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heat treated material is not degraded by the process and satisfactory results are obtained. Additionally, independent test instrumentation/temperature measuring devices, also called field test instruments, come in many configurations and accuracy levels. A substantial amount of the difficulties encountered in heating aluminum alloys is due to improper or inadequate temperature control and circulation of the heating medium. When difficulties arise, the function of these systems should be checked prior to performing other system checks. Basic heat treatment equipment requirements are detailed below.

3.13.1 Air Furnaces/Ovens.

NOTE

SAE-AMS-2750, Pyrometry, is the control document for equipment used to heat treat aerospace materials. AMS- 2750 covers temperature sensors, instrumentation, system accuracy tests, and temperature uniformity surveys. For a complete description of pyrometry requirements for heat treating equipment, refer to the latest issue of SAE- AMS-2750. In case of conflict with this manual, the discrepancy will be negotiated with the responsible technical/ engineering activity for resolution and updating.

The term furnace and oven are interchangeable terms when talking about heat treating equipment. Generally, furnaces operate at higher temperatures and ovens operate at lower temperatures. Air furnaces/ovens are ideal for precipitation (aging), thermal treatments and annealing. These furnaces are also very good for solution heat treating. The initial cost of these type furnaces is higher than for the salt bath types, but they are usually safer, cleaner, more flexible, and more economical to operate. If gas or oil fired, the products of combustion must be excluded from the furnace atmosphere to help avoid high temp oxidation and atmosphere contamination. This is not an issue with electric furnaces and ovens, however, if the atmosphere is intentionally altered or inerted, care must be taken when the operator off-gasses the chamber prior to part removal. Air furnaces also come in two forms with regards to air movement inside the heating chamber; still air and recirculating air (convection). The still air type is capable of achieving higher temperatures than the circulating air type, but it also suffers greater temperature uniformity variances. Still air furnaces are generally designed to operate above 1000 °F and are better suited for ferrous alloy heat treating operations. The circulating air furnace is ideal for aluminum heat treat operations as it is generally capable, depending on build quality and process controls, of holding very tight temperature uniformity as a result of the convection process. The primary limitation of this type of furnace is the maximum effective operating temperature of around 1200 °F.

3.13.1.1 Air furnaces/ovens used for heat treatment of aluminum alloys used on aircraft shall be of the convection or recirculating air type. The heated air in this type furnace is recirculated at high velocities to obtain a rapid heating cycle and uniform temperatures. The ideal set up for an aerospace maintenance facility that performs solution and precipitation heat treatment is a dual chamber oven with both chambers capable of independent operation and circulating air, or two separate circulating air ovens. This set up helps reduce overall heat treat time by eliminating the wait time required for equipment to stabilize at different temperatures for various heat treat processes.

3.13.1.2 Ovens used for solution and precipitation heat treatments of aircraft parts shall be a minimum of class 2, capable of maintaining ±10 °F temperature uniformity with Type D controlling, monitoring, and recording instrumentation. These ovens can also be inherently used for annealing and stress relieving. See Table 3-19 and Table 3-22 for other aluminum heat treat process temperature uniformity and instrumentation requirements.

3.13.2 Salt Bath Furnace. The salt bath method has certain advantages over the air furnace. However, the advantages are usually confined to solution heat treatment only. Associated advantages are uniform temperature without excess danger of high temperature oxidation and more efficient heat transfer which reduces the time required to bring the load to temperature.

This method is adaptable for solution heat treating varying part thickness and complex shapes. The above advantages may be completely nullified by the slower quench caused by the necessary arrangement of equipment, molten liquid burn hazards, explosion hazards, increased hazmat footprint, and decomposition of the sodium nitrate which when dissolved in quenching water forms a compound that attacks aluminum alloys.

3.13.2.1 Salt baths must be operated with caution to prevent explosions or spatter as any water on the material being treated is instantly transformed to steam upon immersion in the salt bath.

