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Basics of Pyrometry, Industry Specificationsand Temperature Uniformity SurveysPyrometryIn its simplest sense, pyrometry is the measurement of temperatures. Practically speaking, inthe business of heat treatment, the term also refers to the equipment, standards, andspecifications that make it possible to measure high temperatures accurately.Take a standard run-of-the-mill thermometer, for example – the kind you might have on yourfront porch. Most people don’t give this device a second thought, because it can’t be adjusted,and it doesn’t need to be very accurate. After all, does it really matter whether it’s 60 or 65 outside?But sometimes, measurement accuracy is more important – for example, in a meatthermometer. You rely on that device to ensure that your food is cooked enough to kill offharmful bacteria. If your thermometer is inaccurate, you or your family could become very ill.In heat treatment, the need for accuracy even more critical. If the temperature readings foryour furnace are inaccurate, those aircraft parts you’re heat treating could literally put lives atrisk.To make matters even more complicated, you must have blind faith in the instruments thatmeasure such extreme temperatures. If your porch thermometer reads 70 but it’s snowing,you know that something’s wrong. If your meat thermometer reads 165 but the chicken is stillpink, you know it’s not safe to eat. But if your furnace reads 1900 and it’s actually 1875 , howcan you even know that there’s a problem?And even if you do somehow realizethat something’s not right, how doyou determine where the problemis? Measuring temperatures inindustrial applications isn’t assimple as sticking a thermometer ina piece of chicken – rather, it takesan entire system of devices allworking together to give a glimpseof what’s happening inside a furnace(See Fig. 1). Any one of the partscould be to blame for an inaccuratemeasurement.Figure 1 - Typical Measurement and Control SystemPage 1 of 5

AMS 2750To address these complications, industry leaders have developed a number of systems andguidelines that help lead to consistency and quality. One of the most important such programsis the National Aerospace and Defense Contractors Accreditation Program (NADCAP). NADCAPcertification is essential to any business that wants to do heat treating work for the aerospaceindustry.To ensure consistency of temperature measurement in heat treating processes, NADCAP relieson a document known as AMS 2750 (AMS Aerospace Materials Specification). It’s important tonote that although NADCAP and AMS are both based in the aerospace sector, the certificationand specification apply to the overall heat treatment industry and are not limited to aerospacealone. AMS 2570 covers all aspects of pyrometry in heat treatment, including: Controllers (calibrations, specifications and readability requirements)Thermocouples (calibrations, usage, types)Recording instruments (calibrations, accuracy)Calibration requirements for thermocouples and equipmentAccuracy requirements and tolerances for acceptanceCalibration proceduresTemperature survey proceduresFrequency of activitiesTemperature Uniformity SurveysOne of the key components to AMS 2750 is the Temperature Uniformity Survey (TUS). A TUSverifies the classification of your furnace and its qualified working zone, and this in turndetermines your required ongoing testing schedule in order to maintain conformity with AMS2750.Furnace ClassificationFurnace classification is a keypiece of information that theTUS will use to determinerequired frequency of testingactivities. Furnace class isdetermined by theFigure 2 - Furnace Classificationstemperature uniformityrange within the qualifiedworking zone – put simply, in the area of furnace that you will be using, how close are youstaying to your desired temperature? The temperature uniformity range is described as aplus/minus degrees value, as shown in Figure 2. So, for example, if a furnace is meant to run at1900 F, and it ranges between 1887 and 1913 , it would qualify as Class 3. A higher-ratedPage 2 of 5

furnace classification means that the furnace is able to stay closer to its target temperaturewithout variation.Instrumentation TypeIn addition to classification, AMS 2750 also uses Instrumentation Type to determine a furnace’smandatory testing schedule. Instrumentation type can be look at as the number of sensorsused for control/recording in a furnace. All of these sensors, controls, and recording systemshave strict accuracy requirements and calibration intervals. The five instrumentation types (AE) are defined clearly within the specification (see figure 3 for a sample).Figure 3 - Instrumentation Type DescriptionsTesting IntervalsPut together, furnace classification and instrument type clearly determine the testing schedulenecessary for your furnace in order to maintain certification. So, for example (See Fig. 4) aClass 2 Furnace with Instrumentation Type B is required to undergo a TUS on a monthly basis,whereas a Class 5 Furnace with Instrumentation Type B only requires a quarterly TUS.Figure 4 - TUS Interval ChartPage 3 of 5

However, AMS 2750 rewards TUS success. If a furnace completes a designated number of TUSssuccessfully, the interval between testing can be increased. In our above example, if the Class 2Furnace with Instrumentation Type B were to undergo four consecutive successful TUSs, itsrequired testing would stretch from monthly to quarterly. Having to perform fewer TUSs is ahuge benefit, leading to: Lower cost of labor/materials,Increased production/reduced down time, andDecreased time spent on documentation of the process.It is also important to note that AMS 2570E provides different timelines based on what is beingtreated in the furnace. Figure 5 deals specifically with parts, while a furnace treating rawmaterials would be subject to another chart, and would have a less rigorous TUS interval tomaintain.TUS LogisticsA TUS requires time and organization since there are many pre-requisites prior to performingthe TUS. Even though a survey may only take two hours to run, it takes planning andcoordination to get all the pieces in place. All the while, management and production areprobably waiting for that valuable furnace time. To minimize this downtime, work to streamlinethe activities that surround the TUS in order to create an efficient process: Organize paperwork.Ensure quick and easy access to maintenance logs.Have a prefabricated TUS rack wired in advance.Communicate dates for calibration of equipment.Develop a quick reporting technique for easy review and signoff.Ensure that the survey device is calibrated and certified (3 months per AMS 2750E).All these preparations put together can shave off hours off the process. When multiplied by anumber of furnaces, the financial benefits are obvious.When the time comes to actually perform the TUS, there is a great deal to consider. Items suchas thermocouple wire requirements, instrument calibration and more are covered in greatdetail in AMS 2750E, but it is critical to know and understand your equipment (e.g.,thermocouple type, noble or base metal, primary or secondary use), so that you can make useof the guidelines included in the specification (see Fig. 5, next page).Page 4 of 5

Figure 5 - Sensor Calibration ChartThermocouplesThermocouples in particular can present a challenge. By their very nature, they haveassociated calibration errors that must be corrected to determine a true temperature. Manyareas in AMS2750 cover these correction factors, which are used to adjust the readings ofthermocouples. It is absolutely mandatory to use these corrections when performing TUSs.While they can be entered after the survey, it strongly recommended to capture this informationbeforehand. This will enable the technician to immediately identify any potential issues withthe survey, allowing actions to be taken immediately. It is important that the individuals thatare performing activities understand how to use these correction factors and how to documentthe results for future reference, such as would be required during a NADCAP audit.ConclusionUnderstanding the TUS process as it applies to AMS 2750E will allow you to be proactiveregarding your requirements and responsibilities as a reliable heat treater. If you are familiarwith your equipment and the specification, you can prepare for efficient transitions, minimizedowntime, and streamline the process of TUS testing. And if you take the time to document theprocess for future surveys, you can save time and money in the long run by enabling yourself toidentify problems before they are too late to prevent.Page 5 of 5