended to provide gentle ambient heat.
This sends power to the device in
pulses, rather than being simple on for a while, then off
for a while, then on for a while, then off for a while etc.
They are usually used with ceramics - if used with a light
bulb, they will cause the bulb to flash on and off fairly
quickly. They are usually much more accurate than Mat Stats
though, and are useful for non-light-emitting heat sources
that need to provide constant high temperatures near to the
heat source, as in a basking spot heated by a Ceramic Heat
These reduce the power to the heat source (but
don't turn it off completely) when the desired temperature
is approached. They are commonly used with lightbulbs.
Compared to the other types of thermostat, they prolong the
life of a bulb by not switching it on and off, and cause
less confusion to your pet as the light levels don't
drastically and rapidly change.
For an animal that requires a
temperature gradient, a stat should be seen as a safety
feature, NOT a control device. For an animal that needs a
single ambient temperature it's slightly different, but for
an animal wanting a gradient, they should not be used to
control the warm end temperatures.
A stat is fitted to ensure that there is always a cool end to retreat to. There's a reason why most only go up to 92f; they weren't intended to be directly controlling 110f bearded dragon basking spots!
If a Bearded Dragon (or any other diurnal basking reptile) gets too hot, it will seek a cool area to cool off in. If you have your stat right under the basking light, set to 92f and it's a hot day, the rest of the vivarium may get to 92f, as there will be very little heat loss due to there only being a small thermal gradient to the rest of the room. If your Dragon has been basking and has got itself up to 100f (desired temperature is 93-96f, depending on the individual) and tries to cool off, it will take a very long time to lose heat with only an 8f difference, even in the cool end. Staying at too high a temperature for a prolonged period is not good for them - when they want to cool off, they have to be able to do it quickly! A cool end of 80-85f gives a 15-20f gradient for heat loss; this will allow much faster cooling (read up on thermodynamics if you want to know why...)
The stat / probe needs to be in the cool end, to ensure that, even on the hottest day, there is somewhere to cool off. A Dragon that is too cool on a particular day will just slow down a little, but won't be otherwise concerned. A Dragon that gets too hot and can't cool down is at risk of neurological damage and even death.
Once you have the cool end under control, the cage is "safe" and you can then think about setting the basking spot...
Once you have a nice, stable cool
end (which may take an hour or two to settle down), you can
start to think about creating a basking spot. The stat
controls the total wattage of energy going into the cage;
the basking spot is controlled by the intensity of the light
on the basking spot - think of it in terms of energy per
square inch of basking spot. A lot of energy means a hot
basking spot; a little energy means a cooler one.
Using a narrower beamed light can help - the wattage is the same, but the light is more focussed onto the basking spot, so the intensity is higher. Changing the distance between the light and the basking spot also helps - it increases the intensity of energy hitting the basking spot. Often this will mean suspending the light from the top of the vivarium, or buying a less tall vivarium to being with, to avoid having to build amazingly tall and complex basking spots.
The majority of our vivariums are 18" tall. We have found that in a 4' x 2' vivarium this allows the basking light to be mounted directly onto the top of the cage to give the correct temperature on a 3-6" tall basking rock under the light. Cages less than 15" tall or more than 20" tall may have trouble getting the proper basking temperatures with roof-mounted bulbs.
There are a number of
commonly-encountered problems / faux-pas associated with the
use of stats:
a) They only control the temperature at the probe. A stat can't control the rest of the cage. You have to set the cage up - with the thermostat - under normal operating conditions, then tinker around with the rest of the temperatures.
b) Heat rises. Having a stat placed 18" up the backwall in a cage for a 1" tall baby Dragon doesn't make much sense - the Dragon doesn't care how warm it is 18" above its head; it cares how warm it is on the ground! Although this is a "safe" kind of error, it leads to inefficiency in heating - if the temperature is controlled at 85f 18" up, it may only be 75-80f on the ground. Whilst this is good in that there's a decent cool spot, it will make setting the basking spot harder, and can lead to premature dimming of the bulb on hot days - the ground level where the Dragon is may still only be 75-80f, but the stat will sense 85f, and will turn the basking light (and hence the basking spot) right down. A prolonged period without a basking spot isn't ideal for a Bearded Dragon.
