Abstract:
A humidity sensor arrangement for providing temperature compensated measurements. A first sensor is configured internal to a housing for the sensor arrangement, and a second sensor is configured external to the housing. The first sensor measures a first relative humidity at a first temperature, and the second sensor measures second temperature external. A processor determines saturation pressures for the first and second sensors and compensates the first sensor by adjusting the relative humidity for the first sensor to be a product of the relative humidity for the first sensor and a ratio of the saturation pressure of the first and second sensors. Further compensation may be produced by applying a time-based filter algorithm to the outputs of the first and second sensors. Temperature sensors can be compensated to determine room temperature and compensated temp can be used for precise humidity compensation.

Description:
TECHNICAL FIELD 
       [0001]    The present disclosure is directed to temperature and/or humidity sensors. More specifically, the present disclosure is directed to temperature compensation for temperature and/or humidity sensors configured for building automation systems (BAS) applications. 
       BACKGROUND INFORMATION 
       [0002]    The demand for multi-variable sensors for the BAS market is increasing steadily, Sensors used for this purpose are generally wall mounted in the zone of a building and comprise one or more sensors, such as temperature, humidity, carbon dioxide (CO 2 ) and volatile organic compounds (VOCs). Additionally, sensors may require the ability to communicate over a wired or wireless communication network. The combination of these sensing abilities and electronics requires more power and therefore results in additional heat being generated in the wall mount case that contains the sensors. 
         [0003]    Certain sensors, such as humidity sensors, may be particularly sensitive to temperature differences and gradients surrounding the sensor. When temperatures are artificially increased in the wall mount case, this in turn elevates the temperature of the internal humidity sensor, and may cause errors relative to the humidity in the room. While electrical and thermal design can minimize the increase in heat, there is a need to compensate the humidity sensor inside the wall mount case to match the actual humidity in the room containing the wall mount case. 
       SUMMARY 
       [0004]    Accordingly, under one exemplary embodiment, a temperature-compensated humidity sensor arrangement is disclosed, comprising a first sensor, arranged internal to a housing for the sensor arrangement, wherein the first sensor is configured to determine a first relative humidity at a first temperature internal to the housing. The sensor arrangement further includes a second sensor, arranged external to the housing, wherein the second sensor is configured to determine a second temperature external to the housing. A processor is operatively coupled to the first and second sensor, wherein the processor is configured to compensate the first sensor, based on a function of the second temperature. 
         [0005]    In another exemplary embodiment, the first sensor of the sensor arrangement is configured to determine a first relative humidity at a first temperature along multiple points of a time base, and the second sensor is configured to determine a second temperature along the multiple points of the time base. The processor may be configured to apply a time-based filter for the multiple points of the time base. The time-based filter may be applied sequentially or symmetrically for the multiple points of the time base. 
         [0006]    In another exemplary embodiment, a method is disclosed for temperature compensated humidity sensing for a sensor arrangement, comprising the steps of determining a first relative humidity at a first temperature internal to a housing for the sensor arrangement via a first sensor; determining a second temperature external to the housing via a second sensor; and compensating the first sensor based on a function of the second temperature. 
         [0007]    In another exemplary embodiment, the method includes the step of determining a first saturation pressure for the first sensor and a second saturation pressure for the second sensor. The function for the method comprises a corrective value based on a ratio between the first saturation value and the second saturation value. Compensating the first sensor may include the steps of producing a corrected relative humidity value for the first sensor, where the corrected relative humidity value comprises a product of the first relative humidity and the corrective value. 
         [0008]    In another exemplary embodiment, the first relative humidity at a first temperature is determined along multiple points of a time base, and the second temperature is determined along the multiple points of the time base. A time-based filter may be applied for the multiple points of the time base. The time-based filter may be applied sequentially or symmetrically for the multiple points of the time base. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]    The present invention is illustrated by way of example and not limitation in the figures of the accompanying drawings, in which like references indicate similar elements and in which: 
           [0010]      FIG. 1  illustrates an exemplary embodiment of a temperature-compensated humidity sensor, which is configured to account for ambient area temperatures and internal temperatures; 
           [0011]      FIG. 2  is an exemplary embodiment illustrating a processing and communications configuration for the sensor of  FIG. 1 ; 
           [0012]      FIG. 3  is an exemplary schematic diagram for the sensor of  FIG. 1 , illustrating the arrangements of physical components for temperature and humidity sensing; and 
           [0013]      FIG. 4  is an exemplary graph illustrating compensation of the sensors reference (room) temperature. 
       
