How
are you able to measure temperatures? Temperature can be measured with a wide
variety of sensors. Each one of them infers temperature by sensing a change in
a physical characteristic. There are six different methods which an engineer is
likely to come in contact with: change-of-state devices, liquid expansion
devices, bimetallic devices, infrared radiators, resistive temperature devices
(thermistors and RTD's), and thermocouples.
Thermocouple Temperature Measurement
Sensors
These
generally consist of two wires or strips that are made from different metals
joined at one end. Changes of temperature at this junction create a change in
the EMF, or electromotive force, between the opposite ends. As the temperature
goes up, the thermocouple output EMF will rise, although not always linearly.
Thermocouple sensors are regarded as among the best temperature measurement
devices, along with the RTD's.
RTD or Resistance Temperature Devices
These
devices will capitalize on the idea that the electrical resistance of the
material will change when the temperature rises. Two main forms are metallic
devices (generally known as RTD's) and thermistors. Just like their name says,
the metallic devices will rely on only a change of resistance in the metal,
with this resistance rising linearly with the temperature. Thermistors are
mainly a resistance change within a semiconductor made of ceramic; the
resistance will drop non-linearly with the rise of temperature.
Infrared Temperature Measurement Devices
These
sensors are unique in being non-contracting devices. They generally infer the
temperature by measuring the amount of thermal radiation that a material is
emitting. Though less accurate than thermocouple or RTD sensors, infrared
devices still provide information regarding the change in temperature. These
are generally only used as a backup method for measuring temperature.
Bimetallic Temperature Measurement Devices
These
devices will take advantage of the variations in rate of the thermal expansion
between various metals. Two different metals are bonded as one. When they are
heated, one side expands a little more than the other, and the bending result
is translated into a reading of temperature by mechanical linking to a point.
These types of devices are portable and don't require any supply of power.
However, they are not generally as accurate as RTD's or thermocouples and they
don't lend themselves readily to recording of temperature.
Fluid Expansion Temperature Measurement
Device
These
kinds of devices, typically the thermometer you find at home, come in two
categories: the organic-liquid type and the mercury type. There are also some
versions available that use gas rather than liquid. Mercury is known as an
environmental hazard, so there are some regulations against the shipping of it.
The fluid-expansion devices require no power, do not pose as a hazard for
explosion, and are stable after many uses. On the other hand, they don't
generate information that is easily transmitted or recorded, and they won't
make good point or spot measurements.
State Change Temperature Measurement
Devices
These
types of sensors generally consist of liquid crystals, lacquers, crayons,
pellets or labels that change appearance when a specific temperature has been
reached. They are usually used, for example, with steam traps. When a trap
exceeds the specified temperature, a small white dot on the label of the sensor
that is attached to the trap will change to black. The time of response is
usually several minutes, therefore these devices cannot respond to quick
temperature changes. Accuracy is also much lower than that of the other
sensors. Moreover, except in the case of liquid-crystal displays, the change in
the state is irreversible. Even so, these types of sensors may be very helpful
when an individual needs a confirmation that the temperature of a piece of material
or equipment has not exceeded a specific level, for example for legal or
technical reasons during shipping of a product.
Editor's Note: This article, written by Camia Sidle,
explains the different methods of temperature measurement that are often used.
When you are in need of calibration services,
ensure that you choose a service and a type of temperature measurement that is
best for your business, for your equipment and for your price.
|