I often print with ABS which requires a warm print chamber to prevent delamination and warping. I have found that getting the chamber to at least 45C does the job, but how do you know when it's 45C? I ordered this:http://www.amazon.com/gp/product/B00DVI14CQdigital thermometer module from Amazon for $6 and then shortened the cable to the thermistor.I designed two different mounts, one of which I printed and used.  I attach it to my printer using magnetic tape, but you can use velcro tape or screws.I printed with ABS in 0.25 mm layers, 20 % infill, no brim/raft.