c) A 100W bulb dimmed to 40W is much less bright than a 60W dimmed to 40W. If your bulb is dim all the time, switch down to a lower wattage. You want the basking light to be as bright as possible, whilst keeping the correct temperatures. I use 60W bulbs in my 4' x 2' x 18" vivariums. The number of people using 100W bulbs in tiny cages that give off almost no light is pretty high! Remember that Dragons need bright light to promote normal activity; a cage with a high-power but extremely dimmed bulb may be at the right temperatures, but it is likely to be a lot darker than one that uses a bulb with a more appropriate wattage bulb.
d) Ventilation is needed to allow heat loss. If heat can't escape from the cage, the cool end will get to 85f, the light will dim...and will stay dimmed. The whole cage is now at 85f, and your Dragon has just lost its thermal gradient and basking spot. A couple of 75mm vents in the cool end allow heat to be lost at that end. This makes sure that as heat is transferred from the warm end, it is lost and doesn't just build up in the cool end. Heat loss is vital for keeping a good thermal gradient. If you are having problems with the cage getting to temperature then staying there, add more ventilation to the cool end. More ventilation in the cool end allows a larger thermal gradient, whilst more ventilation in the warm end will give a cooler basking area. This can be useful if your ambient temperatures are perfect but the basking spot is getting too warm and you can't increase the distance between the basking rock and basking light.
Hopefully this has cleared up some misconceptions. My intention with this guide is to improve safety in reptile cages, by greatly reducing the risk of overheating, especially with summer on its way. People who only got into reptiles in the last 18 months may never have experienced a "proper" summer (as the last one scarcely counts in terms of temperatures) with their reptiles, and may not be aware of what an exceptionally hot cage can do to temperatures and gradients.
There are a lot more things that
can be done with thermostats than what is described above.
I'd really recommend that if you've never kept reptiles that
need basking spots / cool ends, and aren't familiar with the
concept of thermostats, you stick to what's described above.
It's simple, it's safe and it's functional.
If you are using large, ventilated cages, you could replace the dimming stat with an on-off, and have it as a true safety feature. Set it to 85f, place it in the cool end as usual, but set up the rest of the cage (ventilation, size of cage etc) so that, even on the warmest day, the cool end remains below 85f. You are likely to need a large cage to do this.
This means that your bulb is now "uncontrolled", with only a safety cut out in place (incase something goes wrong / we get an unexpected heat wave). What this means is that your cage will now track room temperature, and you'll get a bit more temperature variation, as you would in the wild. If you use this in an unregulated / only slightly heated room, the cage temperature will now track even more closely to outside temperature, giving true temperature fluctuations, depending on the weather. For animals that can sense air pressure (and hence weather) changes, such as Bearded Dragons, it may be less confusing to them if they sense that there will be rain, and the cage gets a little colder than usual, than if they sense rain and the cage retains it's 110f basking spot...
Note: the on-off safety cutout described above is the only way to "stat" a Mercury Vapor Bulb (MVB) - they cannot be used with dimming / pulse stats.
If you are using a large cage, how about having 2-3 basking lights, each linked to their own timer with the timers all linked to a single thermostat. Put the probe to create one permanant cool end. Set one timer to come on first thing in the morning. Set the first timer to off at lunchtime, and the second timer to come on. Your basking spot will move, representing the movement of the sun across the sky (in nature, reptiles have to move during the day to track the sun and the best basking spots). If you have a 3rd light, do the same, but with it coming on in the late afternoon. It may be a solution to all of the lazy bearded dragons that are only too happy to sit around by their light and not move for an entire day...
I have used a similar principle when building a Fake Rock Display Cage. The cage has a series of lights which come on in combinations to give a more realistic daily lighting sequence. These photos were all taken whilst the cage was still being built, so apologies for the unfinished look!
For early morning and late evening the cage is given a reddish glow and no real basking spot:
For mid-morning and late afternoon a basking spot is present along with a second, low-energy bulb to provide extra light:
For the midday period, another bulb comes on to add a bit of extra brightness and light to the basking spot. Although the cool end is still cool and somewhat shaded, on the surface right under the basking spot the temperatures can hit 120f, as they would on the hottest Australian days:
By giving the cage a daily temperature and light cycle, it forces the occupant to move around during the day to track their preferred temperature and light range, rather than being able to lazily sit in a single place all day.