Winters advises on thermometer selection

When selecting a thermometer, it is important to consider the dial or case size, stem or capillary length, the connection type and the temperature range, according to Winters Instruments.

While there are three basic operating types of thermometers, bi-metal, liquid-in-glass, and Bourdon tube, the list can be expanded to include RTD and solar digital thermometers.

All five types may appear suitable for any typical applications, however, the correct selection depends upon the industry being served and the application specified.

The following is a general overview of each of these thermometers.

Bi-metal 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.

Two different metals with different coefficients of thermal expansion are bonded together.

As temperature changes, the unequal expansion of the two metals will cause the bimetal strip to curl, causing a displacement.

This displacement is transferred from a ridged shaft to a delicate spring that drives the pointer.

Liquid-in-glass thermometers, also known as liquid expansion thermometers, are perhaps the most popular type of thermometer.

There are two types of liquid-in-glass thermometer: industrial and laboratory.

These thermometers indicate temperature by measuring expansion and contraction (as the fill liquid is heated, it expands and rises).

The temperature is indicated on the vertical scale next to the fill liquid in the glass tube.

Remote reading thermometers work by means of a 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 10m away.

The capillary of the thermometer 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).

Factors such as ambient temperature, humidity, installation, indoors or outdoors, presence of dust, corrosive atmosphere, mechanical shock, frequency and magnitude of vibration should be considered.

When an application requires remote reading capability combined with high accuracy, a resistance temperature detector (RTD) may be the best option.

RTDs work by reading the resistance charge produced across a metal wire from each temperature input.

They can be supplied with a transmitter fitted inside the RTD head to provide an industry- standard 4-20mA output signal.

Digital readouts are sometimes preferred in some industrial environments.

Electronic sensing thermometers are available both with digital readouts and solar-powered.

The temperature reading is captured by a sensor that relays the data to a digital display.

No additional power supply is required to power these thermometers, which require 16 to 35 lux of illumination.

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