AMS 2750 Compliance Checklist: Complete Guide for Furnace Pyrometry

Understanding AMS 2750: The Pyrometry Standard for Heat Treatment

AMS 2750 (Pyrometry) is the governing specification for thermocouple calibration, instrumentation, temperature uniformity surveys (TUS), and system accuracy tests (SAT) in heat treatment furnaces. Compliance is mandatory for aerospace Nadcap-accredited facilities and is widely adopted across automotive, defence, and general engineering sectors.

This checklist covers AMS 2750 Revision G requirements. Whether you are preparing for a Nadcap audit, setting up a new furnace, or reviewing your existing pyrometry programme, use this guide to ensure every element is addressed.

1. Instrumentation Classification

AMS 2750 defines five instrumentation types based on the number of sensors and recording requirements. Your furnace classification determines minimum survey frequencies and sensor counts.

Checklist item: Confirm your instrumentation type is documented and matches the physical setup. Verify that all control, recording, and test instruments are identified by serial number.

2. Thermocouple Requirements

Thermocouple Types and Tolerances

AMS 2750 references ASTM E230 for thermocouple tolerances. The most commonly used types in heat treatment are:

• Type K (Chromel-Alumel): Up to 1260°C, standard tolerance ±2.2°C or ±0.75%
• Type N (Nicrosil-Nisil): Up to 1260°C, improved stability over Type K at high temperatures
• Type S/R (Platinum-Rhodium): Up to 1480°C, required for many aerospace applications
• Type B (Pt-30%Rh / Pt-6%Rh): Up to 1700°C, used in vacuum and sintering furnaces

Checklist items:

• All thermocouples are traceable to a national standard (UKAS, NIST, or equivalent)
• Expendable thermocouples are single-use only (no re-insertion after removal)
• Non-expendable thermocouples are replaced at intervals not exceeding the supplier's rated life or when drift exceeds tolerance
• Thermocouple wire lot numbers and certificates of conformance are on file

For detailed guidance on selecting the right thermocouple for your furnace, see our Instrumentation Reference tool.

3. System Accuracy Tests (SAT)

A System Accuracy Test verifies that the entire measurement chain — from thermocouple tip through extension wire, connectors, and instrument — reads within the allowable tolerance.

SAT Frequency

SAT Procedure

1. Place a calibrated reference sensor adjacent to the furnace sensor (within 76 mm / 3 inches)
2. Allow the furnace to stabilise at a normal operating temperature
3. Record the difference between the reference instrument and the furnace instrument
4. The combined error of the reference sensor, the furnace sensor, and the instruments must not exceed the SAT tolerance for your furnace class

SAT tolerances by furnace class:

Checklist items:

• SAT performed at the required frequency with results documented
• Reference sensor has a current calibration certificate (traceable to national standard)
• Offset corrections, if applied, are documented and within the maximum permissible correction
• Out-of-tolerance SAT results trigger corrective action before the furnace is used for production

4. Temperature Uniformity Surveys (TUS)

The TUS is the definitive test proving that your furnace maintains the required temperature uniformity across the qualified work zone.

TUS Sensor Placement

Minimum sensor locations depend on the work zone volume:

Sensors must be placed at the extremes of the qualified work zone (corners and centre). For continuous furnaces, survey locations must include the entry, centre, and exit of each zone.

TUS Frequency

Initial surveys are required before the furnace enters production. Periodic surveys are typically:

• Monthly for the first three consecutive successful surveys (initial qualification)
• Quarterly after three consecutive successful monthlies
• Semi-annually after four consecutive successful quarterlies (maximum extension)

Checklist items:

• TUS performed at each qualified temperature (or the extremes of the qualified range)
• All survey sensors calibrated and traceable
• TUS report includes furnace ID, work zone dimensions, sensor locations, survey data, pass/fail result, and surveyor signature
• Failed TUS triggers furnace lockout until root cause is resolved and a passing re-survey is completed

5. Instrument Calibration

All instruments in the measurement chain require periodic calibration:

• Controllers/recorders: Calibrated at least annually (or per manufacturer recommendation)
• Test instruments: Calibrated at least annually
• Reference standards: Calibrated at intervals not exceeding 12 months, traceable to a national metrology institute

Checklist items:

• Calibration certificates on file for every instrument in the pyrometry system
• Calibration due dates tracked with a reminder system (no overdue instruments)
• Calibration performed by an accredited laboratory or using standards traceable to UKAS/NIST

6. Documentation and Record Retention

AMS 2750 mandates that all pyrometry records be retained for a minimum period. Nadcap typically requires records to be available for the audit period plus one cycle.

Records to maintain:

• TUS reports (all surveys, including failures)
• SAT results and correction offsets
• Instrument calibration certificates
• Thermocouple purchase records and certificates of conformance
• Furnace classification documentation
• Corrective action records for any non-conformance

7. Common Non-Conformances at Nadcap Audit

Based on industry experience, the most frequently raised findings include:

• Thermocouple wire used beyond its rated life or re-used after removal
• SAT performed with an overdue or out-of-tolerance reference sensor
• TUS sensor placement that does not cover the corners of the work zone
• Missing or incomplete correction offset documentation
• Furnace used for production after a failed TUS without a passing re-survey