Thermocouples in Furnaces and Ovens

The temperature on the inside of furnaces and ovens are commonly monitored and controlled by thermocouples inserted into the heated chamber.  The one common feature of all furnaces is the fact that there are isotherms within the heated chamber.  Isotherms are regions of equal temperatures.  They are similar to the contour lines on a map illustrating areas of equal altitude. Isotherms are caused by gradients within the furnace created by uneven heating, inadequate circulation, uneven distribution of the workload within the furnace, etc.  There are also isotherms within the wall of the furnace since the outside surface temperature of the wall is close to ambient temperature and the inside surface temperature of the wall may be 3,000 degrees Fahrenheit or more.

Thermocouples are installed in the furnace by machining an appropriate hole through the wall.  The thermocouple now cuts across a myriad of isotherms and creates a conductive path for heat to flow from the hot area to the cool area. The thermocouple (since it measures its own temperature) is constantly being cooled by this conduction.  The end result is 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 your furnace and the atmosphere.  We call this error the “Stem Effect.”  This error is influenced by the heat conduction in the wires, insulation and the sheath or thermowell of the thermocouple. It is virtually 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 since the thermal conductivity of all materials varies with temperature.

The basic physical principles involved are as follows:

1. Heat is always exchanged between two objects at different temperatures and it always flows from the hotter to the cooler object and.
2. Heat cannot be exchanged between two objects at the same temperature.

Our objective is to design and install a thermocouple into a furnace so that the thermocouple sensing tip is always in equilibrium with the temperature of interest and therefore accurate measurements are being made.

In order to minimize the “Stem Effect” error one must install the thermocouple parallel to the plane of heat flow for a distance of at least 20 diameters of the probe.  If you use an 1/8” OD probe, then 2-1/2” inches of the tip of this probe should be located parallel to the plane of heat flow.

Accuracy of temperature measurements made within furnaces can be greatly improved if the temperature sensors are installed parallel to the isotherms for a distance equal to 20 times the diameter of the protection sheath.  This will reduce the “Stem Effect” (error caused by conduction in the sheath, wires and insulation) to an insignificant amount.

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.