Furnaces, Kilns, Ovens, Lehrs
and Thermal Oxidizers
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AMS 2750E was replaced in 2020 by AMS 2750F (aerospace) and CQI-9 (automotive) - These specifications cover pyrometric requirements for thermal processing equipment used for metal heat treatment. It covers temperature sensors, instrumentation, thermal processing equipment, system accuracy tests (SAT), and temperature uniformity surveys (TUS). These are necessary to ensure that ferrous and non-ferrous materials like tool steels, Titanium and aluminum alloys are heat treated in accordance with the applicable process specification(s) and standards like AMS 2770, AMS 2771, AMS 2772, MIL-H-81200.

NADCAP AS7102 (Requirement for Heat Treating) auditors certify that heat treaters are following all requirements including pyromotry and process specifications. Common furnaces and ovens used in these industries:

For the heat treater, the most important information is found in the heat treating specifications (examples: AMS 2759E, MS 2759/11, AMS 2770J, AMS 2771E, AMS 2772F) of the materials. This dictates what furnace / oven temperature uniformity (furnace class) and instrumentation type is required.

Sensor(s) Required by Instrumentation Type Instrumentation Type (1)
A B C D+ D E
One control sensor per zone that controls and displays temperature X X X X X X
The temperature indicated by the control sensor in each control zone shall be recorded by a recording instrument. Alternatively, the recording instrument may be connected to a second sensor contained in the same sheath or holder as the control sensor and separated from the control sensor by no more than 0.38 inch or 10mm. X X X X X  
At least two additional recording sensors in each control zone shall be located to best represent the actual coldest and hottest temperatures in each control zone at any temperature of use based on the results from the most recent temperature uniformity survey. It is recognized that certain furnace designs/load configurations can prevent the location of these sensors in the precise coldest and hottest locations, but these sensors shall be located as close as practical (see 3.3.5 and 3.3.6). These recording locations may change over time. See 3.5.16 for relocation requirements. X   X      
At least one recording load sensor in each control zone. During production in multi-zone furnaces, empty zones (i.e., no material is placed in or intrudes into the zone) do not require a load sensor. However, a notation must be made to the furnace load record that the zone was entirely empty. X X        
At least one additional recording sensor in each control zone located at least 3 inches or 76 mm from the control sensor position and of a different sensor type.       X    
Each control zone shall have over-temperature protection (see 2.2.40). X X X X X  
Furnace Class
Temperature Uniformity
Tolerance, °F
Temperature Uniformity
Tolerance, °C
1                            ± 5.0                              ± 3.0
2                          ± 10.0                              ± 6.0
3                          ± 15.0                              ± 8.0
4                          ± 20.0                            ± 10.0
5                          ± 25.0                            ± 14.0
6                          ± 50.0                            ± 28.0

Many Industries have developed their own pyrometry specifications for furnaces and ovens; but most of them refer to AMS 2750E.

Codes, process and pyrometric heat treat equipment specifications:

AMS 2750 E/F Covers pyrometric requirements for thermal processing equipment used for heat treatment. It covers temperature sensors, instrumentation, thermal processing equipment, system accuracy tests and temperature uniformity surveys.
BAC 5621 Establishes equipment classifications and instrumentation types for processes requiring controlled temperatures. (similar to AMS 2750E) Boeing process specification title: “temperature control for processing materials”
RPS953 Rolls Royce specifications for “laboratory control procedure for heating furnaces and Associated equipment and the thermal processing of materials”. Equipment shall meet AMS 2750E Class 2 for temperatures <1380degF
NADCAP AS7102 This aerospace standard (AS) establishes the requirements for heat treating accreditation by the National Aerospace and Defense Contractors Accreditation Program (NADCAP)

A United States defense standard, often called a military standard, "MIL-STD", "MIL-SPEC", or (informally) "MilSpecs", is used to help achieve standardization objectives by the U.S. Department of Defense.