Temperature Measurement Tips
Temperature measurement accuracy goes beyond just the accuracy of the sensor. Here are 10 things you should know about using platinum resistance thermometers for temperature measurement.
Platinum resistance thermometers (PRTs) can provide accurate and stable temperature measurements if they are selected and used properly. Choosing the right platinum resistance thermometer for the application is as critical as the proper use of the device.
The three types of PRTs are direct immersion, indirect immersion and surface mount. Direct immersion sensors allow the sensor to be immersed directly into the process. They generally provide the most accurate temperature measurement. Indirect immersion sensors use a thermowell to provide additional strength to the sensor or provide a means for removal without interfering with the process. Surface-mount sensors can be convenient to use but offer the most limited accuracy.
In addition to thermometer type, other factors within the measuring system affect accuracy and can contribute to the accuracy of the measurement. Here are 10 tips that will allow a user to get the most out of their platinum resistance thermometer.
1. Think Lifecycle Cost
An indicator of the quality of a platinum resistance thermometer is its long-term stability performance. To achieve tight stability performance requires the use of high purity materials and sound manufacturing processes. PRTs that are stable can be used for many years of service before they need to be replaced.
Sensors that use low-quality materials or unproven manufacturing methods can become unstable due to contamination of the platinum, or from contaminants that leak into the sensor through the lead wire seal. Low quality sensors may save money initially, but the savings can be quickly lost when the cost of replacement sensors, excess energy usage, loss of product, and product recall are factored in.
2. Verify Sensors Regularly
Verify that the platinum resistance thermometer is operating properly by performing a calibration at 32°F (0°C) and measuring the insulation resistance (resistance between the lead and the sensor sheath) annually, or as required by your quality system. Tracking the performance is critical. If the sensor has shifted at 32°F (0°C) by more than the amount required for your application, or if the insulation resistance has dropped below the requirement for a new platinum resistance thermometer of similar design, the device should be replaced.
3. Check the Depth
Stem conduction is a source of error that has the potential to cause large errors (greater than 50°F [10°C]) even though the sensor is working exactly as designed and performs as expected when tested in a laboratory. Make sure the immersion depth is adequate. When the difference between the temperature of the media being sensed and the surrounding ambient conditions is large, the potential exists for large errors due to heat conducting into or out of the platinum resistance thermometer along its sheath.
Because the error experienced is highly dependent on the thermal conditions of the installation, it is nearly impossible to state an immersion value that will work in all instances. A conservative estimate is that the immersion depth should be 10 times the diameter of the sheath plus the length of the sensing element. This works out to be 3.5" for most 0.25" diameter platinum resistance thermometers. Longer lengths are required when a thermowell is used.
4. Match Component Specs
The overall system error can be reduced by as much as 85 percent by matching the transmitter to the actual resistance vs. temperature characterization of the platinum resistance thermometer. Periodically recalibrating the thermometer and updating the transmitter to match its new characterization can eliminate sensing errors due to drift and restore accuracy.
5. Avoid Sensing Errors
Long lengths of lead wires can introduce errors when two-wire and three-wire connections are used. Two-wire connections add all of the lead wire resistance in series with the PRT element, resulting in large errors. This type of connection only should be used for short lead lengths (less than 6") or when significantly large tolerances are acceptable.
Three-wire connections are much better, but they rely on all three wires having equal resistance. The error will be directly related to the maximum resistance variation between the three leads, so long lengths should be avoided. Lengths longer than 50' of 22 AWG wire have resulted in platinum resistance thermometers that are out of tolerance. Additionally, long lengths of leads can pick up electromagnetic and radio frequency interference (EMI/RFI), which can result in measurement errors.
These pitfalls all can be avoided by using a transmitter located in the connection head near the thermometer. This will minimize lead length, and because the 4 to 20 mA signal produced by the transmitter is much larger, it is less susceptible to showing an error from the noise.
6. Remember Less Equals Fast
If a fast response time is desired, select a sensor with minimal mass in the sensing element location. Before a sensor can react to a temperature change, the surrounding material temperature must change first. The time for this surrounding material to change temperature is directly proportional to its mass. For example, a sensor used in a thermowell will have a slower response time compared to the same sensor used in a direct immersion scenario.
7. Think Big Picture
Temperature measurement accuracy goes beyond just the accuracy of the sensor. To improve accuracy, one must remember to examine the whole temperature measurement system and surrounding environment. Contributors to the total measurement error will include components from the transmitter/control system; lead wire connections; readout display; calibration uncertainty of the thermometer and transmitter; and thermal isolation of the PRT from the ambient conditions.
8. Prevent Thermowell Failures
When specifying a thermowell for your application, consider these common causes of thermowell failure:
Pressures greater than the design pressure of the thermowell.
Temperatures exceeding the limit of the thermowell material.
Drag forces on the well from the fluid rushing by the thermowell.
Vibrational effects of the fluid.
The pressure, temperature and drag force effects are well known and easily accounted for with good thermowell design. The vibrational effects are less familiar but pose an equally large threat to the durability of the well. Fluid flow around the well forms a turbulent wake called the Von-Karman Trail. The wake alternates from side to side at a defined frequency, determined by the velocity of the fluid and the diameter of the thermowell tip. If the wake frequency coincides with the natural frequency of the thermowell, the well could vibrate to destruction and break off in the piping. Therefore, it is important to choose a thermowell of sufficient stiffness such that the natural frequency of the well will be greater than the wake frequency. Larger diameters and shorter lengths result in higher natural frequencies of the thermowell.
9. Recognize Errors
Accurate temperature measurements are necessary for product quality and efficient energy use. Recognizing the errors and accounting for them are critical aspects of an accurate measurement system. A list of possible errors can be found in the web-exclusive content for this article (see sidebar link at bottom of page).
10. Respect Hazardous Areas
Some applications require that the sensor assembly has additional shielding from the environment to avoid igniting explosive gases and dust. External to the process, a connection head commonly is used to provide a transition from the RTD leads to the facility wiring and to house a transmitter or digital indicator. This head and the sensor assembly can be rated for use in explosive atmospheres and to keep water and other contaminants away from the sensor.
In conclusion, platinum resistance thermometers are a valuable tool to consider when determining which temperature sensor to use in your process application. The tips above will go a long way in getting the most out of a PRT.