Thermocouple Standards and Calibrations

Thermocouple Standards

As shown in Table I, there are seven thermocouple standards applicable to heat-treating furnaces.  These include Reference Standard, Primary Standard, Secondary Standard, Temperature Uniformity Test Standard, System Accuracy Test plus Working and Load Standards.  This chart summarizes the type of thermocouples, which can be used for each category in addition to calibration frequency and accuracy requirements.  These standards were established by SAE (Society of Automotive Engineers) specification SAE-AMS-2750 Rev. C in 1990.  This specification has been adopted by the U.S. Department of Defense.  It is a valuable reference and we suggest that anyone who manufactures or uses heat-treating furnaces should have a copy of this specification in their Quality Control Department.

Table I

Outline of Sensors
Nomenclature	Description	Calibration		Use/Max Error Limit
		Period	Against	Correction Factor (°F)
Reference Standard	Platinum Platinum-Rhodium	5 years	NIST Reference Standard	Primary Standard Calibration None
Primary Standard	Platinum Platinum-Rhodium	3 years	Reference Standard	Secondary Standard Calibration ±2.7° or ±0.25%**
Secondary Standard	Base or noble metal	1 year base 2 years noble	Primary Standard	Test Sensor Calibration Base: ±2° or ±0.4%** Noble: ±2.7° or ±0.25%**
Temperature Uniformity Test	Base of noble metal	3 months base 6 months noble	Primary or Secondary Standard	Temperature Uniformity Tests ±4° or ±0.75%**
System Accuracy Test	Base of noble metal	3 months base 6 months noble	Primary or Secondary Standard	System Accuracy Tests ±2° or ±0.4%**
Working	Base of noble metal	Before installation	Primary or Secondary Standard	Installation in Equipment Class 1: ±2° or ±0.4%** Class 2: ±4° or ±0.75%**
Load	Base of noble metal	3 months N, R, S 6 months other	Primary or Secondary Standard	Insertion in Loads ±4° or ±0.75%**
*   Sensors of Equivalent or Greater Accuracy are Acceptable **   Percent of Reading, if Greater Than Correction Factor in Degrees
Aerospace Material Specification - SAE AMS-2750 Rev. C. issued 1980-04-15, Revised 1990-04-01 Superceding AMS-2750B. Society of Automotive Engineers, Inc., 400 Commonwealth Drive, Warrendale, PA 15096 (1990).

Calibration Services

Nanmac’s calibration laboratory will calibrate bare or insulated thermocouple wire, assembled thermocouples, RTD’s, thermistors and instruments.  All of our calibration equipment is calibrated against National Institute of Standards and Technology (NIST) standards.  Our calibration data are traceable to NIST standards.  Calibration costs are listed in the chart below.  The maximum temperature range of our standard services is 2,100 degrees Fahrenheit.

Notes:

All temperature sensors must be at least 12 inches long to minimize conduction errors.Calibrations to 2,950°F can be made on a special basis (contact factory for details).  Also, calibrations at cryogenic ranges can also be made on a special basis. Your instruments and sensors can also be calibrated and certified (contact factory for details).

Efficient Vibration Condition Assessment

Methodology Overview
 DLI Engineering’s 40 years of experience in providing early prognosis of incipient faults in rotating equipment have taught us that customers need a proven methodology, recognized by international standards, that provides the following capabilities in the most efficient manner:

• Data Acquisition (Sensor Selection & Mounting)
• Data Manipulation (Signal Processing)
• State Detection (Baseline Profiles)
• Health Assessment (Automated Fault)
• Diagnostics)
• Prognostic Assessment (Prioritized Repair)
• Advisory Generation (Reports/Documentation)

Data Acquisition / Sensor Selection & Mounting
DLI Engineering introduced the first commercially available triaxial accelerometer over 20 years ago and still supports its use with all of its portable data collectors. A triaxial accelerometer with mounting pad is the most efficient method available to collect a complete set of vibration data in three mutually perpendicular directions. This method of data collection for portable, periodic data collection offers the following advantages:

• FAST – Collect vibration data in all three directions in one step instead of moving a single axis accelerometer from one location to the next. The DLI Watchman® DCA-50™ or DCX™ can collect data in all three directions simultaneously. 
• COMPLETE DATA SET – Besides collecting data in three directions all of DLI’s portable data collectors support the collection of two frequency ranges, typically, low range (10 x machine speed) and high range (100 x machine speed).
• REPEATABLE – The use of a permanent mounting pad affixed to each measurement location (e.g. inboard & outboard bearing housing) for mounting the triaxial accelerometer insures repeatable data that can be accurately trended over time. DLI also facilitates repeatable data by supporting barcode-based data collection which minimizes the danger of storing data on the wrong machine or measurement location.

For online monitoring of critical or inaccessible machines it may not be economical to mount a triaxial sensor at every desired machine location. The DLI methodology is flexible enough to support the data collection and analysis of single axis accelerometers, velocity probes, proximity probes or a wide variety of process sensors such as speed, motor current, temperature or pressure.

