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M.I. Cable Technologies Manufacturing to AMS 2750E specification

Mailing Address: Telephone: E-mail:Bay 5 1-403-571-8266 (International) sales@micabletechnologies.com5905 – 11 Street S.E. 1-800-425-5118 (USA and Canada)Calgary, Alberta, Canada Website:T2H 2A6 Fax: www.micabletechnologies.com

1-403-571-8267Office Hours: 7:30am – 4:00pm (Mountain Time)

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Notations on AMS 2750EThe applications in which mineral insulated, metal sheathed cables may be used and to what extent MI Cable Technologies (MICT) can meet the provisions of the specification are summarized below. In many cases, it seems that customers are requesting performance which exceeds that required by the specification itself, and our purpose in issuing this summary is to encourage frank discussion between MICT and our customers in regard to the availability of complying products.

ApplicationsPer Table 1 of AMS 2750E, base metal (types J, E, N, K, and T) thermocouple cables are suitable for use in the following sensor applications:

• Secondary Standard, provided the cable meets ASTM special limits of error and provided the coil length does not exceed 2000 feet, in accordance with section 3.1.2.6.1c. Care must be taken when ordering cables in diameters of 0.125 inch, 3.0 mm, or smaller as MICT’s coil lengths for those cables are often greater than 2000 feet (610 meters).

• Temperature Uniformity Survey, provided the cable meets at least ASTM standard limits of error. Coil length is not a concern.• System Accuracy Test, provided the cable meets ASTM special limits of error. Coil length is not a concern.• Control, Recording, & Monitoring, provided the cable meets ASTM special limits of error. Coil length is not a concern.• Load, provided the cable meets at least ASTM standard limits of error. Coil length is not a concern.

Note that some applications require only standard limits of error and that in general, for temperatures above zero degrees Celsius, ASTM’s special and standard accuracy limits are slightly more stringent than IEC’s class 1 and class 2 limits respectively. MICT’s experience is that end users, as a matter of course, will request ASTM special limits cables regardless of the application and that to avoid misunderstanding, special limits cables should be recommended for all applications.

ConstructionAMS 2750E section 2.2.34 refers specifically to ASTM E 608, which in turn points to ASTM E 585 section 7.2.1, requiring all thermocouples to be made with high purity type 1 (> 99.4% pure) MgO insulation, unless permission is specifically granted otherwise by the end user. MICT’s experience is that end users, as a matter of course, will request high purity MgO and that to avoid misunderstanding, high purity MgO cables should be recommended for all applications.

The cable’s sheath material will normally be specified as Inconel 600 although there are applications, particularly for thermocouple types J, E, and T, in which 316L stainless sheathed cable would suffice.

While type K thermocouples have historically been the choice of most end users, there has been a relatively recent movement towards greater use of type N thermocouples. Emf drift of type K thermocouples is a long recognized characteristic; MICT’s type N thermocouples can offer a reduced drift alternative. Refer to the appendix to this document for additional information regarding type N thermocouples emf drift characteristics.

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Notations on AMS 2750E cont…CalibrationRegrettably, AMS 2750E does not stipulate the beginning calibration temperatures but instead in section 3.1.2.5 states only: “Temperature sensors shall be calibrated in the nominal temperature range within which they are to be used.” Maximum calibration temperature is dependent on the cable’s type designation and outside diameter and reference should be made to ASTM E 608 Table 1 when choosing a calibration range.

Section 3.1.2.5.1 requires calibration at intervals, in order of ascending temperature, not to exceed 140º Celsius.

Standard practice for MICT is to calibrate both ends of each coil manufactured for AMS 2750E applications at the six following temperatures: 100oC, 300oC, 500oC, 700oC, 1093oC and 1200oC. Almost all end users choose to perform their own calibrations. MICT’s calibrations will not meet the maximum temperature interval requirement but we believe that cables meeting ASTM special limits tolerances at the six points selected by MICT will regularly pass all requirements for calibration accuracy imposed by the end user. Calibration results from each end of the coil are provided on the cable’s certification report, together with the coil’s accuracy classification, and a raw materials report.

HomogeneityPer section 3.1.2.6.2 each thermocouple cable must be calibrated at both ends of the coil. This is the standard operating practice within MICT not only for coils manufactured for AMS 2750E applications but for all of our thermocouple cables.

For all but secondary standard applications, section 3.1.2.6.3.2 requires that the calibrations from both ends of the same coil must fall within 1.1º Celsius of one another and both ends must meet the standard or special limits accuracy requirement as defined in Table 1 (see Applications above).

