Group B: Temperature Sensors

There are four major types of temperature sensors: thermocouples, thermistors, resistance temperature detectors, and infrared sensors. Thermocouples, thermistors, and resistance temperature detectors are classified as contact temperature sensors, meaning that they directly measure the temperature of an object by physical contact, as the name would indicate. Noncontact temperature sensors such as infrared sensors indirectly determine temperature by detecting the intensity of thermal radiation being emitted.

Each type of sensor has its own set of advantages and disadvantages. Thermocouples are the most popular type of temperature sensor due to their low cost, durability, and wide range of operation, performing at temperatures up to 3000 C ("Temperature and Temperature Sensors"). They are composed of a pair of junctions made of two dissimilar metals and work by calculating temperature based on the thermoelectric voltage generated between those metals (Mathas). Their simplicity allows a nearly immediate response to temperature variations and means that the units themselves are small and physically durable ("Sensor Selection Guides"). Disadvantages include requiring direct contact with the object being measured and errors due to interference from unaccounted radiation or electric current. Thermocouples also tend to lose accuracy over time as the resistance of the insulated wire deteriorates (Mathas).

Thermistors are the least expensive type of temperature sensor. They are semiconductors formed from metal oxide beads that are coated with glass or epoxy. To measure temperature, a known current is passed through the thermistor and the resulting voltage measured. The temperature can then be determined as a function of the calculated resistance (“Temperature and Temperature Sensors”).  These sensors are highly sensitive and using small thermistor beads will produce very precise readings. However, their range of operation is limited and they do not function well at temperature over 300 C (“Sensor Selection Guides”).

Like thermistors, resistance temperature detectors (RTDs) rely on changes in resistance to determine temperature. Instead of being made from metal oxide beads, the circuit consists of a thin films or coils of platinum (Mathas). They operate over a fairly wide temperature range and can perform measurements over large areas. The greatest advantage is their accuracy and stability over time; RTDs experience very little drift, consistently returning precise measurements (“Sensor Selection Guides”).

Infrared or non-contact sensors measure temperature by converting the thermal energy emitted by an object to an electrical signal. Units typically include a lens to concentrate the radiated heat onto the measurement device. Advantages of infrared sensors include faster response times and the ability to detect temperature of moving, remote, or irregular objects (“Sensor Selection Guides”). They can be used in sensitive applications where contact sensors might contaminate the heat source or interfere with a delicate process. Primary limitations are susceptibility to environmental conditions and restricted measurement areas.

Steve's post covers common methods that pressure sensors use to convert pressure into measurable outputs. Similar to temperature sensors, most rely on electrical signals like voltage, current, or capacitance. However, there is typically an intermediate step in which pressure is first measured in physical displacement prior to conversion to an electrical output. Mike's post touches on ways in which these pressure sensors are being utilized in other fields, including aviation and biomedical industries. I like that it includes a discussion of how similar technology might be applied in intelligent buildings.

References:

Mathas, Carolyn. "Temperature Sensors." Digi-Key. N.p., 27 Oct. 2011. Web. 13 Feb. 2014.

"Sensor Selection Guides." Watlow. N.p., n.d. Web. 13 Feb. 2014.

"Temperature and Temperature Sensors." National Instruments. N.p., n.d. Web. 13 Feb. 2014.