Data Manipulation / Signal Processing

All portable and online data acquisition instruments support the following signal processing:

• Spectra/FFT (general fault determination)
• Time Waveforms/Orbits (impacting/sleeve bearings)
• Envelope Demodulation (rolling element bearings)
• Overall Amplitude (overall machine severity)
• Phase (troubleshooting)
• Cepstrum (harmonic family analysis)

State Detection / Baseline Profiles
The efficiency of ExpertALERT is based on its ability to use statistical baseline data from specific machine(s) to compare with current data. By comparing incoming spectra to statistical baseline spectra, ExpertALERT effectively uses over 500 frequency “bands” in its initial data screening process to identify machine faults. This technique of data comparison is far more sensitive and selective than traditional methods that use only 6-12 frequency “bands”.

Health / Prognostic Assessment
DLI’s ExpertALERT™ Condition Assessment software includes a rule-based, automated diagnostic module as well as all of the necessary graphical analysis tools to confirm or analyze a wide range of machinery faults. Its diagnostic system identifies even the most subtle patterns in the vibration data and provides repeatable, quantifiable and detailed diagnostics. Identified faults are trended over time allowing you to track actual faults rather than just vibration levels. Over 4,700 individual rules can recognize over 956 specific machine fault patterns in 47 different machinery components.

Advisory Generation
Another real efficiency of our methodology is its unique ability to generate a report listing the following critical machine status/condition information:

• Specific machine fault(s)
• Severity of fault (OK, Slight, Moderate, Serious or Extreme)
• Specific repair recommendation
• Repair priority (Desirable, Important or Mandatory)
• Details of specific vibration peaks and frequencies contributing to fault

This actionable information is far easier to interpret than the raw spectral data that is typically presented with other vibration analysis systems. The entire process of data screening, data analysis, fault diagnostics and report generation is completely automated which can save hours of labor compared to other systems currently on the market. This is not to say that human expertise is removed from the DLI methodology. DLI recommends that all significant automatic machine fault diagnoses be reviewed by a trained vibration analyst. We put this recommendation into practice with our PdM Express™ remote vibration analysis service where a senior engineer reviews all results to ensure 100% accuracy.

Information Everywhere
DLI believes that the distribution of the actionable information generated automatically by ExpertALERT software is critical. As such, we have developed a variety of delivery mechanisms ranging from the traditional hard copy reports to email alert notifications to machine condition status results available via a standard web browser. DLI is also on the forefront of database synchronization and replication via the internet. We currently have customers electronically transmitting data to us for analysis from around the globe while we distribute their results back to them as well as optimize their databases remotely.

NANMAC NOTES: Eliminate Temperature Errors Caused by Conduction

http://ow.ly/16r0n
Jacob Nanigian*
Nanmac Corp. Framingham, Mass.

Control of any heat treat process, whether basic or advanced, is only as good as the accuracy of temperature measurement. The accuracy of contact-type temperature sensors is affected by calibration, response time, and conduction.

Calibration errors are generally insignificant. Sensors can be calibrated to an accuracy of ±0.25°C (0.5°F), and standards can be calibrated to the sixth decimal place at laboratories such as MST (National Institute of Standards and Technology). Response time of sensors is very important in transient or cyclical applications, but in furnace applications, rapid transients are not usually encountered. However, the conduction phenomenon is of primary concern in furnaces because it can generate large errors in temperature measurements.

Causes of conductance errors

Conduction errors are caused by temperature gradients within the furnace and the furnace wall, and the ambient temperature outside the furnace. Since the temperature sensor is installed through the furnace wall, it creates a conductive path along which heat flows from the inside of the furnace to the exterior, thus creating an error in the observed measurement. Temperature gradients are also caused by uneven heating, poor circulation, and uneven distribution of the workload within the furnace. 
 
To provide accurate measurement, a temperature sensor must:

• Have reproducible and stable calibration properties (temperature vs. EMF and temperature vs. resistance) over the measured temperature span;
• Not disturb the local temperature of the medium by its very existence; and
• Have sufficiently fast response to follow temperature changes accurately.

Figure 1 illustrates the temperature profile across the wall of a heated chamber at any instant of time. In this illustration, the wall is made of a homogeneous material such as steel or ceramic. If the wall contains several materials, such as steel and insulation plus graphite or ceramic, each homogeneous material will have its own temperature profile.

Several observations can be made in connection with Fig. 1:

• Heat always flows from the hotter medium to the cooler medium.
• Heat energy is continuously absorbed by the wall at its hot side and liberated at the cold side to the cooling medium (air, water, etc.). Under steady-state conditions, the heat absorbed must be equal to the heat liberated or the wall will begin to melt.
• The temperature profile at each interface approaches exponential conditions.
• No region exhibits constant temperature over a given cross-section. Constant-temperature zones are called isotherms, and they appear only parallel to the plane of heat flow. 