3.13.2.2 Hollow core castings or wrought machined parts where the salts are likely to be difficult or impossible to remove shall not be heat treated in a salt bath.

3.13.2.3 Nitrate charged salt baths should not be used to heat-treat aluminum alloys types 5056 and 220 due to the fact that the bath compounds will attack the alloy.

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3.13.3 Field Test Instruments. Field test instruments are used in conjunction with thermocouples to measure the operat-ing temperature inside the oven chamber. They are used to monitor load thermocouples (TCs), perform System Accuracy Tests (SAT), and temperature uniformity surveys (TUS). They range from hand held single input units to suitcase style multi-input units. Many modern units come with software to log and record data. Field test instruments used to perform SATs and monitor load TC shall have a minimum of 1 input channel, a calibration accuracy of ±1 °F or ±0.1% of the reading, whichever is greater, and the ability to log/record data. Field test instruments used to perform TUSs shall have a minimum of 9 input channels, a calibration accuracy of ±1 °F or ±0.1% of the reading, whichever is greater, and the ability to log/record data. National Stock Number (NSN) 6625-01-649-1136 is an example of a multi-point field test unit that logs data and can be used for multiple heat treatment process test, measuring, and recording applications such as SATs and TUSs.

3.13.4 Controlling, Monitoring, and Recording Equipment. Instrumentation type for controlling, monitoring, and re-cording equipment is broken down into five categories. Each category is based on how many controlling, monitoring, and recording TCs are operational in an oven chamber. The most common instrumentation type is Type D. Oven controllers, monitoring, and process recording equipment shall be digital and have a calibration accuracy of ±2 °F or 0.2% of the reading, whichever is greater. Field test instruments can temporarily be used as monitoring/recording equipment to monitor and record heat treatment processes when on-board oven equipment is inoperable or non-digital, ie; paper chart recorders.

See Table 3-22 for oven instrumentation types.

3.13.5 TC. A TC is a thermoelectric device used to accurately measure temperatures. TCs consist of a wire with two leads of dissimilar metal. The leads are joined at one end by welding or tightly twisting the ends together. Heating the joint produces an electric current. This current is converted to a temperature reading with a field test instrument or other TC monitoring unit.

3.13.5.1 TCs may be purchased individually or as a spool and locally manufactured. Welded TCs (Figure 3-2) are less prone to problems and should be used when available. Twisted TCs (Figure 3-3) do not form as reliable a junction as a welded TC and their reliability decreases with repeated use. If necessary, a twisted TC may be made by overlapping the bare ends of the two TC wires 1/4 inch and at 90 degrees to each other. Tightly twist the wire four to five times and cut off the excess wire (safety wire pliers will aid in producing a tight twist). The temperature reading will be measured at the twist closest to the field test instrument.

3.13.5.2 The most common type of TC used in SATs, TUSs, and load TC monitoring is Type K with a high temperature insulation, such as Silica or Nextel fiber. Type K TCs are nickel based. The wire color coding conforms to ANSI MC 96.1.

The positive (+) lead is nickel-chromium (Nickel-Chromium) and has yellow insulation. The negative (-) lead is nickel-aluminum (Nickel-Aluminum) and has red insulation. In some cases, TCs are required to be within a specified range of length to ensure their accuracy; consult the temperature monitoring equipment owner’s manual for guidance. If no guidance exists, TCs should be between 10 and 50 feet long. See Table 3-20 for more information about base metal TCs.

Figure 3-2. Welded Thermocouple Wire

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3.13.5.3 Inspection, Prior to Use. Perform the following inspection steps prior to each use:

a. Inspect the junction for breaks or looseness. If the TC junction is questionable, test for continuity with a thermocouple tester or multi-meter to measure the resistance between the two ends on the connector plug or bare wire ends; if the resistance is greater than 1 ohm (Ω), repair or replace as necessary.

b. Inspect insulation for tears or abrasions that expose bare wire. If bare wire is exposed, trim or replace TC as necessary.

c. Ensure TCs are compatible with the units reading them (e.g., a Type-k thermocouple should be attached to a Type-k monitoring unit). The monitor should be as close as practical to the end of the TC. For standard 24-gauge wire, the distance shall not exceed 100 feet. Distances greater than 100 feet can produce incorrect readings.

d. Plug the TCs into the monitor and ensure they read ambient temperature. If your hands are warmer than the ambient temperature, place each TC tip between your fingers and observe the temperature rise. If it does not respond properly, replace the TC.