    
    
     DETAILED DESCRIPTION 
       [0014]    Humidity sensors or hygrometers are instruments used for measuring the moisture content in the atmosphere. Typically, humidity sensors rely on measurements of some other quantity such as temperature, pressure, mass or a mechanical or electrical change in a substance as moisture is absorbed. Using calibration, calculation and processing, these measured quantities can lead to a measurement of humidity. A temperature of condensation, or dew point, or changes in electrical capacitance or resistance may be used to measure humidity differences. For capacitive humidity sensors, the effect of humidity on a dielectric constant of a polymer or metal oxide material is measured. For resistive humidity sensors, the change in electrical resistance of a material due to humidity is measured. Typical materials may include salts and conductive polymers. For resistive sensors, the material properties may to depend both on humidity and temperature, which means that the sensor may be combined with a temperature sensor. In other sensors, such as thermal conductivity humidity sensors, the change in thermal conductivity of air due to humidity is measured. These sensors may measure absolute humidity rather than relative humidity. Still other sensors include psychrometers, which may include a dry thermometer and a wet thermometer, which may be kept moist with water on a sock or wick. These thermometers are sometimes referred to as a dry-bulb and a wet-bulb, respectively. At temperatures above the freezing point of water, evaporation of water from the wick lowers the temperature, so that the wet-bulb thermometer may show a lower temperature than that of the dry-bulb thermometer. Relative humidity may be computed from the ambient temperature determined in the dry-bulb thermometer and the difference in temperatures as determined in the wet-bulb and dry-bulb thermometers. Relative humidity can also be determined by processing and locating the intersection of the wet and dry-bulb temperatures on a psychrometric table, chart or scale via microcontroller. The two thermometers coincide when the air is fully saturated, and the greater the difference, the drier the air. 
         [0015]    Regardless of the type of humidity sensor being used, the present disclosure is directed to any humidity sensor utilizing a temperature component. Turning to  FIG. 1 , an exemplary embodiment is disclosed where a sensor  102  is positioned in a zone  101 , which may be a room, enclosure, or other area. Sensor  102  comprises a case which may include wall mounts, electronic circuitry  103 , and sensor circuitry  104  comprising an internal sensor/thermistor  105 . Sensor  102  advantageously comprises a second sensor/thermistor  106 , which may be coupled to electronic and sensor circuitry ( 103 ,  104 ) utilizing a thermocouple or any other suitable means. It should be understood by those skilled in the art that, while circuitry  103 ,  104  and sensor/thermistor  105  are depicted as separate blocks in  FIG. 1 , the circuitry and sensors may be integrated into one circuit, circuit board or substrate, or may alternately be separated into multiple different components, depending on the needs of the electronic designer. 
         [0016]    Sensor circuitry  104  may comprise an internal humidity sensor and an associated temperature sensor ( 105 ) that is configured to measure a local temperature, which, in this embodiment, includes the temperature in the wall mount case. Under normal operating conditions, the internal temperature (T 1 ) of the case is elevated by the internal electronics, which can introduce error in the humidity sensor with respect to room humidity (RH). Accordingly, under one embodiment, a second temperature sensor  106  is extended through the wall mount case enclosure  102  such that it can accurately measure the room temperature. The thermocouples for sensor/thermistor  106  are preferably small and should require two wires for operation, so they can easily be positioned under an edge of wall mount case  102  and not be easily visible. Alternately, the thermocouples may be covered by a secondary encasement to protect and hide the wires from sight. 
         [0017]    In the embodiment of  FIG. 1  the second (external) temperature sensor  106  may be used to compensate for potential inaccuracies in sensor measurements within enclosure  102 . Under one illustrative example, the sensor arrangement is configured to measure relative humidity, which may be defined generally as a ratio of the actual amount of water vapor present in a volume of air at a given temperature to the maximum amount that the air could hold at that temperature, expressed as a percentage. Since warm air can hold more water vapor than cool air, a particular amount of water vapor will yield a lower relative humidity in warm air than it does in cool air. As can be seen in the figure, sensor  102  needs to take into consideration and compensate relative humidity (RH 1 ) at temperature (T 1 ) for the area  102 , to measure relative humidity (RH 2 ) at temperature (T 2 ) for area  101  surrounding sensor  102 . Relative humidity for sensor  102  may be determined by 
         [0000]        RH ( T   1 )= P   p   /P ( T   1 ) sat    
         [0000]    where
       RH(T 1 )=the relative humidity and temperature (T 1 );   P p =the partial pressure of water in the air; and   P(T 1 ) sat =the saturation pressure of water at temperature T 1 .
 
