Data processing: measuring – calibrating – or testing – Measurement system – Temperature measuring system
Reexamination Certificate
2002-09-13
2004-08-03
Barlow, John (Department: 2863)
Data processing: measuring, calibrating, or testing
Measurement system
Temperature measuring system
C073S713000, C219S497000, C219S667000, C374S057000, C374S131000, C374S161000, C374S179000, C706S003000
Reexamination Certificate
active
06772085
ABSTRACT:
TECHNICAL FIELD
This invention relates generally to temperature sensing techniques, and more particularly to a self-verifying device and method for temperature measurement and control.
BACKGROUND OF THE INVENTION
Many important properties of chemistry, physics, thermodynamics and heat transfer can only be determined by accurately measuring temperature. The measurement of temperature is fundamental to most modern industries. However, errors in temperature measurement can result in energy inefficiencies, creating scatter in product quality, shortening plant life, and limiting plant safety. The effects of temperature measurement error accumulate over days, weeks, and months of operation and are a significant cost to many industries.
One problem frequently encountered with conventional temperature measurement results from drift. Drift in temperature sensors can present a major roadblock to an industry's effort to create new product types or increase energy efficiency. For example, new gas turbine engines for the aircraft industry must operate at hotter temperatures in order to achieve increased fuel efficiency. Temperature sensors are required to monitor the hotter gas flow temperatures for the new engines in order to operate properly. In general, hotter temperatures tend to degrade sensors more rapidly and cause more rapid aging. Aging increases the probability of erroneous temperature readouts.
In fact, jet airliners must be taken out of service in order to re-check the calibration of thermocouples that monitor the temperature within gas turbine engines. Gas turbine temperature measurement becomes essential to establishing fuel combustion ratios. The cost to take airliners out of service for such check-up is considerable, but necessary, because there previously has not existed a convenient method of confirming the calibration of temperature sensors while they remain installed in a gas turbine engine. The requirement for calibration checks for new gas turbine engines that will operate at hotter temperature will compound the costs that result from temperature sensor decalibration. Furthermore, high maintenance and product quality costs are caused by temperature sensor decalibration that is common to a wide variety of modern industrial and transportation applications.
Sometimes the consequences of poor temperature measurement can result in catastrophic failures. One such accident destroyed the Three Mile Island-Two nuclear power plant, which could have been prevented by a quick and accurate measurement of reactor core temperatures. During such accident, reactor operators were unable to determine whether the reactor core was overheating. Ignorance over core temperature allowed the plant operators to make an erroneous conclusion about the state of the reactor core. The control decision error caused the loss of the Three Mile Island plant, with some total cost estimates running as high as $3 billion.
As another example, a recent DC-10 airplane crash was traced to failure of an engine support strut. The strut failure was subsequently traced to improper temperature control in a metal annealing process. Temperature sensors that controlled the annealing process had decalibrated, and the operator was unable to detect temperature drift in sensor readout.
The importance of accurate and reliable temperature measurement to modern processing and transportation industries is well documented by the above and similar examples that demonstrate the vulnerability of modern industry to temperature sensor drift. Furthermore, a number of techniques exist for monitoring temperature, but existing techniques each have associated problems.
For example, there exist a number of schemes and techniques for monitoring the intensity of heat by measuring temperature. One early technique entailed the monitoring of thermal expansion in order to sense a temperature. Such physical phenomena forms the basis for liquid-in-glass thermometers. Several other techniques involve electrical transduction which is employed to sense temperature. Among these are resistive, thermoelectric, semiconductive, optical and piezoelectric detectors. Temperature measurement involves the transmission of a small portion of an object's thermal energy to a sensor, the sensor functioning to convert that energy into an electrical signal. For the case where a contact sensor is used, the contact sensor is placed inside or on an object, with heat conduction taking place through an interface between the object and a probe. The probe warms up or cools down, exchanging heat with the object. Through careful design of a probe, the measurement site will not be disturbed significantly and error is minimized by appropriate sensor design via correct measurement techniques.
One problem associated with a significant number of such measurement techniques occurs when temperatures have to be measured under tough or hostile environments. Such tough or hostile environments can involve strong electrical, magnetic or electromagnetic fields, or very high voltages which make measurements either too susceptible to interferences, or too dangerous for an operator. Hence, one technique for solving such problems is to use non-contact techniques for measuring temperature. However, non-contact techniques do not work in many environments. Additionally, there exist contact sensors which can sense temperature in a hostile environment, such as thermocouples which measure resistive coupling of different materials when exposed to a temperature environment.
For the case of a thermocouple, comprising a thermo-electric contact sensor, at least two dissimilar conductors are used to make a sensor. A number of different thermocouples are known for use with different applications such as TypeT, TypeJ, TypeE, TypeK, Types R and S, and TypeB. Depending on the temperature and/or chemical environment encountered, a suitable thermocouple can be selected. However, one problem with thermocouple sensors results from “drift”, as discussed above, which can adversely affect accuracy by causing measurement errors.
Another problem results from the aging of temperature sensors which can result in increases in temperature system error. Gregory K. McMillan,
Advanced Temperature Control
, published by The Instrument Society of America (1995), discusses mathematical methods to estimate the magnitude of error that has accrued on an aged temperature sensor. However, mathematical estimates of error are not adequate to correct for temperature sensor drift, in most industrial processes.
Modern industrial processes are carried out at ever-increasing temperatures. Such rise in operating temperature requires the measurement of temperature in service conditions that are increasingly corrosive or otherwise hostile to measurement instruments. For these reasons, temperature sensors increasingly age, or otherwise degrade, while operating under the combined stresses of modern service conditions. Such aging process causes errors in calibration to creep into the readout of temperature measurement instruments. Therefore, plant operators are required to detect drift in temperature sensor readouts, and to correct for such drift, or replace sensors that are known to have decalibrated. Replacement of such sensors usually requires a scheduled shutdown of the process. Accordingly, the detection of drift in temperature sensor readout is generally not easy. Furthermore, the scheduled shutdown of a process is undesirable and costly.
For example, thermocouple drift is usually caused by trace contaminants which migrate into the thermal element wires, changing their composition. The alteration in composition likewise changes signal output for a given temperature. Changes in such signal output cause an error in temperature readout. Other causes of thermocouple drift include breach of the outer protection sheath, which usually results in deterioration of the electrical insulation. Hence, errors are caused from shunting of the signals being generated by the thermal elements. Drift, or deviations in signal output, occur in mo
Cannon Collins P.
Tolle Charles R.
Watkins Arthur D.
Barlow John
Bechtel BWXT Idaho LLC
Le John
Wells St. John Roberts Gregory & Matkin
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