Abstract:
The present invention has to do with a method and system for a high precision electronic psychrometer operable at low temperatures and high humidity environments. The electronic psychrometer includes thermistors for measuring wet and dry bulb temperatures and a wicked cage surrounding one of the thermistors. The wicking action of the wicked cage is controlled by an evaporation controller in conjunction with the wick&#39;s physical parameters. The electronic psychrometer determines relative humidity and provides a readout display and/or a control signal.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application is related to, claims the earliest available effective filing date(s) from (e.g., claims earliest available priority dates for other than provisional patent applications; claims benefits under 35 USC §119(e) for provisional patent applications), and incorporates by reference in its entirety all subject matter of the following listed application(s) (the “Related Applications”) to the extent such subject matter is not inconsistent herewith; the present application also claims the earliest available effective filing date(s) from, and also incorporates by reference in its entirety all subject matter of any and all parent, grandparent, great-grandparent, etc. applications of the Related Application(s) to the extent such subject matter is not inconsistent herewith: 
     U.S. provisional patent application 62/043,746 entitled “Electronic Psychrometer and/or Humidistat with Low Temperature and High Humidity Capability”, naming Christopher W. Callahan as inventor, filed 29 Aug. 2014. 
    
    
     BACKGROUND 
     1. Field of Use 
     This invention relates to an improved apparatus fir measuring relative humidity. More specifically, the invention relates to a high precision electronic. Psychrometer operable at low temperatures and high humidity environments. 
     2. Description of Prior Art (Background) 
     In general a psychrometer is an instrument consisting of two thermometers which are used in the measurement of the moisture content, or relative humidity (RH) of air or other gases. The bulb or sensing area of one of the thermometers either is covered by a thin piece of clean muslin cloth, or other wick material, wetted uniformly with distilled water or is otherwise coated with a film of distilled water. The temperatures of both the bulb and the air contacting the bulb are lowered by the evaporation which takes place when unsaturated air moves past the wetted bulb. An equilibrium temperature, termed the wet-bulb temperature will be reached; the equilibrium temperature closely approaches the lowest temperature to which air can be cooled by the evaporation of water into the unsaturated air. Moisture parameters, such as relative humidity and dew-point temperature, can be evaluated from the wet- and dry-bulb measurements by means of psychrometric tables and generally accepted closed form formulae for calculating water/air mixtures. 
     Relative Humidity (RH) is a measure of the degree to which air is saturated with water compared to the highest level of saturation at a given temperature. This is a ratio of the partial pressure (proportional content) of water in air at the actual conditions to the partial pressure of water in air at saturation (100% RH). Partial pressures of water in air are related to temperature. 
     The traditional method for determining RH is to use a manual sling Psychrometer which has two thermometers, one with a dry bulb and one with a wet bulb. The dry bulb thermometer is typical of thermometers in use in other applications and simply measures the air temperature. The wet bulb thermometer has a water saturated wick around it. When the thermometer is swung in the air to move air over the wet bulb, evaporation of water from this wick depresses the temperature of the bulb to a degree that corresponds to the saturation partial pressure of water in the air at the dry bulb temperature. Comparison of these two temperatures can provide an indirect measure of RH. 
     However, the long-term (6-12 month) storage of crops requires control of both storage temperature and humidity. Storage temperature is depressed to 32-40 degrees F. (crop dependent) in order to minimize the rate of respiration in the crops. Humidity is generally raised to 80-98% RH to reduce desiccation yet still avoid liquid water condensation on the crops. In recent field research pertaining to improved crop storage methods, it has been determined that there is a lack of suitable equipment for humidity measurement and control at low storage temperatures and high humidity. 
     The vast majority of humidity sensing equipment available is based on moisture absorbing, materials whose capacitance changes depending on the material moisture content. These sensors tend to have a precision of +/−2% RH from 20-80% RH at 70 degrees F., but then lose precision in the higher RH range and lower temperature range, straying, to +/−5% RH. It is this range that is most needed by those storing winter crops. Some sensors exist which demonstrate +/−2% RH precision up to 98% RH. But in all of these sensor types, excursions to 100% RH results in reduced precision and accuracy and can cause a mechanical failure or a need for recovery (heat and dry) in order to reuse the sensor. Additionally, these sensors may also suffer an unrecoverable electronic failure. 
     Thus, there is a technical challenge which exists in the measurement of high humidity in low temperature conditions; and, therefore control of equipment (e.g., humidifiers, dehumidifiers) based on these measurements. 