3.13.5.4 TC usage shall be tracked and documented. A simple method is a tag attached to the TC or set of TCs, near the cold junction, that can be annotated with each use. A “use” is defined as one cycle of heating and cooling.

3.13.5.4.1 Base metal TCs used solely for SAT and TUS below 1200 °F and is not damaged can be reused up to 90 times or 3 years, whichever comes first. TCs used at 1200-1800 °F may be reused if “U” in the following formula does not exceed 30: U = Number of uses below 1200 °F + 2 (number of uses 1200-1800 °F). TC used above 1800 °F shall be limited to a single use.

3.13.5.4.2 Base metal TCs used for load monitoring below 1200 °F may be used for up to 90 calendar days after first use or for no more than 30 uses, whichever comes first. Load TCs used above 1200 °F are limited to one use.

Figure 3-3. Twisted Thermocouple Wire

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3.13.5.5 Thermocouple calibration and accuracy. TCs that are to be used with calibrated indicators do not require calibra-tion, other than manufacture’s calibration, if they bear the designation E, J, K, R, S, or T. These designations show that the thermocouple or wire has been manufactured in accordance with one or more of the following standards for the wire type indicated, ANSI/MC96.1, BS 1843, DIN 43714, JIS C 1610-1981 or NF C 42-323. Thermocouple wires manufactured to these specifications have been certified by the manufacturer and do not require special initial or subsequent calibration.

3.13.5.5.1 TCs must be supplied with the manufacturer’s initial calibration certificate or deviation limits certification. The calibration or deviation limits supplied with each individual TC or TC spool must be entered into to the field test instrument for proper temperature offset and indication each time TCs are changed or replaced.

3.13.5.5.2 Maximum allowable error for TUS and load TCs is ±4 °F. Maximum allowable error for SAT TCs is ±2 °F.

3.14 TEMPERATURE CONTROL AND UNIFORMITY TESTING.

Precise temperature control is essential to produce the exact material properties and temper requirements necessary for modern aviation manufacturing and maintenance. Periodic surveys and tests of the internal chamber temperatures must be conducted, documented and compared to the set point temperatures of the oven/furnace controller to ensure accurate equip-ment operation. The two methods used to ensure accurate operation of heat treating equipment are the SAT and the TUS.

The SAT is a quick and simple user test to ensure the oven temperature remains accurate in between TUSs. The TUS is a more thorough user test to ensure temperature accuracy and uniformity in the entire oven chamber as compared to the oven control sensor and SAT. These two tests work in conjunction with each other as a checks and balance system to ensure accurate and uniform oven operation. If one is accurate and the other is not, that is a sign that your oven needs trouble-shooting or maintenance to correct a deficiency.

3.14.1 System Accuracy Test (SAT). A SAT is performed to assess the accuracy of the heat treat oven’s resident thermocouple and controller. This is done through the use of an independently calibrated field test instrument and thermo-couple. By placing the test thermocouple within 3 inches of the resident thermocouple and taking a reading with the field test instrument, the accuracy of the oven’s controller and resident thermocouple is validated. If the difference between the field test instrument and the oven controller is greater than ±3 °F, then that is an indication of required maintenance or adjustment of the oven, oven controller, and/or resident thermocouple.

