Noncontact Temperature Sensors: What You Need to Know Up FrontApril 1, 2006 By: G. Raymond Peacock, Temperatures.com Inc. Sensors
Give noncontact IR thermometers the special attention they need, and they will give you the accurate temperature measurements you need.
Are you sure a contact-type sensor can't handle your application? If it can, stay away from noncontact devices. They present special challenges. And don't be led in either direction solely by initial purchase and installation costs. You will need to factor in maintenance expenses as well, especially if your sensor is going to be working in a nasty environment, or if downtime would be a disaster.
Noncontact temperature sensors are available in many configurations (Figure 1), each bearing its own impressive spec sheet. These devices are called by several names, but technically they are all thermal infrared radiation thermometers. We will call them IRTs in this article. They can be portable or fixed-mount, high-end or low-end, more expensive or less so. (You might find pages 18 and 19 of your Sensors January issue helpful here, or visit www.sensorsmag.com/articles/0106/17/ for tabular comparisons of the various types.)
Figure 1. Four IR temperature sensors, Marathon MM, Raytek, www.infrared-usa.com. Modline 5, Ircon Inc., www.ircon.com. Cyclops 100, Land Instruments Intl., www.landinstruments.net. M770, Mikron, www.mikroninfrared.com.
Finally, all IRTs operate in accordance with Planck's law of thermal emission of radiation. Most of the radiation they mea-sure lies in the IR portion of the electromagnetic spectrum.
What You'll Need to Do
There are three major challenges you will encounter when you select and use a noncontact temperature sensor:
- 1. Filling the field of view
- 2. Overcoming object transparency problems
- 3. Achieving the correct emissivity adjustment
Field of View. An object's thermal radiation "tells" the temperature of the surface, almost like a radio beacon. The difference is that in IRT measurement, the thermal radiation comes from a spot of known size on the surface. You need to capture all that radiation and not allow any to get blocked along the way.
The spot diameter at a given distance (i.e., the cross-sectional area of the "cone of vision" at a given distance) is defined by the optics of the device. Generally speaking, the farther from the sensor, the larger the spot diameter. The spot size is often expressed as a ratio such as 50:1 or 10:1. That means the minimum target spot diameter is 1 /50 or 1 /10 the distance from the sensor to the object of interest (Figure 2).
Figure 2. The spot diameter at a given distance is determined by the sensor s optics and is defined as a ratio; as a rule, the farther the target is from the sensor, the larger the spot
Most Read Articles