Thermal measuring and testing – Temperature measurement – By a vibratory effect
Reexamination Certificate
1999-11-05
2003-02-11
Gutierrez, Diego (Department: 2859)
Thermal measuring and testing
Temperature measurement
By a vibratory effect
C374S119000
Reexamination Certificate
active
06517240
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to a system used for measuring and monitoring temperature using ultrasonic thermometry, and more particularly to a system that measures temperature using solid rods of material that function as the temperature sensor.
Currently, thermocouples are used extensively in the monitoring of industrial process temperatures. Thermocouples are a pair of bare wires, each of which may be in the range of 0.005″ to 0.030″ in diameter, made from two dissimilar metals, joined at one end to form a temperature sensitive electro-voltaic junction. The junction at the tip of the wires is the part of the thermocouple that is used to perform the temperature measurement. The thermocouple assembly, generally constructed as a long, rod-like assembly, is typically from ⅛ inch to 1 inch in overall diameter, with a functional length of from 1 or 2 inches, to several feet. It is partially inserted into the process, the temperature of which is being measured, with the temperature-sensitive junction at the tip being placed in the area of interest in the process. Access to the process is typically obtained via a port in the furnace or reactor in which the process is occurring. The two open ends of the wires at the other end produce a voltage curve that is a function of temperature. The voltage curve produced for a given range of temperatures is primarily a function of the types of dissimilar metals, and the accuracy or calibration of it is primarily a function of the purity of the alloys used to make the thermocouple.
The most widely used thermocouples for measuring high temperatures are platinum/rhodium alloy combinations, theoretically useful for measuring temperatures up to 1,700 C. in oxidizing or inert environments, and tungsten/rhenium alloy combinations, theoretically useful for measuring temperatures up to 2,300 C. in vacuum, inert, or reducing environments. The bare thermocouple wire pairs are usually separated along their length by insulation packing or ceramic beads that are strung along the length of the wires. The thermocouples are often sheathed in a protection tube, and the protection tube is sometimes purged with an inert gas to increase thermocouple lifetime. Both thermocouple types have relatively short lifetimes. They also have increasing end-of-life inaccuracies due to inadvertent oxidation, reduction, or other chemical adulterations to the relatively fine wire alloys or electro-voltaic junction from temperature and process exposure. Additional inaccuracies are introduced as the ceramic insulation becomes increasingly conductive at higher temperatures, thus shunting the small voltage produced by the thermocouple. Thermocouples additionally are susceptible to electromagnetic interference often found in industrial environments, which adds uncertainty and error to such measurements.
As mentioned previously, thermocouple wires are typically in the range of 0.005″ to 0.030″ in diameter. Because of their small physical diameter, they are readily susceptible to chemical attack and performance degradation. Thermocouple wires can be made larger, however the cost at $5 to $35 per inch, depending on type and wire gauge, is prohibitive for industry-standard high temperature platinum/rhodium or tungsten/rhenium wire pairs greater than the mentioned diameter range.
Ultrasonic thermometry can substantially improve the shortcomings of the industry standard thermocouple. A major improvement is to lifetime and stability. This is accomplished by using a solid rod of material to measure the temperature rather than two thin dissimilar metal wires. This bare sensing rod is much more substantive than bare thermocouple wires, being approximately in the range of 0.100″ to 0.250″ in diameter, and is similar in functional length to the thermocouple. The ultrasonic thermometer rod is directly substituted into the same process as the thermocouple that it physically replaces. The larger physical size of the bare temperature-sensing rod material, along with selecting application compatible rod materials or materials resistant to very high temperatures, provide a much greater lifetime than with thermocouples. Additionally, the lower cost of ultrasonic thermometer rod material, in the range of $1 to $4 per inch, is much more favorable than that of the high temperature thermocouple wire that it replaces.
Ultrasonic thermometry relies on the fact that the speed of sound in a solid is a function of temperature. The propagation velocity of ultrasound in a solid material is a function of both the density and the modulus of elasticity of the material, both of which are functions of temperature. In a long, solid rod of material, the mathematical relationship can be expressed as V
e
=SQRT(E(t)/P(t)), where V
e
is the extensional wave velocity in a long rod, SQRT is square root, E(t) is Young's modulus as a function of temperature, and P(t) is density as a function of temperature. This physical phenomenon is the basis of ultrasonic thermometry.
Ultrasonic thermometer systems exist that measure temperature with solid rods of material that function as the temperature sensor. Such systems have used both pulse-echo or continuous wave techniques to measure temperature. Examples of pulse-echo systems and temperature sensing probes are shown and described in U.S. Pat. No. 4,772,131 issued to Varela et al, U.S. Pat. No. 4,483,630 issued to Varela, U.S. Pat. No. 3,597,316 issued to Lynnworth, U.S. Pat. No. 3,540,265 issued to Lynnworth, U.S. Pat. No. 3,633,424 issued to Lynnworth, and U.S. Pat. No. 3,717,033 issued to Gordon et al. These ultrasonic thermometer systems and probes generally function by coupling a short ultrasonic pulse into the probe rod with a transducer. Along the length of the rod, circumferential grooves are cut which reflect some of the ultrasonic energy back to the transducer, thus creating an echo signal. These systems rely upon accurately measuring the time between two return pulses as the representative measure of temperature. Two such reflected or echo signals from two adjacent grooves, or a signal from one groove and a signal from the end of the rod, establish a temperature zone. As the temperature of the zone changes, the transition time of the ultrasonic pulse through the zone also changes, thus providing a measurable indication of average temperature and changes in average temperature of the temperature zone defined by the reflections. A zone may be from less than an inch to many inches in length.
The time position of the return pulses in prior art has been established (a) by comparing the pulses to a threshold that is above the noise level, thus triggering a comparator, (b) by detecting a zero crossing of the reflected signals after crossing a threshold, or (c) by some combination of these techniques. In all cases, these systems attempt to establish the accurate time position of the peaks in the energy of the reflected pulses relative to a clock, but only do so indirectly by either setting a threshold somewhere below the actual reflected energy peak, or by detecting an event like a zero crossing that occurs before or after the actual peak. These systems have inherent inaccuracies in that the peak widths or slopes may change due to external factors not related to the temperature being measured, thereby causing an apparent change in measured temperature, which is actually an error. Such inaccuracies can be caused by environmental thermal effects on the exciting transducer, shifts in the superposition of the composite multi-axis components of the transducer response relative to each other, or electromagnetic noise.
In the known systems described above, no mechanism is provided for conveniently delivering the temperature calibration with the probe when it is installed or replaced by the user of such a system. Nor is any provision made for temperature compensating the probe response due to changes in ambient environmental temperature changes.
Further compromising the performance of prior art ultrasonic thermo
Fendrock Charles
Herb Glenn T.
DuraMetrics, Inc.
Gutierrez Diego
Hale and Dorr LLP
Verbitsky Gail
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