The Food Safety and Inspection Service (FSIS), an arm of the United States Department of Agriculture (USDA) has maintained 'best practices' for measuring temperature in different refrigerated/heated environments. These tests will tell you if your equipment is in correct working order. For food safety and equipment accuracy, temperature sensors/probes have a range of applications for different storage/cooking methods.
An average oven for cooking meat and poultry should be set for 325°F or higher. To maintain accuracy, a temperature sensor/thermometer should be used to ensure that the oven is functioning properly. Misleading oven temperatures can comprise both food quality and safety. The FSIS suggests that oven thermometers/sensors should be hung from a rack in the center of the oven. Be sure to test multiple temperatures, apart from 325°F, to ensure continued accuracy with increased/decreased heat. As per the FSIS advice, some ovens may "run hot" and any normal "variation" should be accounted for when measuring the overall temperature.
Albeit a bit tricky, microwave temperatures can be monitored using specialized probes or with built-in hardware. Consumer-grade microwaves often have this feature built-in, highlighted by this ehow.com article on Frigidaire Microwaves. Other consumer brands allow similar measurements. For commercial uses, however, there are more specialized probes for microwave ovens. These probes are typically immune to Electromagnetic Interference (EMI), Radio Frequencies (RF), and microwaves, and have an expanded temperature range, from 10°C to over 950°C. These probes, while often costly, give both accurate and precise readings of temperature for commercial microwaves. For these applications, ensure that yoursensor/thermometer can withstand the various types of interference to maintain accurate readings.
Borrowing a few tips from our article "Where to place a Temperature Sensor: Vaccine Refrigeration", the same applies for Freezers. Each section of the freezer has some temperature variation and this must be taken into account when using a sensor/thermometer. The FSIS recommends placement between frozen food packages in the center of the freezer, with a 5-8 hour waiting period. After the waiting period, the temperature should read between 0-2°F. These "packaged" buffers are a useful variable, as they represent a common occurrence in freezer storage. The presence of the buffers and temperature readout will indicate if the Freezer is within a functional range (based on the controlled variable).
If you missed it, see our article "Buffer Vials for Temperature Monitoring: Propylene Glycol vs Sand" for an accuracy comparison of buffer substances for temperature sensors and probes.