Thermal measuring and testing – Temperature measurement – By electrical or magnetic heat sensor
Reexamination Certificate
1999-05-19
2001-06-12
Bennett, G. Bradley (Department: 2859)
Thermal measuring and testing
Temperature measurement
By electrical or magnetic heat sensor
C374S173000, C338S025000
Reexamination Certificate
active
06244744
ABSTRACT:
BACKGROUND
In 1821, the English chemist Sir Humphrey Davy discovered that all metals have a positive temperature coefficient of resistance. This discovery led to the development of the resistance temperature detector (RTD), which today is a widely used device for the measurement of temperature in industrial processes.
The quantum mechanical explanation for this effect is that the metal nuclei vibrate with an amplitude that depends on the temperature of the metal. These nuclear vibrations generate phonons within the metal, the energy of which depend on the amplitude of the nuclear vibrations. When electrons flow through the metal, they tend to be scattered by these phonons. This interaction, which disrupts the smooth flow of electrons through the metal, is what we measure as resistivity. As the temperature of the metal increases, the nuclear vibrations increase in amplitude and generate higher energy phonons. These higher-energy phonons scatter electrons more effectively, thereby increasing the resistivity of the metal.
An RTD is typically made by wrapping a length of metal wire around a ceramic bobbin, or by depositing a thin film of metal on a substrate. Generally, a metal having high resistivity is used, so as to minimize the amount of metal required. Because of its resistance to contamination and its stable and predictable temperature coefficient, a commonly used metal is platinum.
To measure temperature with an RTD, one exposes the RTD at the site whose temperature is of interest and allows the RTD to reach thermal equilibrium. One then passes a known current through the RTD. Preferably, this current is relatively small to minimize measurement error arising from ohmic heating of the metal in the RTD. One then measures the voltage across the RTD. From the known current and the measured voltage, one can calculate a resistance whose value is indicative of temperature.
In practice, the RTD is often physically inaccessible. For example, the RTD might be placed deep in a caustic chemical bath, remote from the measurement instrumentation. As a result, extended wire leads are generally required to connect the RTD to a voltmeter. In such cases, the resistance measured by the voltmeter is the sum of the RTD resistance and the lead resistance associated with the extended wire leads. This lead resistance introduces error in the measurement.
To the extent that the lead resistance is much smaller than the RTD resistance, the fact that the RTD resistance measurement is corrupted by the lead resistance results in only a small error. However, in most RTDs, even a small change in the RTD resistance translates to a significant change in temperature. For example, in the case of a platinum RTD, the resistance is 100 ohms at 0 degrees C. and changes by 0.00385 ohms per ohm degree C. Hence, a 100-ohm RTD changes its resistance by only 0.385 ohms per degree of change in temperature. Thus, a lead resistance as small as two ohms results in a five degree C. measurement error. Given that the leads to the RTD in many industrial applications can be as much as half a mile long, it is easy to see how the lead resistance can significantly reduce the accuracy of the temperature measurement.
To avoid this difficulty, it is known to provide a first pair of leads extending from the current source to the RTD and a second pair of leads extending from the RTD to the voltmeter. In this configuration, referred to as the four-wire RTD interface, the lead resistance between the voltmeter and the RTD does not introduce an error because there is no current in those leads. Although the four-wire RTD interface eliminates the effect of lead resistance, it does so at the cost of doubling the length of wire required.
Another known method of eliminating the effect of lead resistance is the three-wire RTD interface. This interface uses a first current source connected to a first terminal of the RTD by a first lead and a second, identical current source connected to the second terminal of the RTD by a second lead. A third lead connected to the second terminal of the RTD provides a return path for the current provided by both current sources. In the three-wire RTD interface, one can subtract the voltage measured at the second lead from the voltage measured at the first lead to obtain the voltage across the RTD. However, the accuracy of the three-wire RTD interface relies heavily on the two current sources being identical. As a practical matter, it is difficult to provide two current sources that perform identically.
It is thus an object of the invention to provide an interface for measurement of temperature with an RTD that eliminates the effect of lead resistance without using excessive lengths of wire and without relying on two identical current sources for accurate temperature measurement.
SUMMARY
In the RTD interface of the invention, a single current source generates two different measurements at two different times over across two different pairs of leads. These two different measurements are combined to eliminate the effect of lead resistance in the wire leads connecting the RTD and the interface.
The RTD interface of the invention is attached by three leads to an RTD having first and second terminals. The first lead connects to the first terminal; the second and third leads connect to the second terminal. The RTD interface includes a current source connected to a switch having a first position and a second position. In the first position, current from the current source is directed to the first lead and diverted from the second lead. Conversely, in the second position, current from the current source is directed to the second lead and away from the first lead. In response to a signal from a processor, a switch controller selectively sets this switch to the first position during a first measurement interval and to the second position during a second measurement interval.
The first and second leads are also connected to the terminals of a voltage measuring element which evaluates a first voltage difference between the leads during the first measurement interval and a second voltage difference between the leads during the second measurement interval. The voltage measuring element includes an output terminal carrying a signal indicative of the voltage difference between the first and the second lead. This output terminal is connected to the input of a processor which processes the first and second voltage differences to obtain the RTD voltage.
The switch can be a relay switch or a solid-state switch, however any switch that can direct the current from the current source in the manner described above can be successfully employed in the invention.
The switch controller is typically a microprocessor. Use of a microprocessor is advantageous because the same microprocessor can be used to process the output of the voltage measuring element. However, the switch can also be controlled manually or by a clocking circuit.
The voltage measuring element can be a differential amplifier having a first input connected to the first lead and a second input connected to the second lead. When a microprocessor is used to process the output of the voltage measuring element, it is advantageous to provide an analog-to-digital converter between the output of the differential amplifier and the input to the microprocessor.
The processor is typically a microprocessor. However, any circuit that can evaluate the sum of two voltage differences taken during different measurement intervals can be used to process the output of the voltage measurement element to remove the effect of the lead resistance. When used in the context of a feedback control system, particularly one in which temperature is a controlled variable, it is convenient to use the microprocessor associated with the controller.
The invention also includes a method of determining the voltage across an RTD by taking two measurements during two different measurement intervals. During the first measurement interval, a known current is directed through a first lead connected to the first termina
Bennett G. Bradley
Foley Hoag & Eliot LLP
Liepmann W. Hugo
Oliver Kevin A.
Verbitsky Gail
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