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Definitions & Terms



The maximum deviation of the instrumentation being tested from the corrected readings of a traceable standard.


Over-Temperature Instrumentation: 

A sensor/instrument combination installed in the furnace, which is used to monitor any over-temperature occurrence and generate an alarm and/or cut back or shut down heat input. The purpose for this control is to protect material and/or the furnace from overheating.


Base Metal:

Thermocouple whose thermo-elements are composed primarily of base metals and their alloys. Examples of base metal thermocouples include E, J, K, N, and T.


Preventive Maintenance Program or PM Program: 

A program for evaluating, taking corrective action as required, and documenting the condition of items that have potential to adversely affect furnace temperature uniformity. Frequency of PM checks is established based on experience to ensure that no major problems occur between PM’s.



The process of adjusting an instrument or compiling a deviation chart so its reading may be correlated to the actual value being measured.

Catastrophic Failure of Survey Thermocouple: 

The obvious failure of a survey thermocouple indicated by extreme high readings, extreme low readings, and/or erratic changes in readings.

Continuous Furnace: 

A furnace where product is conveyed continuously from the charge area to the discharge area.

Control Instrument: 

An instrument connected to a sensor and used to control the temperature of thermal processing equipment.

Control Sensor: 

A sensor connected to the furnace temperature controller, which may or may not be recording.

Control Zone or Furnace Control Zone: 

A portion of the working zone in thermal processing equipment having a separate sensor/instrument/heat input or output mechanism to control its temperature. This portion of a furnace is independently controlled.

Correction Factor: 

The number of degrees, determined from the most recent calibration that must be added to, or subtracted from, the temperature reading of a sensor, or an instrument, or a combination thereof (system) to obtain true temperature. The correction factors of sensors and instruments are usually kept separately and added together algebraically when a combination is used.


Data Acquisition System: 

An instrument system used to automatically collect and store process data as an electronic record.

Dead Band: 

The range within which the temperature input can be altered upscale and downscale without registering a change on the instrumentation.


In the context of AMS 2750, the difference between the uncorrected indicated temperature and the true temperature.


Elapsed Usage Time: 

Number of calendar days from the first use of a sensor, (regardless of the number of cycles of use).

Electronic Record: 

Any combination of test, graphics, data, audio, pictorial, or other information representation in digital form that is created, modified, maintained, archived, retrieved, or distributed by a computer system.

Expendable Thermocouples: 

Thermocouples made of fabric or plastic covered wire. The wire is provided in coils or on spools. Insulation usually consists of glass braid or ceramic fiber cloth on each conductor plus glass braid overall .


Failure of Survey Thermocouple: Obviously incorrect or erratic activity of a survey thermocouple indicated by extreme high readings, extreme low readings, and/or erratic changes in readings not reflected by other sensors.

Field Test Instrument: 

An instrument that is portable, that meets the requirements of Table 7 of AMS 2750, has calibration traceable to secondary equipment or better and is used to conduct on-site tests of thermal processing equipment.

For loaded furnaces:

If a survey load is used during the TUS, some survey thermocouples may continue to rise in temperature and slowly approach the set temperature. This rise in temperature of survey thermocouples towards the set temperature meets the requirement for stabilization.


Equipment used for the thermal processing of materials and parts. The term furnace includes ovens.


Qualified Operating Temperature Range: 

The temperature range of thermal processing equipment where temperature uniformity has been tested and found to be within required tolerances as specified in 3.3 of AMS 2750.

Qualified Work Zone: 

The defined portion of a furnace volume where temperature variation conforms to the required uniformity tolerance.

Quality Organization Approval: 

Documented process for review and acceptance/rejection of a report or test which also defines any delegation of this approval.

Quench System: 

A system that facilitates rapid cooling, usually accomplished using oil, water, water/polymer mixtures, or gaseous mediums. Quenchants are usually delivered by immersion, spray, or fog.


Recording Instrument: 

An instrument connected to a controlling, monitoring, load or recording sensor that indicates process equipment temperature data and generates a permanent process record. Examples are a chart recorder, electronic data recorder or a data acquisition system.

Recording Sensor: 

A sensor that is connected to a recording instrument.

Recurrent Temperature Pattern: Cycling of furnace temperature due to temperature control.

Resident SAT Sensor:

 A test sensor that remains resident in the test location between system accuracy tests.

Retort Furnace: 

A furnace that contains a retort or muffle which normally contains a protective atmosphere and the product being heat treated. The furnace normally surrounds the retort.


Semi-Continuous Furnace: 

A furnace where product is conveyed at regular time intervals from the charge area to the discharge area.


The temperature change in the input that is required to activate a change, either upscale or downscale on the instrumentation. It is normally one-half the numerical value of the dead band, such as: a sensitivity of 1 °F (0.6 °C) is equivalent to a dead band of 2 °F (1.1 °C).


In the context of this document, a device designed to detect or measure temperature (e.g., thermocouple, RTD, etc.). As used in this specification, this term is identical with “Temperature Sensor”.

