Dew point hygrometers and dew sensors

Thermal measuring and testing – Transformation point determination – Between gaseous and liquid states

Reexamination Certificate

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Details

C374S016000, C374S021000, C324S658000, C324S661000, C324S664000, C073S335040, C073S170170

Reexamination Certificate

active

06575621

ABSTRACT:

FIELD OF THE INVENTION
The present invention concerns dew point hygrometers for determining the dew point of gas and dew sensors for determining the neutral dew condensation. More specifically the present invention concerns hygrometers comprising optic fibers, or hygrometers based on change of capacitance.
BACKGROUND OF THE INVENTION
State of the art hygrometers used in agriculture, notably in greenhouses, operate by usually falling into one of the following three types. The first is a psychrometer which requires a very high degree of maintenance. The second is a relative humidity meter which principle of operation is based on the change of capacity, which is very problematic when determining relatively high humidity. Cheap instruments based on capacitance measuring are unreliable since the reading changes in time independently of changes in the humidity content of the air. The third type of sensors are dew point hygrometers based on optic mirrors, which are very expensive, and unsuitable for routine work at greenhouse conditions since the require constant cleaning of the mirror's surface.
In the past decade, with the availability of thermoelectric coolers and solid state instrumentations the optical condensation type dew point hygrometer has become one of the most accurate and reliable humidity instruments, offering broad dew point range and excellent repeatability.
In the optical dew point hygrometer, a condensation surface which is usually a mirror is cooled by a thermoelectric or Peltier cooler until dew or frost begins to condense on the mirror. The condensation surface is maintained in vapor pressure equilibrium with the surrounding gas, and the amount of condensation on the surface is detected by optical techniques. The temperature of the condensation surface at which the rate of the condensate exactly equals the evaporation, is defined as the dew point temperature. The temperature of the surface when so controlled is typically measured with a platinum resistance thermometer, a thermocouple or thermistor embedded in the mirror surface.
This condensation-type dew point hygrometer is suitable for applications in which a maximum accuracy of the water vapor content is needed over a fairly wide range of dew points, and is suitable for applications in which there is a chance of routine contamination with oils, corrosive gases, salts or similar contaminants that are known permanently damage other types of hygrosensors. Typically, optical dew points are used in industries where precise determination of water vapor in the gas is necessary, such as in pharmaceutical manufacture, electronic, chemical and gas/oil refinery industries, meteorology and food industries, in greenhouses and the like.
One of the main drawbacks of optical condensation-type dew point hygrometers is contamination by materials other than the water condensing on the cooled surface, for example, contamination by various salt solutes. This contamination generally reduces the accuracy of the dew point measurement to a degree which depends on the amount of the contaminant present and its solubility in water. Both soluble and insoluble materials, if allowed to build up on the condensing surface, will eventually cause the system to go out of control because of reduced mirror reflectance. In prior art systems, heating of the mirror to the dry state for manual or automatic rebalancing of the optical detection circuit, overcomes the loop offset problem associated with reduced reflectance, but does not address the problem of measurement error associated with vapor pressure modification as induced by soluble mater. The soluble materials such as salts precipitate out and form a thin layer on the mirror surface. The salts tend to absorb water vapor at temperatures above the dew point and dissolve back into dew layer when the mirror recools. The temperature of the contaminated mirror therefore does not reach the true dew point even after compensating for the reduced reflectance. The resultant dew layer contains salts which cause the saturation vapor pressure to decrease.
Several patents were designed to address this problem. U.S. Pat. No. 3,623,356 is directed to a dew point hygrometer in which there is manual or automatic disabling of the feedback control system which controls the temperature of the mirror, thus forcing the mirror surface to heat to a dry state at which time an additional current is injected into the control loop amplifier at the photodetector bridge circuit. The bridge circuit compensates for changes in the reflective characteristics of the mirror due to accumulation of contamination.
U.S. Pat. No. 4,216,669 periodically interrupts control of the condensing temperature, by periodically cooling the condensing surface (i.e. the mirror) to a temperature well below the prevailing dew point for a time sufficient to provide a heavy growth and coalescence of the condensate so as to dissolve all the soluble material and create a medium by which molecules of solute can migrate. Immediately after cooling the condensing surface is heated to a temperature well above the prevailing dew point so as to cause total evaporation of the solvent (condensate) and recrystallization or precipitation of solute into relatively large clusters or isolated colonies. This leaves most of the area of the condensing surface clear or solid deposits and extends the time period required between mirror cleanings by a factor of 10 to 100 times.
Pieter R. Wiederhold in “
The Cycling Chilled Mirror Dew Point Hygrometer”.
Sensors, July 1966, pp. 25-27 discusses the cycling chilled mirror (CCM) hygrometer, wherein the mirror temperature is lowered at a precisely controlled rate until dew formation is detected. Before the dew sample can form a continuous layer on the mirror, the mirror is heated and the dew on the mirror surface is evaporated. The mirror is therefore almost always (95% of the time) in the drop state and contains a dew, layer for only 5% of the time, when a dew point is made. The measurement cycle is typically once every 20 s. Because dew is present on the mirror surface for only a very short time, contaminant build-up on the mirror is kept at an absolute minimum. Surrounding the mirror is the cylindrical, 40 micron filter. In contrast to the in-line filters used with conventional hygrometer systems, this filter does not require that 100% of the total sample gas pass through its element. Instead, sample gas circulates around the outside of the element and is measured by means of convection across the filter element. Because most particulates circulate freely around the filter and exit the measurement chamber, the filter is slow to become contaminated. This arrangement has a slow response time and is relatively inaccurate.
For high-temperature applications, a model has been developed that uses fiber-optic bundles that isolate the temperature-sensitive electro-optical components from the high-temperature environment.
Prior art also teaches optical dew point hygrometers in which the ends of optic fibers are used as the condensing surface. Use of optic fibers instead of mirrors features the advantage of high resistance to a wide variety of chemicals and considerably decrease the cost of producing said hygrometers. However, the problems of contaminant, due to build up of solid deposits on the surfaces of the optic fibers, is very similar to that encountered in dew point hygrometers wherein mirrors are the condensing surfaces, and is a major obstacle in providing dew point hygrometers that are accurate and reliable over long periods of time.
Another device for sensing humidity is a dew sensor (as opposed to a dew point hygrometer). This sensor in fact mimics the condensation of humidity on a natural surface such as on leaves, without control of the temperature of the sensor, and thus in fact is very reliable since it directly mimics the natural process of condensation. Various measurements have shown a good correlation between the temperature of the leaf and the temperature of the dew sensor at night. Thus, such a sensor is suita

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