3.14.1.1 A SAT shall be performed and documented using a field test instrument that meets the requirements of Paragraph

3.13.3 and a TC that meets the requirements of Paragraph 3.13.5.5.2, on each oven chamber used to perform aluminum heat treating, at the following intervals/situations:

a. Initial. Upon initial oven installation, prior to first operational use.

b. Periodic. SAT frequency is based on frequency of aluminum heat treating operations and oven instrumentation type.

See Table 3-22 for instrumentation types.

(1) Shops that perform daily heat treating operations, utilizing Type D instrumentation, shall perform SATs on a weekly basis, not to exceed 7 calendar days. If utilizing Type B or Type C instrumentation, the SAT interval may be extended to biweekly, not to exceed 14 calendar days.

(2) Shops performing weekly to biweekly heat treating operations, every 5 to 14 calendar days, utilizing Type D instrumentation, shall perform SATs on a biweekly basis, not to exceed 14 calendar days. If utilizing Type B or Type C instrumentation, the SAT interval may be extended to monthly, not to exceed 31 calendar days.

(3) Shops performing biweekly to monthly heat treating operations, every 15 to 30 calendar days, utilizing Type D instrumentation, shall perform SATs on a monthly basis, not to exceed 31 calendar days. Use of Type B or Type C instrumentation is recommended, but the SAT interval will not be extended.

(4) Shops performing heat treating operations less frequent than every 30 calendar days, regardless of instrument type, will perform a SAT prior to use/heat treating operation.

(5) If utilizing Type A instrumentation, refer to AMS2750 for SAT interval.

c. After any maintenance to the oven, ie; replacement of a sensor/TC, heating element, or controlling, monitoring, or recording instrument.

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d. Recalibration of the controlling, monitoring, or recording instrument, or when parameter/rheostat adjustments have been made.

3.14.1.2 SAT Procedure: A successful SAT reading must be within ±3 °F of oven controller set-point.

a. Set oven controller to 300-500 °F and allow to stabilize. If oven has a dedicated SAT port, higher temperatures can be tested.

b. Insert TC into oven with tip (measuring junction) as close to practical to the controlling, monitoring, or recording sensor tip. The tip to tip distance shall not exceed 3 inches.

c. Allow oven to recover, not to exceed 20 minutes.

d. Record temperature reading.

e. Document and file report.

3.14.1.3 SAT Report Requirements: A paper or digital copy of the completed record shall be retained by the facility performing the SAT for a period of 5 years and disposed of in accordance with applicable Records Disposition Schedule guidelines. The report will contain, at a minimum:

1. Identification of oven/sensor (if multi-zone) being tested.

2. Date and time of the test.

3. Identification of the test sensor.

4. Test sensor correction factors.

5. Identification of the test instrument.

6. Identification of test instrument correction factors.

7. Set point of oven during the test.

8. Observed oven controller instrument reading.

9. Observed test instrument reading.

10. Corrected test instrument reading [will be same as (i) if (d) and (f) are programmed into test unit, if not, (j = i + d + f)].

11. Calculated system accuracy difference (k = h - j).

12. Indication of test pass or fail.

13. Identification of technician performing test.

14. Identification of supervisor performing review.

3.14.1.4 If SAT failure occurs, corrective action may include, but is not limited to replacement of the out of tolerance sensor, rheostat adjustment, recalibration of the out of tolerance instrument/controller, or temperature offset. After any corrective actions, a SAT must be performed prior to any production heat treatments in accordance with Paragraph 3.14.1.1.

3.14.1.5 Temperature Offsets. Adjustment/offset of the control instrument to facilitate desired chamber temperatures based on most recent SAT results is permissible. If manual adjustments or offsets are made to the controller, the effects of these adjustments must be tested and confirmed across the entire operating temperature range. Manual offsets shall be documented, repeatable, approved/signed by section supervisor or Network Centric Operations Industry Consortium (NCOIC) and used in

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3.14.2 TUS. A TUS is performed to ensure an oven chamber’s operating temperature is accurate and uniform, within given tolerance standards, throughout the entire volume or qualified work zone in relation to the oven set point. This is done through the use of an independently calibrated field test instrument (multi-point data logger) and multiple TC all simultane-ously measuring temperature from multiple points in the oven chamber over a period of at least 30 minutes.

