Patent Publication Number: US-2002005068-A1

Title: Capacitive sensor condensation-type hygrometer

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
     [0001] This application claims priority to U.S. Provisional Application Ser. No. 60/203,554, filed on May 11, 2000. 
    
    
     
       BACKGROUND  
       [0002] Hygrometers may be used to measure the dew point of air, which indicates the amount of moisture in the air. This information may be used for weather observation and forecasting, and for environmental control in industrial applications.  
       [0003] Chilled mirror hygrometers employ an optic sensor to determine the dew point of the ambient air. A metallic mirror is chilled using a thermoelectric heat pump until dew just begins to form. A beam of light, typically from a solid-state light emitting diode (LED), is aimed at the mirror surface and a photodetector monitors the reflected light. As dew drops form on the mirror surface, the reflected light is scattered, which decreases the output of the photodetector. The output of the detector controls the thermoelectric heat pump, forming a feedback system which may be used to bring the mirror to the temperature at which the water on the mirror surface is in equilibrium with the water vapor pressure in the air above the mirror, i.e., the dew point temperature.  
       [0004] Chilled mirror hygrometers provide precise humidity measurements and may be useful in extreme operating environments. However, because chilled mirror hygrometers depend on the optical characteristics of the mirror, they may be sensitive to contamination.  
       SUMMARY  
       [0005] A hygrometer according to an embodiment includes a capacitive sensor. The capacitance of the sensor changes in response to water or ice forming on its surface. A controller controls a heat pump, e.g., a thermoelectric module, to cool the sensor until dew (or frost) forms on a surface of the sensor. The controller then controls the heat pump to heat or cool the sensor in response to a sharp change in the capacitance of the sensor, which occurs at the dew (or frost) point temperature of the ambient air, until the sensor reaches equilibrium at the dew (or frost) point.  
       [0006] The capacitive sensor may include an array of interdigitated electrodes deposited or otherwise formed on a thermally conducting and electrically insulating substrate, for example, a glass or sapphire substrate.  
       [0007] The hygrometer may include a thermistor to measure the temperature of the sensor and a thermistor to measure the temperature of the ambient air. A bridge circuit may be connected between the thermistors and used to determine the difference in temperature between the sensor and the ambient air. The controller may use this information to calculate the relative humidity of the ambient air.  
       [0008] The controller may use a capacitive bridge circuit coupled between the sensor and a reference capacitor to monitor changes in the capacitance of the sensor. 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0009]FIG. 1 is a perspective view of a capacitive sensor hygrometer according to an embodiment.  
     [0010]FIG. 2 is a plan view of an electrode array according to an embodiment.  
     [0011]FIG. 3 is a schematic diagram of a capacitive sensor hygrometer including a controller according to an embodiment.  
     [0012]FIG. 4 is a flowchart illustrating a dew point measurement operation according to an embodiment.  
     [0013]FIG. 5 is a schematic diagram of a capacitive sensor hygrometer including a controller according to an alternative embodiment. 
    
