USP Toolkit: ± 0.5oC but all vessels should agree within 0.4oC
ASTM: ± 0.5oC
Temperature is critical because it has a linear affect of dissolution results. With current technology there is no reason why temperature should not be controlled extremely accurately to reduce one of the primary variables.
Using a calibrated thermometer, the temperature should be tested in each vessel prior to each test. It is not enough to measure in the waterbath and assume that the temperature will be correct because it may not be. The temperature of the waterbath may well be higher than 37oC in order to achieve 37oC inside the vessels. If plastic vessels have to be used, their heat transfer is poor compared with glass and the waterbath temperature will need to be correspondingly higher.
Bear in mind also that factors such as heat loss from one end of the bath to the other, inadequate water pumping and external factors such as air conditioning can effect the temperature around the dissolution tester.