I have a 3ft x 3ft compost bin and have been putting garden and vegetable waste into the compost on a regular basis. I have also been turning the pile every week. I have started to measure the temperature of the pile but wanted to find out what is the temperature I should be maintaining at the center of the bin? Also, how can I control the temperature if it is not in the recommended range.
There isn't really a simple answer to the question of temperature, except to say between 55 deg F up to 160 deg F - different aerobic bacteria operate at different temperatures, but in order to have a fairly sterile product at the end, the temperature should reach up to 160 deg F for a period of time. At that temperature, it will be thermophilic bacteria doing the work - mesophilic operate at lower temperatures, and psychrophylic at 55-70 deg F. Keeping an even temperature is difficult - it will fluctuate according to water content, how often its turned, whether you continue to add new materials and what those materials are, and the outer parts of the heap will always be cooler than the interior. In larger heaps in hot climates, it's possible for the temperature to get much higher than 160 deg F - at higher temperatures, bacteria cannot operate and start to die out. More information in the link below regarding the three main types of bacteria and the temperature ranges they operate at
The link below suggests turning as a measure to take should the temperature get too high, though there is some argument as to how effective more frequent turning is when a pile is too hot - turning increases air supply which initially cools the heap, but also increases aerobic bacterial activity, so the heap (in trials) has been shown to be just as hot, or hotter, three or fours after turning as it was beforehand. This is more of a problem with large heaps in hot climates than it is with smaller ones in cooler regions, but there isn't really any choice but to turn it frequently to try to keep temperatures down.
For small heaps (the size of yours) in cooler climates (like in the UK) a layer of old carpet on top of the heap is recommended to help maintain temperature. It's not intended to rot down or contribute to the compost, but to act as a sort of lid to trap more heat.
With a smaller pile, it's important to confine the compost. I found this article http://www.vegetable-gardening-with-lorraine.com/homemade-compost-bin.html a couple years ago and I've used it to great effect. Makes turning and harvesting easy as pie, provides enough oxygen, and confines/insulates the pile. (Mine is only 30" on a side and I get my 160).
Sun always helps, especially morning sun. The inside temperature of your pile is always competing with the exterior temperature. Sunlight is often the difference between a good pile and a dud (at least that's what I read when I had a dud pile going in full shade a few years ago when I first started composting).
Water may cool the pile initially, but it adds weight, and pressure and temperature go hand in hand. Don't let your pile get too dry.
I turn every other weekend, and stab the pile a few times with the pitchfork on the non-turn weekends to introduce some oxygen. I water after the turn/stab and on Wednesdays.
The site I linked has a lot of great info about small-scale composting, and the DIY bin is the bee's knees.
The point of having a temperature probe is to confirm that the bacterial procession in your pile is proceeding correctly. With a pile that size, the absolute minimum for a hot pile, you won't get extremely hot temperatures.
The initial rise in temperature is what you're looking for to confirm that the pile has been constructed correctly. If it doesn't rise, you have too much carbon material.
It should reach a maximum temperature for a few days, and then start to cool down as the material in the centre of the pile is exhausted.
You then need to move the pile so that the outside of the pile forms the new centre of the new pile. The centre of the old pile forms the outside, insulation for the new pile.
You can use sticks, or whatever, to push through the pile to ensure adequate oxygen reaches the centre of the pile to allow the oxidating reactions to continue.