Contact Thermometers

Contact thermometers measure temperature by making contact with the object being measured—via a thermal probe of some type. These can be penetration probes (the most common type, with a pointed tip, often for insertion into food), immersion probes (for liquids), air probes (usually for measuring ambient air in refrigerated/cooling/oven environments), and surface probes for measurements on flat surfaces, e.g. heated plates, where pointed-tip probes (thermocouples) may have difficulty achieving successful contact.

There are various types of contact sensors used in these types of probes to obtain a measurement, e.g. thermocouples (thermocouple), thermistors (thermistor), or PT100 sensors. Thermocouples are commonly used due to their wide temperature range and their measurement over a broad spectrum. Thermocouples are usually low cost, can be integrated into small probes, and have a fast response time. Thermistor-type sensors are generally used when higher accuracy is required. They have a smaller measurement range, but are considered among the most accurate temperature sensors, offering high-accuracy measurements of +/- 0.4°C or better. PT100 sensors can offer even higher accuracy and are often used in laboratory applications or as a reference-type measurement.

Non-contact thermometers measure only surface temperature and are often referred to as IR (Infrared) or Laser thermometers, due to the fact that many have a laser for aiming. These thermometers operate on the principle that all objects above absolute zero emit infrared energy, and this is what they measure.

The fact that no contact with the object being measured is required can be an advantage for several reasons, e.g. safety (if the object is electrically “live”), accessibility (not all probes can access difficult measurement areas), and also product integrity (foods that may be damaged by inserting a probe).

From a practical point of view, the main differences between contact and non-contact thermometers can be summarized as follows:

• Accuracy: In general, contact thermometers are much more accurate than infrared thermometers
• Response time: Infrared thermometers respond faster than probe thermometers—almost instantly
• Measurement capability: Infrared thermometers will measure only surface temperature (not internal), therefore they are not always suitable for certain food applications