Selecting a Thermometer


When selecting a thermometer, it is important to consider the dial or case size, stem or capillary length, and the connection type and 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 may be detrimental to the thermometer, causing failure and possible personal injury or property damage.

Knowing the environment and media that the thermometer will be subjected to is also essential. This information will determine what type of thermometer is required and if thermowells are necessary.

Temperature measurement errors to keep in mind: conduction, convection, radiation, response time, noise, grounding issues and shorts (especially on metal surfaces).


Thermowells

Thermowells must be used on any application where the stem of the thermometer may be exposed to pressure, corrosive fluids or high velocity. A thermowell allows for a closed system which permits instrument interchange for calibration or repair without disturbing or closing down the process. Thermowells are available in brass or stainless steel with standard thread connections. Other materials and flange options are available upon request.

Bi-Metal Thermometers

Bi-Metal thermometers are direct sensing instruments. They are hermetically sealed and therefore, completely waterproof. For accurate readings, the stem should be immersed at a minimum 2.5” (63 mm) in the media. All Bi-metal thermometers are made of stainless steel to protect against corrosive conditions. An external adjustment screw is conveniently located on the back of each case for field recalibration.

How it works

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 bi-metal strip to curl, causing a displacement. This displacement is transferred from a ridged shaft to a delicate spring that drives the pointer. As the shaft twists the spring, the spring movement is relayed through gears to the pointer that indicates the temperature change. An increase in temperature results in an expansion or a positive twist; a decrease in temperature results in a contraction or negative twist. Bi-metal strips can be fabricated into coils, spirals and disks. They are frequently used in on-off temperature controls (thermostats).

Liquid-in-glass Thermometers

Liquid-in-glass thermometers are perhaps the most popular type of thermometer. Liquid-in-glass thermometers indicate pressure by measuring expansion and contraction (i.e. 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.

Industrial Thermometers

Winters’ industrial thermometers are available in 5” (127 mm) and 9” (230mm) scales. These thermometers have a glass tube filled with liquid which expands and contracts according to reflections in the temperature. The industrial thermometers come in three different installation types: fixed bottom, fixed back and fully adjustable. Brass separable thermowells are standard on both sizes of industrial thermometers.

Please note that the selected temperature range should be approximately twice the normal operating range anticipated and the maximum temperature should not exceed 75% of the full scale.

Laboratory

Laboratory thermometer scales are available from 5” (127mm) to 20” (500mm). Standard ranges are available in Celsius or Fahrenheit. The thermometers have a glass tube filled with liquid which expands and contracts according to the temperature. Protective armors are available in various materials, including stainless steel.

Remote Reading Thermometers

Remote reading thermometers are instruments used to measure temperature from a remote source. 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 30’ (9m) away.

Remote reading thermometers are filled with either gas or vapour depending on the specification. As temperature changes, the gas or vapour expands/contracts, creating pressure that is measured by a Bourdon tube.

The medium being measure should be considered when selecting a remote reading thermometer. Consider factors such as ambient temperature, humidity, installation, indoors or outdoors, presence of dust, corrosive atmosphere, mechanical shock, frequency and magnitude of vibration.

Cases are available in various sizes and made of steel, stainless steel, phenolic and cast aluminum. Capillaries are available in copper braided and stainless steel (with or without flexible armor), in an assortment of lengths. Winters recommends using a stainless steel bulb and capillary for all process temperatures above 500°F (260°C).

Winters also has a non-contact thermometer that utilizes an infra-red beam to measure surface temperatures. This portable point-and-shoot thermometer gives temperature readouts on its digital display.

Gas thermometers

Gas-filled thermometers are designed for industrial applications requiring accurate and uniform response over the entire range. The entire system is filled under pressure with inert gas for positive movement, accuracy and sensitivity. The accuracy delivered by this system is ±0.5% of the full scale value.

Vapour thermometers

Vapour-filled thermometers have non-linear scales (non-equal graduations across the entire scale). To ensure exceptionally close readings, the operating temperature should fall in the 2/3 of the scale. The accuracy delivered by this system is ±2% of the full scale value. Vapour thermometers provide economical and reliable remote reading service for most industrial applications.

RTD Thermometers

When an application requires remote reading capability combined with high accuracy, a Resistance Temperature Detector (RTD) may be the best option. An RTD works by reading the resistance charge across a metal wire that is produced from each temperature input. RTDs can be supplied with a transmitter fitted inside the RTD head to provide an industry standard 4-20 mA output signal. RTDs are very rugged but cannot be used to measure extremely high temperatures.

Solar Digital Thermometers

Digital readouts are sometimes preferred in some industrial environments. Electronic sensing thermometers are both available 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. These thermometers require between 16 to 35 lux of illumination.

Note: When selecting a thermometer, always refer to ASME B40.200 (2008)

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