Industrial temperature readout and probe selection guide

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Industrial temperature readout and probe selection guide (1.06 MB)

Industrial temperature sensors are typically calibrated by placing them in a stable temperature source (dry-well, furnace, calibration bath) and comparing their output to a reference standard probe connected to a thermometer readout. This document provides a guide for selecting a thermometer readout and companion reference standard probe that will provide adequate system accuracy required to calibrate common temperature sensors such as PRTs and thermocouples.

This guide covers the most common applications of precision thermometers in calibration such as choosing a thermometer to improve calibration accuracy in a bath or dry-well or to compare against process instrumentation in-situ (e.g. a thermowell next to a gauge or transmitter). Please consult a Fluke Calibration temperature specialist to assist with your equipment selection if you have a special application such as measuring surface temperatures, liquids with high pH, air temperature, or temperature inside an enclosure such as a freezer or oven.

Five steps in choosing a thermometer readout and reference probe:

Step 1: Choose the best thermometer readout for the industrial sensor application.

Several questions should be considered in selecting the right thermometer readout:

• Which temperature sensors need to be calibrated—PRTs/RTDs, thermistors, thermocouples?
• Will the readout be used in the field or in a calibration lab?
• How many channels are needed on the readout?
• What level of data logging, graphing and recording features are required?
• Is temperature source control of dry-wells, baths, or furnaces desired to help automate sensor calibration?

The following table provides a guide for selecting a readout with these technical needs in mind.

Step 2: Select the reference probe considering the temperature range of the sensor application and immersion depth required.

It is important to select a reference probe that covers the full temperature range of the sensor application. Table 2 summarizes temperature ranges for selected reference probes.

Consider the length

Make sure the reference probe is long enough to reach the bottom of the dry-well or the sensing element of the unit under test in a bath. The sensing element of a PRT is usually in the bottom one inch of the probe. A thermistor sensing element is only a few millimeters at the bottom of the probe. The measurement junction of a thermocouple is where the two dissimilar wires connect.

To ensure the reference and the unit under test are at the same temperature during comparison calibration, the sensing element of the unit under test needs to be vertically aligned with the center of the sensing element of the reference probe. Also, inaccurate measurements can occur if either the reference probe or the unit under test is not sufficiently immersed in the dry-well or bath.

Consider the diameter

Minimum immersion is the minimum depth the probe needs to be inserted into the bath or dry-well for accurate measurement. It is determined by the diameter of the selected probe and the length of its internal sensing element. A general rule is the minimum probe immersion needs to be 15 times the probe diameter plus the sensor length. Fluke Calibration 6-inch and 9-inch PRTs have a 3/16 inch diameter rather than a 1/4 inch diameter and can be a better choice for calibrating shorter probes. See Table 2 for minimum immersion depths for select probes.

Safety and other considerations

Some applications may require exposing more of the probe to extreme temperatures than is desirable. Exposing the probe handle to extreme temperatures poses safety concerns for the user, since it may be too hot or cold to touch without safety gear. Also, the transition junction is located inside the probe handle base where the probe connects to the cable and can be damaged by extreme temperatures. Finally, if high temperatures in the transition junction cause the insulation resistance to decrease below 100 MΩ, the performance of the probe might also decrease.

For example, a 5615-12 Secondary Reference PRT can operate over a range from –200 °C to 420 °C. However, the 5615-12 transition junction range is –50 °C to 200 °C. This means the probe is designed to measure temperatures from –200 °C to 420 °C, but the probe will be damaged if the handle is exposed to temperatures outside the range of –50 °C to 200 °C. Even if the probe is not damaged, touching a probe handle that is extremely hot or cold with bare hands could result in burns.

In this example, the 5615-12 can be used to calibrate sensors as low as -200 °C, but would be damaged if placed in a freezer at -80 °C since the transition junction lower limit is -50 °C. For a freezer application, the 5606 Full Immersion PRT would be the right choice since the probe and transition junction can operate at a lower limit of -200 °C.

Step 3: Determine the combined system accuracy of the readout and reference probe selected.

Table 3 shows the system accuracy for Fluke Calibration 1523/1524, 1502A/1504, 1529, and 1586A Super-DAQ thermometer readouts and selected reference probes (5615, 5627A, 5628, 5605, 5610) or Type T and K thermocouples. For example, the 1586A Super-DAQ with DAQ-STAQ Multiplexer and a 5628 Secondary Standard PRT has a system accuracy of ± 0.011 °C at 0 °C.

Reference probes are connected to the thermometer readout, but readouts don’t all share the same connection scheme. When a readout and probe are paired, be sure to choose a model terminated with the right connector. For your convenience, probe models with the correct termination for the readout are shown in Table 3. Note that the readout accuracy with a 5606 probe assumes that the probe has received an optional calibration.

Step 4: Verify that the readout and probe system selected will provide the accuracy needed to calibrate the sensor under test.

The calibration system comprised of a readout and reference probe needs to have a higher level of accuracy than the temperature sensor being calibrated. A “test accuracy ratio” (TAR) of 4:1 or 3:1 is commonly used as a guideline. A 4:1 TAR means the calibration system is four times more accurate than the sensor being calibrated. In this example, the system with a 4:1 TAR would be more accurate than a system with 3:1 TAR.

Table 4 shows the minimum system accuracy required to calibrate common temperature sensors (Grade A and B PRTs, Type T and K thermocouples). For example, a system (readout, reference probe, and temperature source) with a combined accuracy of ± 0.06 °C would be needed to calibrate a Grade B PRT at 0 °C with a 4:1 TAR.

Step 5: Evaluate if additional calibration is required.

Factory calibration

It is standard practice for all Fluke instruments to include a factory calibration that is traceable to national standards. Traceability means that there is an unbroken chain of comparisons between the instrument and a national standard providing assurance that measurements obtained with the instrument correlate to a national standard at a particular level of uncertainty.

In a few cases, probes such as the 5606 do not include a factory calibration, but a calibration is an available option. If you purchase an uncalibrated probe, then the chain of traceability is broken until a calibration is performed.

With many Fluke instruments, the factory calibration is also accredited to ISO 17025. Table 5 summarizes the factory calibrations for the instruments discussed in this guide. Typically, type T and type K thermocouples are provided uncalibrated by the manufacturer. Check with your distributor about temperature instrument calibration options available.

System calibration

In addition to a factory calibration for both the probe and readout, you may desire to verify the performance of the probe and readout together with a “system calibration.” This system calibration provides a higher level of assurance that the instruments are performing as expected when combined together and all probe coefficients are entered correctly into the readout. Check with your distributor about system calibration options available.

Summary

This guide has covered the steps to follow when choosing a readout and probe appropriate for your application. Temperature range of the application and accuracy required are key considerations, but other factors discussed in this guide should be evaluated. If you have a special application such as measuring surface temperatures, liquids with high pH, air temperature, or temperature inside an enclosure such as a freezer or oven, please consult a Fluke Calibration temperature specialist to assist with your equipment selection.