Measuring and testing – Instrument proving or calibrating – Gas or liquid analyzer
Reexamination Certificate
1999-10-21
2001-05-15
Raevis, Robert (Department: 2856)
Measuring and testing
Instrument proving or calibrating
Gas or liquid analyzer
Reexamination Certificate
active
06230543
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to methods for manufacturing electrical humidity detectors, and more particularly to techniques for calibrating such detectors.
Humidity detectors are incorporated into heating, ventilation and air conditioning (HVAC) systems which control the environment within a building. These detectors emit an electrical signal which indicates the level of relative humidity in the zone of the building in which the detector is located. A typical detector comprises a humidity sensor which has a material with an electrical characteristic that varies with variation of humidity to which the sensor is exposed. The sensor is connected to an electrical circuit, often referred to as a transmitter, which normalizes the signal from the sensor and compensates for differences between the actual and ideal electrical characteristics of the sensor.
U.S. Pat. No. 5,408,381 describes one type of a humidity sensor. In this sensor, conductive layers are bounded to opposite faces of a film core. The core is made of a polyimide that has a dielectric constant that varies in a substantially linear manner with humidity. Thus, the capacitance of the sensor changes with the ambient humidity. Because of variations in the polyimide core and manufacturing tolerances, the specific relationship of the capacitance to humidity differs from sensor to sensor and typically differs from the ideal relationship characteristic.
As a consequence of the difference between the actual sensor capacitance and the ideal capacitance, the transmitter circuit to which the sensor is connected provides a mechanism for altering the electrical signal representing humidity in order to compensate for such differences. Conventional transmitters are manufactured with two signal adjustments, offset and gain (or sensitivity). If the output signal from the transmitter is not a linear function of the humidity, a third adjustment may be provided to make the transfer function fit a desired linear characteristic.
During the manufacture of the humidity detector, the electrical signal characteristics are calibrated to match the desired characteristics over the range of relative humidity (RH) with which the detector is intended to operate (e.g. 10-80%). The calibration is performed by adjusting the settings at two widely separated humidity levels within the intended operating range. The procedure requires that the assembled detector be placed within a test chamber that exposes the sensor to an accurately maintained first humidity level for a long enough period for the performance to stabilize. Once the sensor has stabilized, the electrical signal produced by the detector is compared to the ideal signal level indicative of the first humidity level and the offset of the transmitter circuit is adjusted so that the actual signal level matches the ideal.
The gain of the transmitter circuit then is adjusted at a second known humidity level. Because it may take an hour or two for the sensor to stabilize at each humidity level, the detector calibration process is relatively time consuming.
SUMMARY OF THE INVENTION
A humidity detector has a sensor with an electrical characteristic that varies as a function of relative humidity to which the sensor is exposed, and has a transmitter circuit to which the sensor is connected, wherein the transmitter circuit produces an output signal in response to the electrical characteristic of the sensor. The present invention provides a method for calibrating a humidity detector which requires that the sensor be exposed to only a single reference humidity level. This method is predicated on the sensor property that there is a mathematical model of an ideal sensor such that the variability existing in real sensors is related to the model by a constant which multiplies the mathematical model.
That method includes connecting a first reference device to the transmitter circuit in place of the sensor. That first reference device simulates the performance of a model sensor at a first relative humidity level. Preferably the first relative humidity level is chosen to represent a specific value at which the signal offset of the circuit can be adjusted independently of the signal gain. Specifically, the first reference device has an electrical characteristic value which corresponds to the electrical characteristic of the model sensor at the first relative humidity level. The transmitter circuit is adjusted so that the output signal indicates the first relative humidity level. Thereafter, the first reference device is disconnected from the transmitter circuit.
A second reference device then is connected to the transmitter circuit in place of the sensor to simulate performance of the model sensor at a second relative humidity level. Selection of the second relative humidity level is not critical, but preferably should be widely separated from the first relative humidity level. The second reference device has an electrical characteristic value which corresponds to the electrical characteristic of the model sensor at the second relative humidity level. The transmitter circuit is adjusted so that the output signal indicates the second relative humidity level. Thereafter, the second reference device is disconnected from the transmitter circuit.
The sensor then is connected to the transmitter circuit and exposed to a known level of relative humidity. The transmitter circuit is readjusted so that the output signal indicates the known level of relative humidity.
The values of the electrical characteristic for the first and second reference devices can be determined empirically by measuring the performance of a plurality of sensors at the first and second relative humidity levels and then averaging the measurements for each level. Alternatively, a mathematical. model of the relationship between humidity and the electrical characteristic of the sensor can be employed to derive the values of the electrical characteristic for the first and second reference devices.
REFERENCES:
patent: 4800749 (1989-01-01), Merrick
patent: 5205151 (1993-04-01), Shimamura et al.
patent: 5408381 (1995-04-01), Thoma et al.
patent: 5656928 (1997-08-01), Suzuki et al.
patent: 5672806 (1997-09-01), Hung
Linear Technology, LTC1043 Dual Precision Instrumentation Switched-Capacitor Building Block specification document, Not dated.
Froehling Paul H
Hoefert Kurt A
Lewis George
Oman Gary F
Schultz Bruce
Haas George E.
Johnson Controls Technology Co.
Quarles & Brady LLP
Raevis Robert
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