Getting a Handle on Temperature

by Pete Leal

Temperature is an important variable in manufacturing, quality control, and maintenance processes. Monitoring temperature insures that processes are operating consistently under optimum conditions which results in improved product quality, increased productivity, and reduced downtime.

Temperature can be measured using a wide variety of sensor types. Some of the more common types found in manufacturing plants are thermocouples and RTDs (resistive temperature devices) contact and infrared non-contact measurement.


Contact Measurement

Thermocouples
Thermocouple sensors consist of two dissimilar, joined to produce a specific voltage at a given temperature. This voltage is then measured and interpreted by a thermocouple thermometer, and then displayed for a user to view. There are many types of thermocouples, which are made of different types of materials. Some of the more common types are J, K and T. Choosing the correct thermocouple type depends on the applications’ required temperature range and accuracy.

Resistive temperature device (RTD)
RTD sensors measure temperature by measuring the change in electrical resistance across the metal wire. The resistance value is then measured and interpreted by a RTD thermometer, and displayed for a user to view. While RTD wire can be made of any metal, platinum is the metal of choice due to its excellent repeatability, stability and resistance to corrosion and chemicals.


Non-Contact Measurement

Infrared
All objects emit infrared energy, and the hotter an object is, the more active its molecules are, and the more infrared energy it emits. Infrared thermometers sense emitted, reflected, and transmitted energy from an object and translate this information into a temperature reading. Energy passes through the infrared thermometer’s optical system and is converted to an electrical signal, which is then converted and displayed as a temperature reading.

Infrared thermometers offer some distinct advantages over contact thermometers, since they can measure temperature without physically touching the object. Infrared temperature measurement is ideal for measuring objects that:

  • Are too far or to difficult to reach
  • Require non-contamination from one reading to the next
  • Are moving, rotating or vibrating
  • Are too time consuming for contact sensors and require quick temperature measurements
  • Have high temperatures up to 3,000 F
  • Can be scratched or damaged by physical contact
  • Have curved, distorted or varying surface conditions
  • Are too thin and soft for contact sensors
  • Are too hot to touch or approach
  • Are electrically active and too dangerous to touch

Temperature Measurement Applications

Measuring and interpreting temperature has many MRO applications. Measurements throughout the plant can be done quickly and easily and without complex analysis. This information can then be stored electronically for later data trend analysis.

An electrical application is plant wiring. Anytime electrical current flows through a cable or connection, electrical resistance generates heat. Increased current flow through a cable generates more heat. If two similar sized cables have different temperatures, then the cable with the higher temperature carries the higher current. Resistance and heat build-up also occurs at loose or defective wiring connections.

Some typical applications for technicians and service personnel are:

  • Watching bearing temperatures to predict failure
  • Detecting motor overload
  • Finding defective transformers
  • Detecting loose and improper wiring connections
  • Checking electrical panels for hot spots
  • Measuring the surface of HVAC ductwork to reveal temperature variations, poor insulation and duct leakage
  • Diagnosing blown or plugged steam traps from a distance
  • Evaluating temperature differences between supply/return registers in a HVAC system
  • Checking for coil sweating on condensate lines in refrigeration systems
  • Checking the temperature of emergency relief valves
  • Diagnosing water leaks by measuring wallboard temperatures. Moist spots on walls will experience water evaporation and therefore will measure at a lower temperature
  • Testing furnace performance by checking the flue and supply grill temperatures