     BRIEF SUMMARY 
     The foregoing and other problems are overcome, and other advantages are realized, in accordance with the presently preferred embodiments of these teachings. 
     An electronic device for measurement of dry bulb and wet bulb space temperatures is disclosed. A microprocessor contains necessary software to calculate relative humidity from the dry and wet bulb space temperatures and to adjust an output as necessary to control humidity and/or temperature of the space. 
     The invention is also directed towards an electronic psychrometer having wet and dry temperature sensors, wherein the temperature sensors are substantially 10 k Ohm+/−0.05 deg. C. thermistors. The invention also includes a fan-less evaporator cage surrounding the thermistors, wherein the evaporator cage, or wick, comprises pick dimension P, wherein pick dimension P is the number of carrier crossings per longitudinal inch of the evaporator cage. Also included is a programmable controller and a computer readable medium, operatively coupled to the programmable controller. The computer readable medium contains a set of programmable controller instructions that, if executed by the programmable controller, are operable to: calibrate the wet and dry temperature sensors; and determine relative humidity with an accuracy of substantially +/−1% RH at 32 degrees F. 
     In accordance with one embodiment of the present invention an electronic psychrometer is provided. The electronic psychrometer includes a dry temperature sensor and a wet temperature sensor. An evaporator cage surrounds the at least one wet temperature sensor, wherein the evaporator cage comprises pick dimension P, wherein pick dimension P is the number of carrier crossings per longitudinal inch of the evaporator cage. Also include is a programmable controller and a computer readable medium, operatively coupled to the programmable controller. The computer readable medium contains a set of programmable controller instructions that, if executed by the programmable controller, are operable to determine relative humidity with an accuracy of substantially +/−1% RH at 32 degrees F. 
     The invention is also directed towards a method for calibrating an electronic psychrometer. The method includes providing a reference fluid having a known temperature. The method also includes providing wet and dry temperature sensors. The wet and dry temperature sensors are immersed or enveloped within the reference fluid and the temperatures reported by the sensors is compared to the known temperature of the reference fluid. A calibration temperature offset is determined from the difference between the reported temperatures and the known temperature. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The subject matter which is regarded as the invention is particularly pointed out and distinctly claimed in the chums at the conclusion of the specification. The foregoing and other objects, features, and advantages of the invention are apparent from the following detailed description taken in conjunction with the accompanying drawings in which: 
         FIG. 1  is a pictorial illustration of an electronic psychrometer system in which the invention is implemented; 
         FIG. 2  is a pictorial illustration of a self-ventilating and adjustable ventilation cover plate in accordance with the invention shown in  FIG. 1 ; 
         FIG. 3  is a pictorial illustration of a system of psychrometer systems in accordance with the invention shown in  FIG. 1 ; and 
         FIG. 4  is an illustration of one method for calibrating the wet/dry thermistors in accordance with the invention shown in  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION 
     The following brief definition of terms shall apply throughout the application: 
     The term “comprising” means including but not limited to, and should be interpreted in the manner it is typically used in the patent context; 
     The phrases “in one embodiment,” “according to one embodiment,” and the like generally mean that the particular feature, structure, or characteristic following the phrase may be included in at least one embodiment of the present invention, and may be included in more than one embodiment of the present invention (importantly, such phrases do not necessarily refer to the same embodiment); 
     If the specification describes something as “exemplary” or an “example,” it should be understood that refers to a non-exclusive example; and 
     If the specification states a component or feature “may,” “can,” “could,” “should,” “preferably,” “possibly,” “typically,” “optionally,” “for example,” or “might” (or other such language) be included or have a characteristic, that particular component or feature is not required to be included or to have the characteristic. 
     As noted earlier psychrometry is the principle whereby the measurement of a gas (often air) humidity is determined from simultaneous dry bulb thermometer and wet bulb thermometer measurements. The dry bulb thermometer measures the temperature of the gas. The temperature of the wet bulb thermometer depends on both the dry bulb temperature (e.g., ambient temperature) and humidity of the gas. The rate of evaporation of water from the wet bulb thermometer depends on the amount of water vapor present in the surrounding gas. The temperature of the wet bulb thermometer results from a balance between the evaporative cooling and convective heating by the ambient gas flows. 