For secondary standard applications, section 3.1.2.6.3.1 requires that the end to end calibration difference must fall within 0.6º Celsius rather than 1.1º Celsius. MICT does not in general intend to manufacture cables to meet this very small end to end calibration tolerance. Special selection of coils will be required to meet this requirement and the supply of cable cannot be guaranteed.

However, if a coil should fail the end to end calibration difference test, section 3.1.2.6.3.4, permits cutting the coil into two or more pieces and so long as the individual piece meets the end to end calibration requirement, that piece may be used. For MICT, we would in general for any cable failing the end to end requirement, cut the coil in half and retest each half coil. Any further failure will disqualify the newly created partial coil.

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Calibration LaboratoryMICT provides calibration results for all thermocouple cables manufactured in our factory with each shipment. Our laboratory uses the comparison calibration method in general accordance with ASTM E220. Standard calibration temperatures for types K, J, E, and N are 300oC, 500oC, 700oC, and for type K and N thermocouples in either 310S or Inconel 600 sheath, 1093oC. Type T calibration is performed at temperatures of 100oC, 200oC, and 280oC using a silicon oil bath. Type K and N thermocouples which are to meet the AMS 2750E specification are also calibrated at 100oC (using silicon oil bath) and 1200oC.

In addition, MICT can supply type T thermocouple cable for cryogenic applications with calibration results provided at the liquid nitrogen boiling point (approximately -196oC).

MICT works continuously to reduce the measurement uncertainty at each calibration temperature and new estimates are made at six month intervals. The uncertainty estimates for each temperature are reported on each coil’s calibration report.

For the most recent k=2 expanded uncertainty of measurement values contact our Sales department.

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K-type thermocouples – typical calibration performance

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Calibration - 3.0K600HP, coil #123328

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Calibration - 1.6K600HP, coil #124308

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N-type thermocouples – typical calibration performance

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Calibration - 1.5N600HP, coil #117435.7

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Calibration - 4.5N600HP, coil #124855

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Calibration - 6.4N600HP, coil #125708

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Appendix N-Type thermocouple drift

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N-type thermocouple - emf driftMICT manufactures a range of N-type thermocouple cable especially designed for rigorous AMS2750 applications:

• Sheath diameter of 0.059” up to 0.375”• Provided as simplex, duplex, triplex cable• ASTM type 1 high purity MgO insulation• Sheath material: Inconel 600• ASTM special limits or IEC class 1 tolerances• Front end to back end temperature (emf) differential < 1.1oC (2oF)

N-type thermocouple emf driftN-type thermocouples are subject to emf drift from both high temperature and prolonged use. Through studies conducted by MICT and Yamari Industries of Japan, the drift phenomenon was plotted after short term and long term use. Through MICT’s experience, we’ve learned that careful selection of raw materials and controlled processing are critical in minimizing this effect. The combination of wire match to sheath material has also shown itself to have a substantial influence on the amount of drift experienced.

Short term drift (refer to figure 1)• Five sample coils of type N thermocouple cable were manufactured, each one using a unique pair of matched type N wire provided by four different wire manufacturers. All

wires were designated “thermocouple grade” by the respective wire manufacturer.• Heated to 1250oC in electric furnace.• Measured emf values for 50 hours at 10 minute intervals after 1250oC temperature reached. (short term drift to positive values occurred in the initial hour at 1250oC and then drifted negatively)

Long term drift (refer to figure 2)• Five sample coils of type N thermocouple cable were manufactured, each one using a unique pair of matched type N wire provided by four different wire manufacturers. All

wires were designated “thermocouple grade” by the respective wire manufacturer.• Heated to 1000oC in electric furnace.• After total length is heated to 1000oC, emf values were recorded after 3, 285 and 500 hours time. (continuation of the negative drift occurred over time, confirming the trend seen in the short term test)

The above tests were conducted using wire matches from various suppliers. As each supplier has slight differences from one another, we are able to identify which NP/NN wire combinations work best in high temperature critical applications.

MICT employs only N wire matches manufactured by suppliers 1, 2 & 3 within Figure 1 and Figure 2. In addition we carefully select the MgO and Inconel 600 materials. MICT sources its raw materials from only European, North American, or Japanese manufacturers. We do not employ low cost, poor quality materials readily available from developing country manufacturers in any of our products.

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N-type short term drift

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N-type long term drift

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