Temperature probes are devices used to measure temperature using contact-style sensing methods. These methods infer temperature by using probes to measure changes in a temperature-sensitive characteristic (such as voltage difference or resistance).

Temperature probes consist of sensor junctions in direct contact with a metal or plastic casing. The casing of the probe makes contact with the system/media being measured and the sensors within the casing typically measure a voltage difference or resistance. Depending on the configuration, one sensor may be in contact with a reference system of known temperature for calibration.

Types of Temperature Probes

Temperature probes can be classified by their sensor technology or probe configuration.  There are three main types of temperature sensors used in probes.

Thermocouples

Thermocouple temperature probes are devices that generate a temperature-dependent voltage. The wires are joined at one end to form a measuring junction within a casing. To measure temperature, the probe is inserted into the fluid or medium and a voltage is generated. In one-junction designs, the voltage is measured directly by the incorporated measuring device to interpret the corresponding temperature. In two-junction designs, the other end of the wires is terminated within a reference probe at a known temperature, and the voltage difference between the two junctions is measured to determine the temperature of the measured system. Thermocouples are generally simple, rugged and cover a wide temperature range. 

Internationally standard color codes help users select thermocouples with materials and sensitivities appropriate for specific applications. British color codes for standard thermocouples are listed in the table below: 

Table Credit: electronics-tutorials.ws

Resistive Temperature Detectors (RTDs)

Resistive temperature detectors (RTDs) are types of thermal resistors - wire windings that exhibit changes in resistance with changes in temperature. The hotter they become, the higher the value of their electrical resistance. Platinum is the most commonly used material because it is nearly linear over a wide range of temperatures, is very accurate, and has a fast response time. RTDs can also be made of less expensive metals like copper or nickel, but these materials have restricted ranges and problems with oxidation. RTD elements are usually long, spring-like wires surrounded by an insulator and enclosed in a sheath of metal. When platinum is used, they are very linear and accurate, but are typically more expensive than other temperature detection methods.

Thermistors

Thermistor temperature probes are another type of thermal resistor. They tend to exhibit a nonlinear relationship between resistance and temperature - large changes in resistance are proportional to small changes in temperature. There are two types of thermistors: negative temperature coefficient (NTC), where the resistance drops with a rise in temperature; and positive temperature coefficient (PTC), where the resistance increases as the temperature rises.

This table compares the properties of these different types of sensors:

Table Credit: The Engineering Toolbox

Other types of sensor technology include solid state sensors and fiber optic sensors.

Solid state or IC temperature sensors are semiconductor sensors which utilize the temperature-sensitive voltage vs. current characteristics of semiconductor diodes. They are very linear with options of high current or voltage outputs; however they typically are not usable at temperatures exceeding 200°C (~400°F).

Fiber optic sensors use optics to measure temperature changes. They consist of a thin film, typically gallium arsenide, deposited on the end of an optical fiber. Temperature can be deduced from the reflected absorption/transmission spectrum. Other specific types of fiber optic temperature probes use luminescence or infrared sensors.

Luminescence temperature sensors consist of temperature sensitive phosphor deposited on the end of an optical fiber. When the phosphor is excited by changes in temperature the luminescence can be measured.

Infrared temperature sensors absorb ambient infrared radiation given off by a heated surface. The incoming light is converted to an electric signal, which corresponds to a particular temperature. This may be done with various methods such as expansion of a material such as a plastic to change the length of the light path. Fiber optic temperature probes are useful where metal probes could be a problem such as an MRI.

Probe Configurations

The probe configuration determines how the device interacts or contacts the system it is measuring.

• Straight probes measure stationary surface temperatures and can be used in wells.
• Penetration probes or needle probes are used to measure internal temperatures.
• Flexible probes can be manipulated for various temperature measurements.
• Drop probes are designed for well or underwater temperature measurements.
• Curved probes are used to measure temperatures on pipes or other curved surfaces.
• Roller probes or moving probes measure temperatures on moving surfaces.
• Air probes are designed for air-flow temperature measurements.
• Angled probes are used for temperature measurement applications inside pipes and angled spaces.
• Surface probes attach to skin or a flat surface. They are also known as banjo, patch, or bolt-on probes.
• Clamp probes or strap sensor probes clamp or wrap around the surface to be monitored.
• Flat probes or ribbon probes are temperature sensors with a flexible membrane.

Specifications

The most important specifications to consider when selecting temperature probes are temperature range, diameter, accuracy, and the number of sensing elements.

• Temperature range defines the maximum and minimum temperatures that the probe is rated for sensing.
• Diameter dimension defines either the diameter of the probe or the diameter/width of the object being measured, depending on the probe configuration.
• Accuracy defines the average deviation a temperature probe measurement will be from its actual value.
• The number of sensing elements defines the number of elements in the probe. Typically, the more elements in the probe, the more accurate and expensive the probe is. Probes usually have one or two elements for sensing, but some can be configured to contain more.

Features

A number of additional features may be important to consider when selecting temperature probes.

• Insulated wires - wires are insulated rather than bare for added protection.
• Wireless - temperature probe can be operated wirelessly for remote operation and/or automated control. Useful in volatile or time-intensive applications where hand-held device might be less appropriate.
• Quick connect - plug for the device is a quick connect for ease-of-use.
• Metal braided leads - device leads are braided, for a more durable connection
• Additional sensor types - device incorporates additional sensors, such as humidity sensors.

Applications

The application in which the probe is being used needs to be considered in the selection process. For instance, some probe designs are specifically made to measure certain materials. Devices like soil temperature probes and water temperature probes are some examples of products which have specific applications. Temperature probes may also be designed for corrosive applications in which the probe material is plastic instead of metal. In addition, probes for sterile applications such as medical temperature probes and food temperature probes are designed with ease of cleaning in mind.

Image credits:

Chromalox | Dynisco