Temperature Sensors: Contact or Noncontact?January 1, 2006 By: G. Raymond Peacock, Temperatures.com Inc. Sensors
First, answer this question: Can you touch the object or fluid of interest or not? Now you can begin looking for the temperature sensor that will serve your application best.
Temperature is the physical property most widely measured by sensing technology. There are many temperature sensors out there, but you don't want to choose a product based only on its measurement range and its price. The results will not be good. So what's your best course?
These examples of noncontact temperature sensors are available from Raytek Corp. (www.raytek.com), left, and FLIR Systems (www.flir.com)
The great dividing line in temperature sensing—and in many other sensing technologies as well—is this: Can you touch the object or the process fluid of interest or not? Now that you've answered that question as it pertains to your own application, you can move on and evaluate the various devices competing for your approval.
When Should You Use Contact Sensors?
- 1. Whenever you can make good thermal contact with the object or fluid
- 2. If the expected temperature is below ~1700°C (3400°F) or above about –40°C (–40°F)
Good thermal contact means the sensor and the object or fluid are at, or very close to, the same temperature. (This discussion pertains both to process fluids and discrete objects. For the sake of convenience, we will use the word object throughout.) This is usually the case when the sensor's dimensions and mass are small compared to what's being measured. You should maintain this physical contact by welding, soldering, clamping, gluing, or using some other reliable method of affixing the sensor to the object.
The contact-type temperature sensors shown here are by RdF Corp (www.rdfcorp.com), left, and Minco, with the sensing elements displayed below and the packagd products above (www.minco.com)
The two temperature limits are somewhat arbitrary. At 1700°C, platinum alloy thermocouples begin to lose calibration rapidly and the wires and common insulating materials may begin to soften. Specialized devices such as Type B platinum and tungsten-rhenium thermocouples can be used at higher temperatures, but only by those who are experts in high-temperature measurements. At the lower limit of –40°C you begin to encounter cryogenics issues. Many contact-type sensors work well below that temperature but, again, such applications are best left to the experts.
The most common contact temperature sensors are liquid-in-glass thermometers, thermocouples, RTDs, and thermistors. They are typically enclosed in a protective metal or ceramic sheath, called a thermowell, so that they can penetrate a process barrier and also be easily pulled out for calibration or repair without exposing the process and/or maintenance personnel to undesirable conditions. When you need to select a contact-type temperature sensor, Figure 1 should help with your initial ranking of the candidate devices.
Figure 1. Contact-Type Temperature Sensors
When Should You Use Noncontact Sensors?
First, bear in mind that even though noncontact temperature sensors are available in a myriad of styles and types, and are known by an assortment of names, they are all radiation thermometers if they operate according to Planck's law of thermal radiation. They are called radiation pyrometers, IR pyrometers, optical pyrometers, IR thermometers, thermal imagers, and so on. They can be battery-powered portables, fixed-mount, or online process-monitoring devices.
You will see that the list of application areas is a bit longer than that for contact temperature sensors because noncontact sensors do not have to be at the same temperature as the object. Choose and use this type when:
- 1. The object is moving
- 2. Contact would damage either the object or the sensor (e.g., extremely hot, corrosive, abrasive, etc.)
- 3. Contact with the object would change its temperature significantly
- 4. A large, observable area measurement is desirable
- 5. The object is too far away or very difficult to access, such as inside a special atmosphere or in outer space, (e.g., stars and galaxies)
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