Thermocouples are installed by machining an appropriate hole through the wall. The thermocouple thus cuts across myriad isotherms, and creates a conductive path for heat to flow from the hot area to the cool area. The thermocouple, which in reality measures its own temperature, is constantly being cooled by this conduction.

The output of the thermocouple is always in equilibrium with the heat coming into the junction and the heat being carried away by conduction via the thermowell to the outer jacket of the furnace and the atmosphere. This is called the stem effect. This error is caused by the heat conduction in the wires, the insulation, and the sheath or thermowell of the thermocouple. It is impossible to predict the magnitude of this error. Even if you could determine this error at a particular temperature, it would change as the temperature changes, because thermal conductivity of all materials varies with temperature.

Therefore, the heat treater's objective should be to install a thermocouple into a furnace so that the sensing tip is always in equilibrium with the temperature of interest and, therefore, accurate measurements are made. Ideally, the sensor should be an adiabatic probe — that is, it should be thermally isolated from the wall.

Testing for conduction We designed an experiment to determine the magnitude of errors produced by the stem effect. We used a container of hot water surrounded by a reservoir of cold water. The hot water was kept hot by an immersion-type heater; the cold water was kept cold by a simple circulating pump and cooling radiator. The temperatures in both the hot and cold reservoirs were continuously monitored by laboratory thermometers with an accuracy of ±0.25°C (0.5°F).

The carbon steel hot water tank had a 12-cm (5-in.) .O.D. with a wall of 1 cm (0.5-in.) thickness. Figure 2 is a schematic of the test setup. Four thermocouples, each with a different style of thermal junction, were installed in the wall of the hot water tank and aligned flush with the inner surface of the tank. Thus, all the thermocouples were in direct contact with the hot water. All the thermocouples had iron/constantin (copper-nickel) elements, and their outputs were monitored by the same meter through a thermocouple selector switch. In addition, all thermocouples were made from the same lot of wire, and resistances were matched. The four thermal junction styles were:

Tl — Exposed-bead weld junction T2 — Grounded junction: sheath welded closed and the thermal junction electrically in contact with the sheath.
T3 — Insulated junction: sheath welded closed but the thermal junction electrically isolated from the sheath. All RTDs and thermistors are similar in design to this insulated junction. The sensing element is electrically insulated and then enclosed in a metallic sheath that protects the sensing element from abuses of the environment such as corrosion, oxidation, erosion, and chemical attack. This protection comes at the cost of reduced accuracy.
T4 — Right-angle junction: exposed-ribbon welded junction.

Tl, T2, and T3 are conventional thermal junction styles, and T4 is a right-angle unit with ribbon elements (Fig. 3). Measurements were taken from these four thermocouples over a 16-hour period. When the hot water temperature was held at 83°C (181°F) and the cold water held at 38°C (101°F), thermocouple Tl read 78°C (173°F), T2 read 69°C (156°F) and T3 read 53°C (128°F), whereas the rightangle ribbon thermocouple read 83°C (181°F). A temperature gradient of 40 to 50°C (70 to 88°F) was maintained over the two days in which this test was conducted. (The test took place in an exhibit hall during an ISA show.) The variations in this gradient were caused by several factors: our cooling system was not very well controlled by our simple heat exchanger — the circulating water pump and our small radiator; and, as the ambient temperature in the hall rose during the day, it caused the temperature of our cold water reservoir to rise, thus putting an additional load on our heat exchanger system. Note that the insulated junction (T3) exhibited the largest error in both absolute measurement and percent of temperature change between the hot and cold water

The right-angle ribbon sensor has a junction thickness of 0.07 mm (0.003-in.). The extension leads in the vicinity of the junction are also ribbons, and lie in the same plane as the sensing junction for a distance of at least 20 times the thickness of the junction. Thus, the sensing tip and the adjacent ribbons are parallel to the plane of heat flow, and since the ribbons on both sides of the junction are heated simultaneously with the junction, no significant error is caused by the stem effect. 

 To minimize the error caused by the stem effect in all sensors, this ratio of 20:1 must be maintained at the sensing tip. For example, a 3-mm (0.125-in.) round stainless steel probe must be positioned in the furnace so that it lies parallel to the plane of heat flow for 6 cm (2.5 in.) This can be accomplished by one of two methods: make a 90° bend in the probe 6 cm (2.5 in.) from its tip, or position the mounting so that the probe is parallel to the plane of heat flow.

Figure 4 is a schematic of a furnace with heating elements at the bottom. The isotherms are shown as broken lines. Figure 5 is a similar sketch showing the isotherms in a furnace with heaters on all sides. In all instances, the correct orientation requires that the thermocouple extend parallel to the isotherms for a length equal to 20 times the probe diameter. A right-angle thermocouple such as the one used in the test described above can be installed into a furnace without any 90° bends because the probe already incorporates such a bend.
   *Member of ASM. International