Measuring and testing – Gas analysis – Moisture content or vapor pressure
Patent
1997-02-26
1998-07-07
Williams, Hezron E.
Measuring and testing
Gas analysis
Moisture content or vapor pressure
73 73, 7333504, 324674, 324689, 338 34, 338314, 422 83, 422 98, G01W 100, G01N 2564, H01L 700
Patent
active
057772061
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The present invention concerns both an instrument for determining the water content of a gas and a method of determining the water content of a gas.
BACKGROUND OF THE INVENTION
A method of measuring condensation point or gas concentration and a device for predicting ice formation are known from German 19 513 274 A1. The known method and device also allow determination of relative and absolute humidity.
The humidity sensor is a capacitive sensor heated by a heating element to a point of maximal sensitivity. This is the point in the particular embodiment where the relative humidity is between 60 and 80%. Relative humidity is then determined by processing two variables, specifically the relative humidity determined by the sensor at operating temperature and the temperature of the sensor itself as determined by a separate temperature probe.
SUMMARY OF THE INVENTION
The object of the present invention is an instrument for and a method of determining the water content of a gas that will allow the determination of humidity from only one variable and will operate more precisely.
This object is attained in accordance with the present invention in the instrument recited in the preamble to claim 1 and in the method recited in the preamble to claim 9 as recited in the body of each of those claims.
Advanced versions and advantageous embodiments are recited in the subsidiary claims.
The instrument in accordance with the present invention employs a single temperature-dependent resistor instead of a heating element and a separate temperature probe. The resistors temperature dependence is accordingly exploited to detect the actual temperature from the change in resistance that accompanies heating. The temperature can accordingly be determined directly, meaning without the transition loss and delay that occur when heat is transmitted from one component to another. Restricting the procedure to a single common temperature-dependent resistor also allows considerable reduction in the size of the overall instrument, which in turn promotes rapid response to any changes due to low heat capacity. This behavior makes it possible to also employ the instrument as a component of a rapid response regulator that will ensure constant humidity.
Thermal coupling of the capacitive sensor to a resistor that is heated to and maintained at a constant temperature will maintain the sensor as well at the same constant temperature. It will be necessary in interpreting the results to take into consideration only one more variable, specifically changes in capacity as a function of humidity. Interpretation can simultaneously be restricted to the interpolation or extrapolation of values determined from prior calibration at the same temperature. Thus the interaction, reproducible at constant temperatures, between capacity and relative or absolute humidity can be exploited for the interpretation. This reproducible interaction can be expressed by a function that is either linear or nonlinear, depending on the type of sensor employed and on the particular operating temperature. The function will represent a measure of relative or absolute humidity. The other humidity parameter in every case, meaning absolute humidity when relative humidity is determined first or relative humidity when absolute humidity is determined first, can be obtained by known methods from the initial result and the partial pressure of the water vapor.
The instrument preferably includes a flat base of highly heat conductive ceramic with the capacitive sensor on one side and the temperature-dependent resistor on the other.
This feature results in a compact structure, whereby the base itself provides electric insulation between the two components while constituting an extensive heat conductor that couples the temperature of the capacitive sensor tight to that of the resistor.
The instrument can also have a thermal insulation wrap that acts as heat insulation although it is discontinuous and moisture permeable on the side the capacitive sensor is mounted on.
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Kahle Gerrit
Schulze Thomas
Zuchner Klaus
Wiggins J. David
Williams Hezron E.
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