     Wet-bulb and dry-bulb temperatures are digitally measured and relative humidity measurement proceeds by standard psychrometric equations. Water vapor pressure is estimated from the wet bulb and dry thermometer temperatures using the psychrometric equation,
 
 e=e   s ( t   w )−γ( t   d   −t   w )  eq. 1
 
where e is the vapor pressure, e s  (t w ) is the saturated vapor pressure at the wet bulb temperature (t w ), t d  is dry bulb temperature, and γ=0.660 (mb/° C.) when barometric pressure is 1000 mb.
 
     Relative humidity is the ratio of actual water vapor present in gas to the maximum quantity which could saturate at the gas temperature. Thus, relative humidity (RH) is given by:
 
RH=100 e/e   s ( t   d )  eq. 2
 
     Referring now to  FIG. 1 , there is shown a diagram layout of an electronic psychrometer system  100  in which the invention is implemented. Included within the system  100  is remote temperature differentiator housing  11 . Differentiator housing  11  includes wet bulb temperature sensor  13 , dry bulb temperature sensor  12 , wick  17 , and evaporation controller  14 . Also, shown in  FIG. 1  is optional fan  500 . 
     Still referring to  FIG. 1 , wet bulb temperature sensor  13  is a high accuracy negative temperature coefficient (NTC) thermistor (e.g., 10 kOhm+/−0.05 deg C.: US Sensor # PR103J2). It will be understood that temperature sensor  13  is referred to as a wet “bulb” temperature sensor and that the term bulb is common language stemming from sensors using liquid thermometers. Similar to web bulb temperature sensor  13 , dry bulb temperature sensor  12  is also a high accuracy NTC thermistor. It will be appreciated that wet and dry bulb sensors  12 ,  13  may be substantially matched (electrical characteristics) NTC thermistors or offset (electrical characteristics) by a predetermined amount. In alternate embodiments the thermistors may be high accuracy positive temperature coefficient (PTC) thermistors, thermocouples (TC), or resistive thermal devices (RTD). 
     Still referring to  FIG. 1 , housing  11  may be any suitable shape or size to facilitate the balance between the evaporative cooling and convective heating by the ambient gas flows discussed earlier. It will be appreciated that in alternate embodiments the color of the housing  11  may be chosen to exploit air mixing by thermal or solar radiation. For example, the housing  11  may be colorized black to increase the internal ambient temperature and further facilitate the balanced discussed herein. Housing  11  may also be variably colorized to promote heating effects within one section of housing  11  and cooling effects in another section, thereby promoting convective air flow through the housing  11 . Likewise, housing  11  may be a lighter color throughout to minimize solar heating by solar radiation. In alternate embodiments convective flow through housing  11  may be induced or facilitated by a heater resistor. It will be appreciated that the dimensions and characteristics (e.g., color) may be incorporated, and/or accounted for by controller  19  discussed herein. 
     Housing  11  also contains evaporation controller  14 . Evaporation controller  14  exerts pressure on wick  17  at point  17 A which controls the flow of moisture from reservoir  15 , along wick  17 , through evaporation controller  14  to be evaporated into the interior chamber  11 A of housing  11 . It will be appreciated that evaporation controller  14  works cooperatively with the characteristics of wick  17  to control the evaporation into the interior of housing  11 . For example the pick dimension P, or Picks per inch—is the number of carrier crossing points per longitudinal inch of wick  17 . Pick dimension P may be any suitable pick dimension, such as, for example, 2 carrier crossings per inch. 
     Still referring to  FIG. 1 , the water reserve  151  may be extended (e.g. less evaporation to the ambient air, by minimizing the length of wick  17  exposed to air. This can be done with placement of the reservoir  15  relative to the wick  17  and/or with a covering or sleeve  502  over the wick  17 . For clarity only a partial covering  502  is shown. 
     Still referring to  FIG. 1 , reservoir container  15  may be any suitable container for holding liquid  151  (e.g. water). In alternate embodiments reservoir container  15  may also include sensor  16 . Sensor  16  may communicate reservoir status to controller  19 . For example status may include liquid level, temperature, or viscosity. Also shown in  FIG. 1  is reservoir heater  15 A. Reservoir heater may be any suitable heater such as for example, electric or solar and may be thermostatically controlled. Similarly reservoir  15  may be painted or otherwise colorized any suitable color for absorbing or reflecting sunlight or any other radiant light in order to adjust the temperature of the liquid  151  held in reservoir  15 . 
