Johnson noise, or thermal noise, is the noise produced by thermal agitation of charges in a conductor. The available thermal noise power produced in a resistance is independent of the resistance value and is proportional to the absolute temperature and the frequency bandwidth over which the noise is measured.
Johnson noise thermometers have been used to measure absolute temperatures, and have the advantage that they are independent of the materials or the past history of the sensing resistors. Moreover, they are drift free and are characteristically linear thermometers.
Johnson noise thermometers are not, however, without drawbacks. For example, they require highly sensitive, low-noise wide-band amplifiers and are subject to electromagnetic (non-thermal noise) interference. They are also susceptible to microphonic noise generated by vibration and are affected by characteristics of their lead wires and require an appreciable time for noise signal integration to obtain an accurate indication of temperature.
Johnson noise thermometers have been used to measure temperature in nuclear reactors, as well as in nuclear fuel irradiation tests and for in situ calibration of resistance thermometers. The use of Johnson noise thermometers in space nuclear applications requires that the thermometer operate unattended, reliably, and with good accuracy (&lt;1% uncertainty) at high temperatures (about 1400K) for at least 7 years at full power. Requirements on the sensor include compatibility with space nuclear environments, modest radiation resistance, vibration and EMI resistance, continuity reliability, and relatively fast time response (&lt;8 seconds).
A remotely operated Johnson noise thermometer system can provide very long term drift-free temperature indications since it is unaffected by changes in the sensor's resistance, within limits. However, the delay in acquiring an accurate temperature, due to the need to integrate a noise signal for a fixed period of time (e.g. 1 second or 10 seconds) limits the value of the noise measurement for fast control systems.
It is expected that a high-temperature resistance thermometer would read inaccurately due to shunting of the measuring current through the insulation material which supports and insulates the metal resistance winding and due to changes in the sensor's properties after long time exposure to high temperatures. Any effects of long term drift of the sensor would be corrected by the use of the Johnson noise temperature measurement. In most applications, this problem of insulator shunting is less significant for the Johnson noise thermometer than it is for the DC resistance thermometer.
Long term drift free operation requires that the gain, bandwidth, and noise contribution of the noise signal processor be validated. Conventional methods for validating the characteristics of the noise signal processor in such locations as the laboratory where the processor is accessible require that the sensor be disconnected from the signal processor and that it be replaced by known noise signal sources. In remote applications, switches could perform the signal substitution, but most switches used for this purpose that could control the low-level input signals to the noise signal processor are either noisy or unreliable or both.
U.S. Pat. No. 3,878,723 to Blalock et al. describes a thermal noise power thermometer in which temperature is determined as a function of thermal noise voltage and thermal noise current sensed by a resistor. Both voltage and current are sensed by two preamplifiers.
U.S. Pat. No. 4,099,413 to Ohte et al. describes a thermal noise thermometer. A resistor is supplied with shot noise which produces a DC and an AC output.
U.S. Pat. No. 4,627,744 to Brixy et al. describes a thermometer which uses a resistive, Johnson noise element and a thermocouple.
In spite of the above-noted systems of the prior art, a continuing need exists for an improved thermometer system which is highly accurate and reliable over long periods of time.
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