Costly Mistakes to Avoid:
Noncontact Temperature Measurement
March 1, 2009
When choosing a non-contact temperature sensor (be it a thermal imaging camera or spot pyrometer), it is important to understand the details of what this sensor must do. The user often makes mistakes by not fully understanding three critical factors: emissivity, wavelength and the object’s parameters. Making a mistake with any one of these factors can cause the user to purchase the wrong sensor for the application, thereby affecting product quality, increasing rejects and/or wasting time and money.
By understanding these factors, operators can help to ensure a more efficient process with highly accurate and repeatable temperature performance.
of the object refers to how well that object emits the energy (heat) which it
contains. A thermal imaging camera or spot pyrometer is designed to receive
energy at a certain wavelength and provide a temperature based on its
calibration at that wavelength. It is important to note that emissivity can
change as an object heats or cools, making a completely accurate measurement
very difficult. The nature of the product and its characteristics during the
critical measurement phase is crucial to understand. There are many charts and
listings (see chart) that will help guide users to a close estimation of the
object’s emissivity. However, if accuracy is extremely important, it is best to
have the object tested to determine its exact emissivity.
- Understanding the wavelength used by non-contact temperature sensors
(thermal imaging cameras or spot pyrometers) is important as you choose a
device that will give you the most accurate temperature measurement of your
target object based on the object’s parameters. The wavelength of the infrared
device can greatly affect the impact of the object’s emissivity on the
temperature measurement. Shorter, or lower, wavelengths are less affected by
emissivity where longer, or higher, wavelengths, are more affected by low
Temperature measurements by typical “long wavelength” detectors (8 to 14 microns) can be dramatically affected by low emissivity targets. For example, an imager with a short wavelength (0.65 micron) imaging a target that is 3,632°F (2,000°C) can have a 1% emissivity shift that creates a 2° temperature difference. Conversely, in the same scenario, a long-wavelength imager (10 micron) could have as much as a 16° difference. These differences underscore the importance of choosing the correct wavelength for the target.
- One must know the object’s parameters in order to choose the correct temperature measurement device. An object’s parameters include temperature, speed (how fast the process is moving or changing), distance between the measurement device and target, minimum spot size needed and the field of view to be incorporated in the measurement. Targets at ambient conditions require a different type of measurement device than ones used for molten metals or other objects at extremely high temperatures. Speed can vary greatly. For example, a high speed food processing line may run 60 ft/min as opposed to a steel production line moving at 1 ft/min. Many applications require a large distance between the measurement device and the target. The optics of the measurement device become critical in ensuring that you have adequate resolution (minimum spot size) while maintaining a comprehensive view of the target (field of view). Keep in mind that pyrometers are good for a spot temperature (or single point temperatures), whereas imagers can provide a temperature reading of a larger area, or many spots. Will a single-point temperature be good enough, or will you miss something if you are only looking at a single point? For example, if you were looking at a flower, do you want to measure the temperature of the petal or of the entire flower head? Understanding the target itself is very important.
More Missteps to AvoidPyrometers are available as single-color (single wavelength) or dual-color systems (two-color wavelengths). A single-color pyrometer will average the spot size of, for example, a 1" dia. area, while a dual-color unit will measure the hottest spot of that same 1" dia. spot. Single-color pyrometers are wavelength-dependent; therefore, using a longwave pyrometer on a highly reflective surface will not work well.
There is a myth that dual-color technology is appropriate for most any application because it is “somewhat” emissivity dependent. But that is not always the best choice. In most instances, moving toward a single-color lower wavelength detector will provide users with a more accurate and repeatable temperature measurement.
A shortwave thermal imaging camera operating in the near-infrared spectrum allows you to view components such as molten titanium or steel billets, with minimal emissivity effects. Shortwave has limitations however, requiring targets to have a minimum temperature of 1,112°F (600° C) in order to provide accurate temperature measurements.
When imaging inside an oven or power boiler, imagers using wavelengths of 3.9 microns or 850 nanometers can filter out the flames, seeing through them to view the internal walls of the boiler or monitor target temperatures in the oven. If a user requires glass surface measurements, which are highly reflective, using 4.8 to 5.2 micron devices can minimize the surface reflections on objects such as float glass and bottle manufacturing.
The question users need to ask is: “What do I need to see and how well do I want to see it?”
Mikron Infrared Products
when a thermocouple cannot do the job, and the process or application would
benefit from a thermal imaging camera or spot pyrometer.
When Thermocouples Must Step Aside
- Object is moving
- Object is inside a controlled atmosphere
- Avoid contamination of a process
- Object is a viscous or granular surface
- Object is too hot or corrosive for contact methods
- High speed is essential
- Induction or microwave fields are present
- Object is too distant from sensing point
- Multiple temperature measurements are required across the entire field of view