Part of Winters Instruments’ “Support Tool” on temperature instrumentation
(Note: We think this is a fine, concise overview of some of the major types of CONTACT thermometers on the market. (It’s been edited a bit to make it easier to read on our web pages and, we think, a little more thorough, precise & clearer.))

Toronto ONT, Canada –  When selecting a thermometer, it is important to consider many parameters such as the dial or case size, stem, probe or capillary length, the connection type and the temperature range. To ensure safety and accuracy, thermometers should be selected while giving consideration to the measured media and the ambient operating conditions.

Improper application can be detrimental to the thermometer, causing failure and possible personal injury or property damage.

While there are 5 basic operating types of contact thermometers, they being Bimetal, Liquid-in-Glass Thermocouples, Thermistors and Bourdon tube (Filled Systems), the list can really be expanded to also include Resistance Temperature Detectors (RTDs) and solar-powered digital thermometers.

The five types considered here do not include thermocouples and thermistors but those covered appear suitable for many applications. However, optimum selection depends upon the application details, which in industrial uses can be very demanding. The following will serve as a general overview of each of the thermometer types supplied by Winters Instruments.

Bimetal Thermometers
Bimetal thermometers are direct sensing instruments. They are hermetically sealed and therefore, completely waterproof. All bi-metal thermometers are made of stainless steel to protect against corrosive conditions.

How they work: Two different metals with different coefficients of thermal expansion are bonded together.

As temperature changes, the differing expansion of the two metals will cause the bimetal strip to curl, causing a displacement in an attached linkage and pointer. This displacement is transferred from a ridged shaft to a delicate spring that drives the pointer. These thermometers are used in many devices, perhaps the best known are the dial type pocket thermometers illustrated above.

Liquid-in-Glass Thermometers

Also known as liquid expansion thermometers, liquid-in-glass (or LIG) thermometers are perhaps the most popular type of thermometer, commonly seen in the form of clinical fever thermometers, but in industry and commerce as glass tubes with a central colored line that changes length with temperature with a “root” in a lager bulb at the base.

There are two types of liquid-in-glass thermometers: industrial and laboratory. Laboratory models often require certification such as ASTM thermometers, which implies traceability to a reference temperature standard defined by the actual specification number.

How they work: These thermometers indicate temperature by expansion and contraction of the liquid in the tiny capillary tube (i.e. as the fill liquid is heated, it expands and lengthens). Temperature is indicated on the vertical scale parallel to the fill liquid in the glass tube.

Remote Reading Thermometers (Filled System Thermometers)

By means of a metallic capillary tube with a sensing probe at one end and an indicating dial on the other, temperatures can be determined from a source that is up to 30’ (100m) away.

How they work: The capillary of the thermometer (often stainless steel) is filled with a gas or vapour. As temperature changes, the gas or vapour expands/contracts, creating pressure that is measured by a Bourdon tube.

Some of the most common capillary fills are nitrogen (in gas thermometers) and isobutene (in vapour thermometers). Consider factors such as ambient temperature, humidity, installation, indoors or outdoors, presence of dust, corrosive atmosphere, mechanical shock, frequency and magnitude of vibration. (Perhaps the most unique feature of these devices is that they are long-lived and require no electrical power source.)

RTD (Resistance Temperature Detector)
When an application requires remote reading capability combined with higher accuracy than a filled system device, a Resistance Temperature Detector (RTD) may be the best option.

How they work: RTDs work by measuring the resistance (voltage drop at a constant excitation current) of the device across the sensor’s terminals. The Electrical Resistance of each device is a known function of temperature. RTDs can be supplied with a transmitter fitted inside the RTD head to provide an industry standard and calibrated 4-20mA output signal.

Solar Digital Thermometers
Digital readouts are sometimes preferred in some industrial environments. Electrical output thermometers are available with both digital readouts and solar power.

How they work: A temperature reading is produced by a sensor that then relays the measurement in electrical format to a circuit that converts the signal to a digital temperature display. There are a very wide range of such devices on the market.

Solar-powered models need no batteries or plug-in power supply for measurement or display. Solar-powered thermometers, however, typically require between 16 to 35 lux of visible illumination.

About Winters

Winters Instruments is a global manufacturer of pressure & temperature instrumentation, with distribution in over 80 countries, operating from 6 global offices,

Go to www.winters.com to view their list of products and distributors, or call 1-800-WINTERS for more information.