Shedding Light on Infrared Pyrometers (Part II)
The most important step in ensuring accurate and repeatable temperature control with an infrared pyrometer is selecting the appropriate unit for a given application. There are three distinct types of infrared pyrometers currently available: handheld, single-color and two-color pyrometers. Each style varies in operation, cost and temperature-measurement capabilities. This article will detail the differences between the types of infrared pyrometers as well as the applications and proper use of each.
Handheld pyrometers are the most common and economical option when it comes to infrared temperature measurement. Generic models can be purchased for as low as $25, depending on the features of the unit. These basic units generally measure infrared energy at 8-14 micrometer wavelengths, which, in turn, gives them a temperature-measurement range of approximately 0-1600?F (-18 to 871?C). Some handheld pyrometers offer adjustable emissivity capabilities allowing the user to set the device to an emissivity setting between 0 and 1 in increments of 0.01. Other handheld pyrometers, however, do not offer adjustable emissivity settings. These units are permanently set to measure all objects at a set emissivity, usually between 0.8 and 1.0.
Overall, the biggest advantage of handheld pyrometers is their low cost and portability. There is a trade-off to using a more portable and less-expensive device, however. Most handheld pyrometers are incapable of measuring hot forging temperatures. Also, because of the wavelength at which handheld pyrometers operate, they are inherently more susceptible to measurement error than single- or two-color pyrometers. Handheld pyrometers are best suited for periodic temperature measurements as part of maintenance or quality control. They should not be used as part of a manufacturing process to continuously monitor or control temperatures.
Single-color pyrometers are a more precise version of infrared pyrometers. The term “single-color” implies that the infrared pyrometer measures the thermal energy emitted from a workpiece at one particular wavelength. The majority of single-color pyrometers operate at wavelengths between 1 and 2 micrometers, which is much shorter than common handheld pyrometers. Consequently, the measurement range of most single-color pyrometers is between 500-3000?F (260-1650?C).
Single-color pyrometers are also more precise and allow the user to adjust the device’s emissivity between a value of 0 and 1 in increments of 0.01. Generally, single-color pyrometers are the instrument of choice in high-volume manufacturing processes in which temperature needs to be continuously monitored. It is important to recall that a single-color pyrometer should be selected based on the lowest minimum temperature that will need to be measured. This will reduce the possible measurement error if an incorrect emissivity value is used.
Two-color pyrometers, also known as ratio pyrometers, are two pyrometers built into a single device. They operate by measuring the thermal energy emitted at two separate wavelengths, generally around 1 and 2 micrometers. The electronics of the unit then combine both measurements in the form of a ratio that is calibrated to equal a particular measurement temperature. Two-color pyrometers should always be calibrated at or near the most commonly measured range of temperatures.
When measuring temperature with a two-color pyrometer, the emissivity of the measured object is not nearly as important as when measuring with a single-color or handheld device. However, it is vital that the emissivity of the measured object be relatively constant at both measurement wavelengths. Therefore, two-color pyrometers are best suited for the measurement of gray-body materials, which, in the context of the forging industry, is limited to oxidized steel.
In some cases, a known variation of emissivity between the measurement wavelengths can be accounted for through the adjustable setting on a two-color pyrometer known as e-slope. One significant advantage offered by two-color pyrometers is the ability to measure objects that are partially obstructed from view. Two-color pyrometers can generally output an accurate temperature measurement of a workpiece in conditions with up to 90% signal loss. Signal loss includes anything from solid objects (such as induction heating coils) to airborne particulates (such as smoke or dust). It is also important to understand that losses due to an emissivity value less than 1 are also included in the total quantity of signal loss.
When measuring a target with a two-color pyrometer in known signal-loss conditions, it is again vital to ensure that the signal at both measurement wavelengths is affected equally, thus maintaining the gray-body nature of the measurement target. The ratio between the thermal energy emitted at each measurement wavelength, as well as the temperature measurement, will remain unaffected even though the individual measurements were reduced due to the obstruction. Most two-color pyrometers will output an error message if the combined signal loss of the measurement target is greater than 90%, thus preventing incorrect temperature measurement. Overall, two-color pyrometers are best suited for the measurement of high-temperature-oxidized steel workpieces that behave as gray-body materials in manufacturing and metal-forming operations.
General Use in Processing Ferrous and Nonferrous Metals
We conclude with some information regarding the general use of infrared pyrometers in measuring ferrous and nonferrous metals, as well as some final guidelines regarding the proper use and selection of infrared pyrometers.
Ferrous steels, commonly used in numerous manufacturing and metal-forming processes, naturally develop an oxidized surface when heated to hot forging temperatures near 2000?F (1093?C). The oxidized surfaces of these materials make them behave as a gray-body material and maintain a constant emissivity value equal to approximately 0.85 when measured by a pyrometer at any wavelength. As a result of this characteristic, infrared temperature measurements are most appropriately conducted with the use of a two-color pyrometer.
However, commonly forged nonferrous materials such as aluminum and titanium do not display the emissivity properties of a gray-body material. Therefore, the emissivity value of nonferrous materials changes with respect to the wavelength at which it is measured. Because of this characteristic, nonferrous forging materials should be measured with a single-color pyrometer with an emissivity setting equal to the emissivity of the workpiece material at the wavelength at which the pyrometer measures. Again, an effort should be made to select the pyrometer with the shortest possible measurement wavelength capable of measuring the desired necessary minimum temperature.
Lastly, the use of any infrared pyrometer involves general guidelines to which an operator should always adhere. First, infrared pyrometers should be verified and calibrated on a regular basis as part of routine maintenance and quality-control programs. If an infrared pyrometer is found to have lost calibration over time, most infrared-pyrometer manufacturers offer calibration and repair services.
Second, an effort should always be made to determine the correct emissivity value of the target workpiece at the measurement wavelength of the pyrometer. Most new pyrometers are accompanied with a datasheet from the manufacturer with suggested emissivity settings for various forging materials at the wavelength measured by the pyrometer. If the measured material cannot be found in the product literature, a general search online should yield an appropriate emissivity setting.
Third, infrared pyrometers should never be used to measure a workpiece that is within a standard gas-fired or electric furnace because the high-temperature walls of the enclosure will actually reflect thermal energy off of the measurement target and artificially increase the thermal energy measured with the infrared pyrometer. When attempting to determine the temperature a workpiece reaches within a furnace, the measurement should be made directly upon exit from the unit.
I would like to add a note of thanks to the Forging Industry Educational and Research Foundation (FIERF), particularly Carola Sekreter, for the funding of the project at Marquette University that resulted in much of the information shared within this article. In addition, I would like to thank my academic advisor, Dr. Joseph Domblesky, as well as Ron Finstad and Ircon Corp. for their continued support and resources contributing to my work.