     Also shown in  FIG. 1  is controller  19 . Controller  19  comprises: memory or computer readable medium  19 B, at least one processor or programmable controller  19 A, analog-to-digital and digital-to-analog converters necessary to process information relayed from sensors  12  and  13  via standard input/output channels or wireless connections; and, if present, from sensor  16 . Controller  19  computes the relative humidity (RH) for display on display readout  191 . It will be appreciated that RH may be computed by controller  19  according to equation 1 and equation 2 discussed earlier; or, any suitable algorithm for determining RH based upon wet and dry bulb temperatures. In alternate embodiments a secondary input of barometric pressure can be included to more accurately calculate saturation, however in mathematical modeling the impact of pressure is generally negligible in RH calculation. 
       FIG. 1  also shows connectors  18  and  161  for transmitting sensor data from housing  11  and container  15 , respectively. It will be appreciated that connectors  18  and/or  161  may be any suitable connector including wireless. 
     Referring also to  FIG. 2  there is shown a pictorial illustration of a self-ventilating and adjustable ventilation cover plate  20  in accordance with the invention shown in  FIG. 1 . Ventilation cover plate  20  includes cover  21  and ventilation cavities  22 . Cover plate  21  is suitably sized and shaped to enclosed housing  11  interior chamber  11 A. Ventilation cavities  22  may be any suitable size, number, and shape to cooperatively work with evaporation controller  14  and wick  17  to control the evaporation of liquid  151  into the interior chamber  11 A of housing  11 . 
     Referring also to  FIG. 3  there is shown a pictorial illustration of a system of psychrometer systems in accordance with the invention shown in  FIG. 1 . It will be understood that any suitable number of enclosed psychrometers  10  may be distributed in a space. Each of the psychrometers is suitably connected to controller  19  via a suitable connector, e.g., wire or wireless. Controller  19  monitors and determines the RH value for each station and displays on display  191 . It will also be understood that controller  19  includes the logic and circuitry necessary to display warnings and or alarms if the RH for any given station is not within a specified range; or, if the liquid at each station is below a predetermined level. Alarms may be any suitable combination of visual or audio alarms. In addition, alarms may be communicated over an internet or cellular connection. It will be appreciated that any suitable configuration may be employed. For example, a configuration where each sensor has the required controller  19  to conduct the RH calculation and sends data, via a wireless connection or hardline, to a main controller which handles output controls. The alarm signal may also include the logic and resources necessary to drive humidifiers and/or dehumidifiers ( 400 ) to bring relative humidity to non-alarm levels. 
     Referring also to  FIG. 4  there is shown an illustration of one method  40  for calibrating the wet/dry thermistors in accordance with the invention shown in  FIG. 1 . It will be appreciated that synchronous calibration of the wet-bulb and dry-bulb temperature sensors is critical to accuracy. The first step  45  immerses the wet/dry sensors in reference fluid with a known temperature, such as for example, a stirred ice bath at 0 C (32 F). It will be appreciated that any suitable reference fluid may be used, such as, for example, a 100 degree C. boiling bath for applications requiring high temperature accuracy. The processor ( FIG. 1-19B ) monitors the temperatures reported by the wet/dry sensors periodically, e.g., every second  42  for ten seconds, for example. If the variance of the array of temperature readings is less than a predetermined amount  44  the processor  19 B determines  46  the calibration temperature offset (from the reference fluid temperature) for each wet/dry sensor. The processor  19 B saves the calibration offset for each wet/dry sensor in non-volatile memory  19 A. Otherwise, if the variance is greater than the predetermined amount another array of temperature values is measured  42 . It will be appreciated that calibration of the temperature sensors as described overcomes two prior art problems. First, manufacturer tolerance on temperature vs. resistance for thermistors (or other sensors) is generally rated at 20 or 25 C, not 0 C resulting in drift in the desired measurement regime. In addition, there is often integration resistance deviation when attaching the sensors or when using wire for remote placement of the sensors. 