3.14.2.1 A TUS shall be performed and documented using a field test instrument that meets the requirements of Paragraph

3.13.3 and a TC that meets the requirements of Paragraph 3.13.5.5.2, on each oven chamber used to perform aluminum heat treating, at the following intervals/situations:

a. Upon initial oven installation, prior to first operational use. See Paragraph 3.14.2.2.

b. Periodically. See Paragraph 3.14.2.3.

c. Annually. See Paragraph 3.14.2.4.

d. After any maintenance to the oven, ie; replacement of a sensor/TC, heating element, controlling, monitoring, or recording instrument, airflow pattern/velocity, replacement of refractory material.

e. Recalibration of the controlling, monitoring, or recording instrument, or when parameter/rheostat adjustments have been made.

f. Work zone volume increase, larger than previously qualified area.

g. Work zone location change, outside of previously qualified area.

3.14.2.2 Initial TUS Requirement. Initial survey temperatures shall be the minimum and maximum temperatures of the qualified operating temperature range. Additional temperatures shall be added as required to ensure that no two adjacent survey temperatures are greater than 600 °F apart. For example, if an oven operating range is 200-1200 °F, the oven must be surveyed at 200 °F, 1200 °F, and at a temperature of 600-800 °F to meet the 600 °F maximum adjacent survey temperature requirement. Temperatures of 200 °F, 700 °F, and 1200 °F would meet this requirement.

3.14.2.3 Periodic TUS Requirement. For single operating ranges greater than 600 °F, TUS temperatures shall be se-lected so that one temperature is within 300 °F of the minimum and another temperature is within 300 °F of the maximum qualified operating range and there is no more than 600 °F in between. For example, if an oven operating range is 200-1200 °F, then the oven must be surveyed at 200-500 °F and again at 900-1200 °F and the selected temperatures must not be more than 600 °F apart. Temperatures of 375 °F and 925 °F would meet this requirement. Periodic TUS frequency is based on frequency of heat treating operations.

a. Shops performing daily heat treating operations, utilizing Type D instrumentation, shall perform TUSs on a monthly basis, not to exceed 31 calendar days. After eight consecutive passing TUSs, the interval may be extended to bi-monthly, not to exceed 61 calendar days. If utilizing Type B or Type C instrumentation, after 4 consecutive passing TUSs, the interval may be extended to quarterly, not to exceed 91 calendar days.

b. Shops performing weekly heat treating operations, every 5 to 14 calendar days, utilizing Type D instrumentation, shall perform TUSs on a bimonthly basis, not to exceed 61 calendar days. After eight consecutive passing TUSs, the interval may be extended to quarterly, not to exceed 91 calendar days. If utilizing Type B or Type C instrumentation, after 4 consecutive passing TUSs, the interval may be extended to quarterly, not to exceed 91 calendar days.

c. Shops performing biweekly to monthly heat treating operations, every 15 to 30 calendar days or less frequently, shall perform TUSs on a quarterly basis, not to exceed 91 calendar days regardless of instrumentation type used. Use of Type B or Type C instrumentation is recommended, but the SAT interval will not be extended.

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d. If at any time a TUS fails, the interval resorts back to standard and the count starts again.

e. If utilizing Type A instrumentation, refer to AMS2750 for TUS interval.

3.14.2.4 Annual TUS Requirement. In addition to the periodic test requirement, at least once per year, not to exceed 365 calendar days, surveys shall also be performed at the minimum and maximum of the qualified operating temperature range.

For example, if an oven operating range is 200-1200 °F, then survey temps of 200 °F, 375 °F, 925 °F, and 1200 °F would meet this requirement.