Sheathed Thermocouple Assembly (MIMS): The cut-to-length finished assembly consisting of thermoelements having one end joined in a measuring junction, contained within and electrically isolated from, a protective sheath closed at the measuring end. The protective sheath has a moisture seal at the reference junction end of the sheath. The assembly may include a thermocouple connector but does not include a reference junction or extension leads mechanically joined to the thermoelements.

Stabilization (also referred to as equalization, equilibrium, steady state or soaked condition): 

Furnace stabilization occurs when all control thermocouples are within the allowable TUS tolerance span and controllers are cycling and/or maintaining the desired temperature in each zone. All survey thermocouples must be within the desired temperature range and must not be changing such that they may drift above or below the maximum or minimum temperature.

For unloaded furnaces: if the temperature readings of any TUS sensor exhibit an upward or downward trend that is not converging toward the set point the test period shall be extended as necessary until the trend is no longer evident.

System Accuracy Test (SAT) or Probe Check: 

An on-site comparison of the instrument/leadwire/sensor readings or values, with the readings or values of a calibrated test instrument/leadwire/sensor to determine if the measured temperature deviations are within applicable requirements. Performed to assure the accuracy of the furnace control and recorder system in each control zone.


Grounded Junction: 

A sheathed thermocouple having the hot junction (measurement junction) end fused or welded to the end closure of the protective sheath.


Heat Sink: 

A mass of material equivalent to the heat transfer characteristics of the thinnest section of the part being heat-treated. Heat sinks may be used during TUS and during production.



The calendar time between 2 tests or calibrations. Examples include: week or weekly requires the due date is the same day in the following week – a span of 7 calendar days; month or monthly to indicate the next test occurs on the same numerical day in the following month. If the monthly test is performed on the 31st day of a month followed by a month with 30 days, the due date is the last day of the following month (similar for tests on January 29-31 – the due date is the last day of the following month. Same definitions apply to the bi-weekly, quarterly, semiannual and annual intervals.


Laboratory Furnace: 

A furnace used only for determining response to heat treatment as required by material specifications.

Load Sensors: 

Sensors that are attached to the production material or a representation of production material, that supply temperature data of the production material to process instrumentation.


Maximum Permitted Error: 

A tolerance band for the thermal electric response expressed in degrees or percentages. Maximum permitted error provides a tolerance within which various types of thermocouples must conform to standard thermocouple reference tables.

Measuring Junction: 

That area of a thermocouple joined together to complete ameasurement circuit, which is used to measure an unknown temperature. Also called the hot junction.


Noble Metal Thermocouple: 

Thermocouple whose thermo-elements are composed primarily of noble metals (e.g., platinum/platinum-rhodium) and their alloys. Examples of noble metal thermocouples include types R, S and B.

Nonexpendable Thermocouples: 

Thermocouples that are not covered with fabric or plastic insulations. One type consists of ceramic insulators over bare thermocouple wire, sometimes inserted in a tube for stability and protection. A second type consists of a combination of thermocouple wires, mineral insulation, and a protecting metal sheath compacted into a small diameter. The thermocouple thus constructed is protected, flexible and, within the temperature limits of the sheath material, may be used many times without insulation breakdown.This type of thermocouple, conforming to ASTM E 608, is available under many trade names.


Temperature Overshoot: 

The amount of temperature rise in the furnace work zone above the applicable positive TUS tolerance that occurs upon first reaching furnace set point temperature.

Temperature Uniformity: 

The temperature variation (usually expressed as ? degrees) within the qualified furnace work zone with respect to set point temperature. For retort furnaces where a sensor in the retort is used to control temperature, the temperature variation is with respect to the sensor in the retort and not to the furnace set temperature.

Temperature Uniformity Sensor: A calibrated and traceable sensor with known deviations if any, that are used for conducting Temperature Uniformity Surveys.

Temperature Uniformity Survey (TUS): 

A test or series of tests where calibrated field test instrumentation and sensors are used to measure temperature variation within the qualified furnace work zone prior to and after thermal stabilization.

Test Instrument: 

Instrument used to perform system accuracy tests, temperature uniformity surveys or calibrations of controllers, recorders, data acquisition instruments, or monitoring instruments.

Test Sensor: Sensor used in conjunction with a test instrument to perform a system accuracy test or temperature uniformity survey.

Thermal Processing: 

Any process in which materials are exposed to controlled heating, soaking, or cooling to change the structural nature, properties or condition of the material or part. The term excludes pre-heating and heating for forging, drying, and hot forming.

Thermal Processing Equipment: A term used to refer to any vessel (such as furnace, oven, freezer, liquid bath, etc.) used to process materials at controlled temperatures.


A type of sensor consisting of two electrically conducting circuit elements of different thermoelectric characteristics joined at a junction.

Thermocouple Calibration: 

The process of determining the emf developed by a thermocouple with respect to temperature established by a national standards organization.

Thermocouple Use: 

One cycle of heating and cooling a thermocouple.

Traceable (or Traceability): 

The ability to relate measurement results through an unbroken chain of calibration to NIST or equivalent agencies in countries outside the United States.

Thermocouple Use: (of a thermocouple / see AMS 2750

One cycle of heating and cooling a thermocouple.


Vacuum Furnace: 

A furnace capable of operation at any pressures lower than atmospheric pressure (760 mm Hg).

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