     Prototype Description 
     A prototype utilized two NTC 10 k Ohm thermistors in a voltage dividing circuit with a fixed 10 k Ohm resistor. With reasonable calibration (see  FIG. 4 ), the temperature of a thermistor changes its resistance in a predictably precise and accurate manner. Using the voltage dividing circuit, this resistance is indirectly measured by the voltage across the fixed resistor. An Arduino Uno microcontroller supplied 5 VDC +/− voltage to the voltage dividers and measured the circuit voltage using a 10-bit analog to digital converter. In this prototype the Arduino Uno microcontroller software assumed 5 VDC for calculating resistance of the thermistors, however an alternate embodiment measures the bus voltage and incorporates this into the calculation to reduce error. One of the thermistors is referenced to air directly to measure dry bulb temperature. The other is wrapped in a wick used for manual sling psychrometers with the far end of the wick placed in a reservoir of water to saturate the wick remotely. This sensor measures wet bulb temperatures. In prototype experiments it was expected that air flow over the wet bulb thermistor would be required, similar to the need for swinging a manual psychrometer. However, it was noted in the first experiment that this was not needed since the thermal mass of the thermistor is considerably less than that of a traditional liquid thermometer and its fluid in the manual sling psychrometer; and, thus requires lower heat transfer rates to reach equilibrium at the wet bulb temperature. The coarseness and other characteristics of the wick are also important in this design element; the wick used initially was quite open and loose allowing for good evaporation and air flow dose to the measurement surface. Regardless, the behavior was repeated and is predictable. 
     Initial Results 
     FIRST PROTOTYPE—Using high precision thermistors a prototype circuit and associated software was developed to measure dry bulb and wet bulb temperatures. The prototype thermistors are mounted on a breadboard, but would eventually be mounted remotely from the main circuit, connected with wire or wireless connections. Thermistors can be made to be moisture resistant with potting (epoxy) and can also be manufactured to very high precision (+/−0.1 deg. F.). The measurement approach used in this design should result in a more rugged, precise, and accurate measurement of RH in low temperature high humidity environments at a material cost under $50. 
     SECOND PROTOTYPE (See  FIG. 1 )—A remote housing  11  having two openings was provided. A rubber stopper was used to plug one of the conduit holes and to allow CAT5e cabling to enter the housing. The evaporation controller  14  was glued into the other opening allowing the connection of the water reservoir  15  and a controlled, wick water supply  151  with minimal evaporation from the bottle. Various size bottles can be used, this prototype used a 1 fl oz size. 
     High accuracy NTC thermistors were used in the second prototype (10 kOhm+/−0.05 deg. C. US Sensor #PR103J2). No other significant changes were made to the circuit in this build. In initial tests of this build, it was found that an optional air flow over the wet-bulb thermistor could be used to stably depress the wet-bulb temperature. A small fan (Orion # OD2510-05HB) was integrated with desired results. The fan can be powered by any suitable means, e.g., battery power, solar powered, etc. 
     Conclusions 
     The prototypes used standard 10 k Ohm fixed resistors in the voltage divider. The actual resistance of the resistors was measured and used in the software-based calculation, but higher precision resistors would provide a more accurate RH calculation. Matching of the fixed resistors to the expected resistance of the thermistors in the measurement range results in maximum precision of the instrument. 
     The prototype or proof of concept used a laptop computer and USB connection for power and logging of results. A local LCD screen and power source were integrated into the prototype design. Other options for reporting sensor data are available for uploading data to cloud based data programs (Mojyle, etc.), email via Ethernet, or direct SMS text message communication via cell. 
     The prototype uses an Arduino Uno 10 bit analog to digital convertor which results in an output precision of about 0.09%. It will be appreciated that higher bit convertors would result in higher precision. 
     Referring to the figures it will be appreciated that item  400  ( FIG. 3 ) represents a controlled device, such as, for example, a humidifier, a dehumidifier, a fan, or the like. It will be understood that devices such as the aforementioned may be controlled by controller  19  according to the calculated RH levels. There are currently no low temperature, high humidity humidistats on the market that are suitable for these applications. The microcontroller  19  may be programmed to provide control of such a system resulting in a very precise and stable control system for RH in storage. 
     It should be understood that the foregoing description is only illustrative of the invention. Thus, various alternatives and modifications can be devised by those skilled in the art without departing from the invention. 
     For example, enclosure of the sensing probes with careful attention to aspiration helps to avoid erratic readings during a compressor cycle in the refrigeration system. When the compressor runs, the air coming off an evaporator in a cooler will be very cold and very dry which may drive the dry bulb temperature lower very quickly. The wet-bulb is enclosed in a moistened wick and takes longer to respond. This results in an RH inversion which sends it above 100% (not possible). In an alternate embodiment a piece of dry wick material, same material as the wet-bulb, may be used to cover the dry bulb to make their dynamic thermal response relatively more equal. The other is using the enclosure lid. Alternatively, software processing by processor  19 B may identify the situation and disregard the data and/or annotate the data stream to clarify it. 
     Accordingly, the present invention is intended to embrace all such alternatives, modifications and variances that fall within the scope a the appended claims.