3.14.3 Tus Procedure.

NOTE

If, for any reason, a TUS cannot be performed, load thermocouple monitoring shall be used for all aircraft heat treating operations, not to exceed two periodic TUS cycles. If a TUS still cannot be performed after two periodic TUS cycles have elapsed, consult the responsible technical/engineering activity for aircraft heat treating process authorization.

During each TUS, the survey parameters shall reflect the normal operation of the equipment used in production. That is, if normal procedure is to load parts into a temperature stabilized oven, then the survey shall be initiated by placing the rack with TCs into an oven stabilized at the survey temperature. Common TUS procedure would be to bring the oven to desired set point, allow to stabilize, load survey rack with TCs into chamber, begin capturing data on data logger, allow oven to recover, mark official start TUS, allow TUS to run for at least 30 minutes, terminate TUS and data capture, interpret results, generate and file report.

3.14.3.1 Number of Required Thermocouples. The number of required sensors varies based on volume of the oven chamber to be surveyed. If the oven chamber volume is less than:

a. 3 cubic feet, 5 sensors are required.

b. 225 cubic feet, 9 sensors are required.

c. 300 cubic feet, 14 sensors are required.

d. 400 cubic feet, 16 sensors are required.

e. 600 cubic feet, 19 sensors are required.

f. Chamber volumes 600 cubic feet or more, refer to AMS2750.

3.14.3.2 Location of TUS Sensors. Sensor location varies based on the volume of the chamber.

a. Volumes less than 3 cubic feet, four sensors shall be located at the lower four corners and the fifth sensor shall be placed in the center. TUS sensors shall be placed to best represent the qualified work zone.

b. Volumes of 3 to 225 cubic feet shall have a sensor placed at each of the eight corners and the ninth sensor shall be placed in the center. TUS sensors shall be placed to best represent the qualified work zone.

c. Volumes greater than 225 cubic feet, sensor placement shall be the same as indicated in Step b with the additional required sensors in Paragraph 3.14.3.1 uniformly distributed to best represent the qualified working zone.

3.14.3.3 Qualified Working Zone. A qualified work zone is the defined portion of the oven/furnace volume where temperature variation conforms to the required uniformity tolerance. If an entire oven chamber cannot meet TUS standards, then it is acceptable to perform a TUS and qualify a working zone. The qualified work zone can be any location in the oven/furnace the operator chooses. The qualified work zone location and volume tested shall be such that no heat treated material extends beyond the defined/qualified work zone boundaries. If the qualified work zone changes, for example if a part or material to be heat treated will not fit inside the currently qualified work zone boundaries, a TUS of the new required zone/volume shall be performed prior to material heat treatment.

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3.14.3.4 TUS Data Collection. Data collection begins once the rack is inserted into the oven chamber and the door is closed. TUS sensors shall record temperature data from all sensors at a frequency of at least once every two minutes for the duration of the survey.

3.14.3.4.1 Secure all TCs to the rack in the specified location based on chamber volume. Connect all TCs to the data logger. Ensure data logger is on and ready to capture data.

3.14.3.4.2 Insert TUS rack into oven chamber and begin logging the recovery process. Oven recovery time is defined as the amount of time required from the time the door is closed for all test TCs to report within ±10 °F, or as applicable for the process being performed, of the temperature set point. At no time shall any test, control, or recording sensor exceed the upper temperature uniformity tolerance. Oven recovery time for aluminum solution heat treating and aging TUSs shall not exceed the following:

a. Ovens tested without a load, 20 minutes.

b. Ovens tested with a load, 40 minutes.

3.14.3.4.3 Upon successful oven recovery, mark official TUS start time and run survey for a minimum of 30 minutes. For TUSs with 9 or less TCs, sensor or recording instrument failures are not permitted. A temporary condition such as a short or loose connection where normal temperature readout is restored shall not be considered a failed sensor. Surveys with 10 or more sensors are permitted sensor failures provided they are not in any corner and meet the following conditions:

a. Survey with 10 to 16 sensors: 1 failure

b. Survey with 17 to 23 sensors: 2 failures

c. No adjacent sensor failures

d. Surveys with 24 or more sensors, see AMS2750

3.14.3.4.4 After a minimum of 30 minutes have elapsed, conclude the TUS and remove the rack/sensors. Interpret the data, and generate/file the report.

