Best Practice for Mounting Thermocouples in Pipes

Diagram 1 - Not recommended.

Immersion type thermocouples are used to measure the temperature of liquid flowing in a pipe or sitting in a vessel. The particular orientation for any installation depends on the application, whether additional hardware is required, and the relative dimensions of the thermocouple sheath and the pipe.

There are however, recommended practices for placing thermocouples in the piping flow stream.

As you can see from diagram 1, placing the thermocouple in a tee, positioned perpendicular to flow is not recommended. This is because the conduction along the sensing area may be non-negligible and could bias the measurement, depending on the liquid and ambient temperatures. In addition it may be hard to know precisely where the measurement junction is located along the cross section of the pipe in this configuration.

Ideally they should be mounted in a tee where an elbow would normally be used as you see in diagram 2. If possible the thermocouple should be oriented along the normal flow direction, and the measurement located downstream of the T-bend (the T will help mix the liquid if it is not thermally uniform).

Diagram 2 - Recommended orientation.

If there is no convenient spot where an elbow would normally be used, a u-shape can be adapted to allow the installation for the tee. See diagram 3.

Diagram 3 - Use U shape if there is no convenient placement for a tee.

For more information, contact Duro-Sense by calling 310-533-6877 or visit their web site at https://duro-sense.com.

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Platinum Resistance Temperature Detectors

Platinum Resistance Thermometers Detectors (RTDs) rely on the fact that platinum, like many other metals, exhibits increased electrical resistance as temperature rises. For example, a conventional RTD designed to provide 100 Ohms at 0 °C has a resistance in the neighborhood of 80 Ohms at -50 °C and 120 Ohms at 50 °C, or a sensitivity of about 0.4 Ohms per degree. RTDs constructed to particularly exacting specifications, termed Standard Platinum Resistance Thermometers, are named as the defining measurement tools for interpolating temperatures under ITS-90. In general, RTDs can have high accuracy (0.01 °C), stability, and repeatability across a wide range of temperatures from -200 °C to 500 °C.

Typically the platinum element is formed into thick or thin films, or the platinum wire is arranged in two, three or four helical coils (see diagram, right) – the more coils, the higher the sensitivity. The film or wire is placed inside a glass or ceramic enclosure, and can be supported by loose or compacted MgO. Platinum-based leads connect the probe unit to the thermometer electronics, which convert the electrical signal to temperature.

RTDs are broadly divided into two groups: Industrial RTDs and Standard Platinum Resistance Thermometers, depending on sensitivity and robustness. ASTM and IEC define several classes of RTDs, each with a different set of specifications. An ASTM "Class A" unit, for example, has an out-of-the-box tolerance — maximum permissible error — that ranges from 0.47 °C at -200 °C to 0.13 °C at 0 °C to 0.98 °C at 500 °C.

Advantages

• Wide temperature range
• Resistance-temperature relationship is well characterized.
• Rugged construction in industrial RTDs
• Available in different shapes and sizes – application specific
• Can be used with a digital temperature read-out device.

Disadvantages

• Mechanical shock and vibration will cause drift.
• Deterioration at elevated temperatures (e.g., >500 °C)
• 2-and 3-wire devices need lead-wire compensation.
• Non-hermetically sealed RTDs will deteriorate in environments with excessive moisture.

Post abstracted from "Mercury Thermometer Alternatives: Platinum Resistance Thermometers (PRTs)" by NIST.