Measuring and testing – Liquid analysis or analysis of the suspension of solids in a... – Vapor pressure
Reexamination Certificate
2000-12-07
2001-08-21
Williams, Hezron (Department: 2856)
Measuring and testing
Liquid analysis or analysis of the suspension of solids in a...
Vapor pressure
C073S335060
Reexamination Certificate
active
06276196
ABSTRACT:
FIELD OF THE INVENTION
The present invention is related to techniques for determining the concentration of a vapor in a gas stream and, more particularly, to apparatuses and methods for determining vapor concentration in a gas stream by measuring evaporative cooling in the gas stream.
BACKGROUND
The measurement of the concentration of a vapor in a gas is important in many situations. For example, it is useful to know the concentration of flammable gases in a gas stream in combustion technology. The humidity of air in an area is of interest to people concerned about the weather. In an organic chemical manufacturing facility, monitoring the concentration of vapors of certain volatile liquids in air is critical to the safety of the personnel in the area. Further, to assess the physiological condition of a patient in surgery, an anesthesiologist would want to know the concentration of an anesthetic in a gas stream administered to the patient. The concentration of water vapor in the exhaled air of a person can indicate the functioning condition of the person's respiratory system. The detection of temperature and moisture content of air being inhaled and exhaled will provide valuable information to health care professionals on aerosol therapy and toxicology of toxic gases inhalation.
Vapor concentration sensors based on measuring the mass of vapor absorbed on polymer films coated on surface acoustic wave devices have been developed. For example, Jay W. Grate and Mark Kluxty,
Anal. Chem
., vol. 63, pp 1719-1727 (1991), describe a humidity sensor in which vapor absorption changes the frequency of oscillation of mass-sensitive resonators. Also, Polymer-based impedance effect humidity sensors are disclosed by S. Tsuchitani et al. in “A humidity sensor using ionic copolymer and its application to a humidity—temperature sensor module,”
Sensors and Actuators
, Vol. 15, No. 4, pp 375-386, 1988. In the Tsuchitani humidity sensors, moisture absorption by ionic copolymers causes a change in impedance in an electrical circuit, thereby causing a change in oscillation frequency. However, vapor concentration sensors by vapor absorption are not very specific and are subject to interference by any absorbable vapor that has not been present in samples used for the calibration of the vapor absorption sensors. Moreover, such vapor sensors do not work well near the condensation point because they may not respond to a fall in humidity quickly. Therefore, a need exists for a highly specific vapor concentration sensor that will function over a wide range of concentrations.
Humidity sensors have been used for many years to determine air humidity for weather reporting. For such applications, one simple kind of humidity sensor has a dry bulb thermometer and a wet bulb thermometer. The wet bulb thermometer has a thermometer with a bulb moistened by a wick. Generally water passes by capillary action against gravity up the wick from a container. Water evaporates from the wick when the air is unsaturated with respect to water vapor. Due to the cooling effect of water evaporating from the wick, the temperature of the wet thermometer will be lower than the true temperature of the air had there been no evaporation. The temperature of the wet thermometer is known as the “wet-bulb temperature.” The temperature that is measured by a dry thermometer, known as the “dry-bulb temperature,” and the wet-bulb temperature are used to determine the humidity in air. See, for example, McCabe and Smith,
Unit Operations of Chemical Engineering
, McGraw-Hill, Ch. 24, 3rd ed., (1956). Such humidity sensors tend to be large. Their response time is typically not very fast.
More recently, moisture sensors employing micro-thermocouple sensors for determining temperature and relative humidity in airstream have been reported, for example, in “Design and development of a micro-thermocouple sensor for determining temperature and relative humidity patterns within an airstream,”
J Biomechan. Eng
. Vol. 111, PP. 283-287, Nov. 1989. In such a device, a wet-bulb thermocouple junction is coated with a sprayed-on boron nitride coating, which is reported to be hard and porous. A sleeve is used to supply water to the boron nitride coating. It would appear that coating a thermocouple junction by spraying is not an easy task and one has to take special care to position the sleeve precisely to wet the boron nitride coating without leakage. It is also difficult to form a boron nitride coating that is stable on metal or glass surfaces. Moreover, to get a porous structure suitable for conducting water adequately one needs to form a boron nitride layer that is quite thick, making it brittle and slow to transfer heat.
Therefore, a need exists for a vapor concentration sensor that is relatively simple to construct, and particularly for a vapor concentration sensor that is sturdy. Recently, we reported a vapor sensor employing micropores, see U.S. patent application Ser. No. 08/878,566, “THERMOMETRIC APPARATUS AND METHOD FOR DETERMINING THE CONCENTRATION OF A VAPOR IN A GAS STREAM,” filed on Jun. 19, 1997, which is incorporated by reference in its entirely herein. However, there is still a need for a vapor sensor that is rugged, simple to make, and can be produced in a small size.
SUMMARY
In one aspect, the present invention provides a sensor for sensing the concentration of a vapor of a vaporizable liquid in a gas stream. An embodiment of the sensor includes a micropore which has an opening into the gas stream. The micropore has an evaporation end at the opening into the gas stream and a lumen which conducts the vaporizable liquid from a supply of the liquid to the opening for evaporation at the evaporation end. A wet-transducer temperature sensor (or simply “wet temperature sensor”) capable of sensing temperature at the evaporative end of the micropore. The wet temperature sensor has a heat sensitive part in contact with the liquid in the micropore near the opening. That heat sensitive part circumscribes the micropore and forms part of the lumen. When the liquid evaporates, the latent heat of evaporation is absorbed from the gas stream and the surroundings thereof, resulting in the cooling of the vicinity of the wet temperature sensor. Such heat loss when the wet temperature sensor wet with the liquid is placed in the gas stream will result in the temperature sensed by the wet temperature sensor being lower than the non-evaporative temperature of the gas stream. This lowering in temperature can be measured to determine the concentration of the vapor in the gas stream.
To determine the lowering in temperature described above, in an embodiment, a dual-transducer temperature sensor is provided to compare the wet-transducer temperature to the gas stream temperature. In such a dual-transducer temperature sensor, a wet-transducer temperature sensor senses the temperature at the evaporative surface and a reference temperature sensor senses the gas temperature without evaporation as a reference.
In an embodiment, multiple micropores can be included to increase the area of evaporation such that steady state temperature can be achieved for determining the vapor concentration quickly. Due to the ability to form micropores of uniform size and shape, and to form small temperature sensors with thin layers of material, fast heat transfer can be achieved to enable fast response time for vapor sensing. Since the micropores are small, capillary action can hold the liquid in the micropores even when the temperature sensors are turned in different orientations. Thus, with the present invention, a fast vapor concentration sensor can be made, even for applications that require small dimensions and independence to the position relative to gravity.
The sensor of the present invention is advantageous over conventional sensors with woven wicks. First, regarding woven wicks, it is difficult to form a woven material that can wrap uniformly around a temperature sensitive unit such as a thermistor head or thermocouple junction to provide adequate liquid without dripping. Also,
Greenstein Michael
Lum Paul
Mauze Ganapati R.
Melton, Jr. Hewlett E.
Agilent Technologie,s Inc.
Politzer Jay L.
Williams Hezron
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