    
     DETAILED DESCRIPTION  
     [0014] A hygrometer  100  according to an embodiment includes a capacitive sensor  102 , as shown in FIG. 1. The capacitive sensor includes an array of interdigitated electrodes  104  formed on a substrate  106 . The substrate  106  is made from a material that is electrically insulating and thermally conducting, for example, glass or sapphire. The electrodes  104  may be formed from a metal film, e.g., copper, that is deposited on the substrate  106  and is patterned and etched using semiconductor processing methods. FIG. 2 illustrates an exemplary electrode array  200 .  
     [0015] The sensor  102  and a sensor thermistor  108  are attached to a thermoelectric module  110 . The sensor and thermistor may be attached to the thermoelectric module  110  with a thermally conductive epoxy such that the electrodes  104  and the thermistor  108  remain at the same temperature. Alternatively, the thermistor may be attached to the top surface of the substrate  106  with the electrodes  104  to ensure that the sensor and thermistor remain at close to the same temperature.  
     [0016] The thermoelectric module  110  operates as a bi-directional heat pump, and may be used to alternately heat and cool the substrate. The thermoelectric module may include one or more pairs of bismuth-telluride pellets  112  sandwiched between two ceramic plates  114 . Each pellet pair includes an n-type pellet and a p-type pellet. Although current travels up one pellet and down the other, the carrier current travels in the same direction in both pellets. Thus, heat may transported in one direction in both pellets, creating a “thermally isolated” cold junction at one of the ceramic plates  114 . Alternating the direction of the current alternates the direction of heat flow.  
     [0017] A sensor module  120 , including the electrodes  104 , substrate  106 , sensor thermistor  108 , and thermoelectric module  110 , is attached to a heat sink  130  to absorb heat from and provide thermal energy to the thermoelectric module. A control thermistor  140 , thermally isolated from the sensor module  120  and heat sink  130 , is used to measure the temperature of the ambient air.  
     [0018] The capacitive sensor hygrometer  100  is connected to a controller  300 , as shown FIG. 3, and may be used to perform a humidity measurement operation  400  according to an embodiment, as shown in FIG. 4. The controller  300  monitors the capacitance of the sensor  102  and controls the temperature of the thermoelectric module  110  in response to that measurement. The thermistors  108  and  140  may be connected by a bridge circuit  310 . The controller is connected to one or both of the thermistors in order to monitor the ambient and/or sensor temperature. The controller is also connected to the bridge to monitor the temperature difference between the thermistors.  
     [0019] To perform the humidity measurement operation  400 , the controller  300  chills the sensor  102 . As the sensor  102  cools, dew droplets (or frost particles) begin to form on the surface of the sensor module, between the interdigitated electrodes. An equilibrium state is achieved when the surface of the sensor module is at a temperature at which a layer of condensed water or ice first forms and covers the sensor surface. For temperatures above 0° C., the equilibrium temperature is the dew point temperature. At this temperature, the water layer formed on the sensor surface is in equilibrium with the air directly above the sensor, which is saturated, i.e., holding the maximum amount of water vapor possible at the existing temperature and pressure. For temperatures below 0° C., the equilibrium temperature is referred to as the frost point.  
     [0020] The capacitance of the sensor is very sensitive to the condensed layer on its surface because the dielectric constant of water (about 88 at 0° C.) is very high compared to air (about 1 at 0° C.). The equilibrium temperature is the temperature at which the condensed water or ice layer first fully covers the area between the electrodes  104 . At this point, the capacitance of the sensor rises sharply.  
     [0021] The controller  300  monitors the capacitance of the sensor (block  404 ). Upon passing the equilibrium temperature, indicated by the sharp rise in capacitance, the controller may control the thermoelectric module  110  to alternately heat and cool the sensor  102  until the sensor is in the equilibrium state (block  406 ). To improve the feedback response of the controller  300  and the thermoelectric module  110 , the substrate  106  should have a high thermal conductivity to prevent lag in the transfer of heat from the thermoelectric module at the bottom of the substrate to the top surface of the substrate and the electrode array  200 .  
     [0022] The controller  300  determines the difference in temperature between the ambient air and the sensor  102  by the information provided by one or both of the thermistors  108 ,  140  and the bridge circuit  310  (block  408 ). The thermistors  108  and  140  provided the absolute temperature of the sensor and the ambient air, respectively, and the bridge circuit  310  provides in the differences between the resistances of the thermistors  108  and  140 . The intrinsic thermistor differences may be corrected using a measurement made at thermal equilibrium. Using the bridge circuit to determine the temperature difference between the ambient air and the sensor may provide a more accurate measure of the difference in temperature between the two thermistors.  
     [0023] The controller  300  may then calculate the relative humidity (RH) from the temperature information using tables and/or equations stored in a memory  320  (block  410 ).  
     [0024] The capacitive sensor hygrometer  100  may be provided on a semiconductor chip, the bulk of which serves as heat sink  130 , using semiconductor fabrication methods. Such an integrated capacitive sensor hygrometer may be have side dimensions on the order of 100 μm or smaller. Reducing the size of the sensor may reduce the response time and increase the sensitivity of the sensor. However, miniaturization of the sensor  102  may also decrease the magnitude of the signal generated by the electrode array and used by the controller to achieve the equilibrium state. For smaller sensor modules, the sensitivity of the system may be increased by coupling a reference capacitor  510  to the electrode array with an AC capacitive bridge circuit  520 , as shown in FIG. 5. The controller  300  can measure small changes in the signal generated by the electrode array by comparing the changing capacitance of the sensor to the known capacitance of the reference capacitor and control the temperature of the thermoelectric module  110  in response to that comparison.  
     [0025] The capacitive sensor hygrometer  100  may be used in demanding conditions over a wide range of ambient pressures and temperatures. Since the hygrometer monitors electric signals rather than optic signals, as in a chilled mirror hygrometer, the sensor  102  can operate with a fairly thick layer of ice on the electrodes  104  because the capacitance of the sensor is less sensitive to surface contamination than the optical characteristics of the mirror in the chilled mirror hygrometer. Also, a thin protective layer, e.g., a polymer film, may be provided over the sensor  102  without significantly affecting the sensitivity and accuracy of the hygrometer  100 .  
     [0026] A number of embodiments have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the invention. For example, steps of the humidity measuring operation may be performed in a different order and still achieve desirable results. Accordingly, other embodiments are within the scope of the following claims.