November 2022 Volume 4

EQUIPMENT & TECHNOLOGY

IV. Errors Prevalent in Pyrometer Temperature Measurement and How to AvoidThem In view of the previous discussion, maintaining strict temperature control is fundamental to producing a high-quality forging. When a pyrometer is used for temperature measurement and control, there are several ‘best practices’ which if observed will reduce problems associated with inaccurate temperature control. No. 1: Avoid Inaccuracy Caused by Poor Focus or FouledOptics Cause of inaccuracy: Poor optical focus and lens fouling diminish irradiance on the detector. Best practice: Check the focus, avoid obstructions, clean the lens, or use a two-color pyrometer. Because pyrometers are optical devices, they require certain maintenance and upkeep. Improper focus causes the target (the heated stock) to be smaller than the measurement area. In this condition the temperature measurement is lower than what the target temperature actually is. Similarly, lens fouling from smoke or die lubricant will reduce the irradiance on the detector which also causes temperature measurement error. One distinct advantage of the two-color pyrometer is that the two detectors averaging scheme eliminates the error. There are of course limitations to the extent to which error is eliminated and is detailed in the product literature. No. 2: Avoid High Angles with Surface Cause of inaccuracy: As pyrometer to target angle decreases from 90 degrees, the apparent emissivity will also decrease. Best practice: Locate pyrometer so that the target surface is close to a 90-degree angle. One challenge of locating a pyrometer on the forging line is placement relative to the measurement target. The tendency is to locate it in the most convenient place without much regard for what the pyrometer needs. If pyrometer to target angle approaches 50-degree θ it will alter the apparent emissivity. Ensuring that the angle to target surface is near a 90-degree angle will minimize temperature error.

No. 3: Specify the Correct Emissivity or Signal Ratio Cause of inaccuracy: Emissivity programmed into the pyrometer does not match the target emissivity. Best practice: Measure and compensate for emissivity using the standard test method in ASTME1933-99a. Having the emissivity parameter set too high or too lowwill result in significant measurement errors. The best way to avoid this is to carry out the procedure(s) given in the ASTM publication E1993-99a. Materials such as brass and aluminum have a very low emissivity compared to oxidized steel which has a very high emissivity. Although emissivity tables are available, they cannot account for every situation. The best way is to determine the emissivity in-situ. No. 4: Calibrate the Pyrometer and Controls Annually, As A System, With NIST Traceable Standard Cause of inaccuracy: Pyrometer calibration can drift over time. Control devices in process loop can change temperature readout Best practice: Calibrate the pyrometer and controls annually, or as often as required by applicable quality standards. Regular (annual) calibration is due diligence in maintaining a quality system. Calibration should include any control devices associated with the pyrometer (control loops, logic controllers, temperature displays etc.). A documented history of calibration creates confidence in accurate pyrometer measurements. As a bonus, many quality assurance auditors love the smell of a freshly printed calibration certificate. No. 5: Training for TeamLeaders on Best Practice andMaterial Temperature Controls Cause of inaccuracy: People. Lack of knowledge leads to uniformed decisions, inappropriate corrective actions. Best practice: Provide team leaders with high level training on pyrometer operation and sources of errors. In terms of character and tenacity, the forging industry has some of the best people I know. Unfortunately, I also have seen more eye-pyrometers (human eye, which is) than in any other industry. A lot can be said about the ability of the human eye to read color temperature, but it is no substitute for an accurate pyrometer. A good practice is to train leaders on pyrometer theory and maintenance to avoid defective forgings and other temperature problems. V. Conclusion: Producing quality forgings depend on accurate temperature controls. Pyrometers have several advantages over thermocouples including:

• non-contact • fast readings • measurement at a distance • durability

Figure 7: Low target angles affect temperature measurement

FIA MAGAZINE | NOVEMBER 2022 11