3.14.4 Temperature Uniformity Pass/Fail Requirements. A survey shall be considered passing if all the following requirements are met.

a. Control or monitoring sensor readings and TUS sensor readings did not exceed applicable positive temperature toler-ance at any time. See Table 3-19 for Temperature Uniformity Allowances.

b. The time required to achieve recovery, stabilization, and maintain set point temperature tolerances did not exceed the time limit specified in Paragraph 3.14.3.4.2.

c. All readings of control/monitor and TUS sensor readings are within the temperature tolerance requirements of Table 3-19 for the process being surveyed after the official start of TUS survey time except as allowed in Paragraph 3.14.3.4.3.

d. TUS sensor data was logged for each sensor at a frequency not greater than every two minutes.

e. TUS is run for the minimum required time of 30 minutes.

3.14.5 TUS Data and TUS Reports. TUS data must be gathered and recorded on a system that creates electronic records that cannot be altered without detection.

3.14.5.1 The TUS system software and playback utilities shall provide a means of examining and/or compiling the record data, but shall not provide any means of altering the source data. The system shall be capable of providing evidence the record was reviewed, such as by recording an electronic review or a method of printing the record for a physical marking to indicate a review.

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3.14.5.2 TUS Survey Report Requirements. A paper or digital copy of the completed record shall be retained by the facility performing the TUS for a period of 5 years and disposed of in accordance with applicable Records Disposition Schedule guidelines. The report will contain, at a minimum:

a. Oven identification name or serial number.

b. Survey temperature.

c. Required temperature uniformity.

d. Oven chamber dimensions or qualified working zone dimensions and location in chamber.

e. TUS sensor and location identification including a detailed diagram, description or photograph of any load or rack used.

f. Time and temperature data from all TUS sensors.

g. TC spool correction factor or correction factors for each TUS sensor at each calibration temperature.

h. Corrected or uncorrected readings of all TUS sensors. Reading shall be identified as corrected or uncorrected.

i. As found and as left TUS offsets (if used in production).

j. Survey start date and time.

k. Survey end date and time.

l. Survey test instrument identification or serial number.

m. Survey test instrument calibration agency.

n. Survey test instrument calibration date.

o. Survey test sensor failures, if any.

p. Indication of test pass or fail.

q. Identification of technician performing survey.

r. Identification of supervisor approving survey.

3.14.6 Failed TUS Procedures. If the temperature uniformity is not within the tolerances of Table 3-19, the cause of the deviation shall be determined, documented, and corrected. The equipment shall not be used for additional processing until the deviation has been corrected and the TUS has been performed successfully.

3.14.6.1 For ovens being tested at an extended interval, failure of a TUS shall cause the extended TUS interval to revert back to the standard periodic interval as applicable in Paragraph 3.14.2.3.

3.14.6.2 Temperature Offsets. Adjustment/offset of the control instrument to facilitate desired chamber temperatures based on most recent TUS results is permissible. If manual adjustments or offsets are made to the controller, the effects of these adjustments must be tested and confirmed across the entire operating temperature range. Manual offsets shall be documented, repeatable, approved/signed by section supervisor or NCOIC and used in production heat treatments, ie;

controller is offset to 380 °F to obtain accurate uniform internal chamber temperature uniformity of 375±10 °F, controller is offset to 910 °F to obtain accurate uniform internal chamber temperature uniformity of 920±10 °F, etc. It is highly recom-mended to post the signed temperature offset chart next to the oven controller or a similar location where the operator will see it and not overlook it. Temperature offsets of ±5 °F are permissible without oven maintenance or troubleshooting, however, it is highly recommended. Temperature offsets greater than ±5 °F shall be troubleshot and corrected by a qualified technician within 180 days.

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