Hi
i use one of these at work for checking fridges and it's great at what it does
http://www.ebay.com.au/itm/USB-Temp...cal_Test_Equipment&hash=item4d12ae1af2&_uhb=1
turn it on chuck in fridge and then connect to PC later to graph the temp over time.
you can configure it to take a reading over a various time scale and it will log for weeks if wanted.
it's surprising to see the temp changing in a fridge overnight or from different power sources
with us it's a essential aid as when can print out it's results and show to customer the problem has been fixed