From this proportionality, the relationship to the relative humidity and temperature of the room (RH(T 1 )) can be expressed as:
       
 
         [0000]        RH ( T   1 )*( P ( T   1 ) sat   /P ( T   2 ) sat )= RH ( T   2 ) 
         [0000]    where P(T) sat  is only a function of temperature. Accordingly, the relative humidity and temperature of area  101  may be accurately determined by taking the relative humidity and temperature (RH(T 1 )), determined by sensor  104 , and making it a product of a ratio between the saturation pressure of water at temperature T 1 , determined by sensor  104 , and the saturation pressure of water at temperature T 2 , determined by sensor  106  P(T 1 ) sat /P(T 2 ) sat . With regard to the saturation pressure of water vapor in  FIG. 1 , this can be expressed as: 
         [0000]        P ( T ) sat   =e   (77.3450+0.0057 T−7235/T)   /T 8.2 
         [0000]    where:
       P(T) sat =water vapor saturation pressure (Pa) at temperature T(K);   e=constant 2.71828; and   T=dry bulb temperature of the moist air (K).       
 
         [0024]    This embodiment described above advantageously compensates humidity particularly for stagnant environments, even if the internal heat is changing due to changes in the operation of the electronic components (current loops, etc.). However, air flow in most areas (such as office spaces) is dynamic, with fans and other air moving systems changing the airflow at arbitrary times. If air stream(s) of different temperatures pass over the wall mount case, the external sensor/thermistor ( ) 106  will react much faster than the internal humidity sensor (T 1 ) in the wall mount case. This dynamic change could cause and extreme error in humidity, causing other elements of the system to erroneously engage. 
         [0025]    In order to compensate for such errors, further processing may be required for the sensor signals. Turning to  FIG. 2 , an exemplary sensor block diagram configuration is shown, comprising inputs from one or more humidity sensors  201  and temperature sensors  202 . The dual-sensor configuration of  FIG. 1  may be combined in the embodiment of  FIG. 2 , or may have separate circuitry to accommodate the readings of each sensor (RH(T 1 ), RH(T 2 )). The signals from humidity sensors  201  and temperature sensors  202  may be multiplexed and transmitted to analog-to-digital (A/D) converter  204  for digital conversion, and processed in digital signal processor (DSP)  205 . DSP  205  may comprise a universal or dedicated processor responsible for all sensor processing. Alternately, DSP  205  may comprise one or more processors in communication with a central processing unit. The DSP and sensor settings may be controlled and/or calibrated by settings module  208 , which may comprises a non-volatile memory, such as EEPROM. Outputs from DSP  205  may be stored in register  206 , and forwarded to output/communication module  207 . 
         [0026]    Module  207  is configured to communicate sensor outputs (OUT) to other devices or peripherals that may be in communication with the sensor. In one embodiment, module  207  comprises a Serial Peripheral Interface, which is a synchronous serial data link capable of operating in full duplex mode. In this embodiment, sensors may communicate with other sensors or devices in master/slave mode, where the master device may initiate a data frame. Multiple slave devices may be allowed with individual slave select lines. In another embodiment, module  207  comprises a Modbus interface that allows the sensor to communicate data packets between many devices using the EIA-232 or EIE-285 protocols. The protocol defines function codes and the encoding scheme for transferring data as either single points (1-bit, coils) or as 16-bit data registers. This basic data packet is then encapsulated according to the protocol specifications for Modbus ASCII, RTU, or TCP. In another exemplary embodiment, module  207  comprises a BACnet interface, or “Building Automation and Control net,” which is an Ethernet based, ASHRAE (American Society for Heating, Refrigeration and Air-Conditioning Engineers) standard. This protocol defines data communication services for sensor and equipment which is used for monitoring and control of heating, ventilation, air conditioning and refrigeration (HVAC&amp;R). BACnet provides comprehensive sets of messages for conveying encoded binary, analog and alphanumeric data between devices. It should be understood by those skilled in the art that other communication protocols may be used as well, depending on the application needs. In one embodiment of  FIG. 2 , output from module  207  may also be fed back they settings module  208  to adjust or calibrate operation of DSP  205 , and/or sensors  201 - 202 . 
         [0027]    Utilizing the configuration of  FIG. 2 , temperature differences due to air flow in an area discussed above may be corrected using a time-based filter algorithm for sensor readings. An exemplary moving average filter operates by averaging a number of points from the sensor input signal to produce each point in an output signal. This may be expressed algorithmically as 
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         [0000]    where x( ) is a sensor input signal, y( ) is an output signal and M is the number of points in the average. Thus, as an example, in a 5 point moving average filter, a sensor signal point  10  in the output signal is given by 
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         [0000]    Alternately, the group of points from the input signal may be chosen symmetrically around the output point, resulting in 
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         [0000]    Accordingly, a time base filter (or “rolling average”) can be assigned to the values of the external sensor/thermistor  106  (T 2 ) to effectively slow down the reaction time to align with the internal humidity senor  105  (T 1 ). 
         [0028]    In another embodiment, the temperature difference between the two sensors ( 105 ,  106 ) may be limited as a boundary condition to the maximum difference between startup and maximum temperature difference in stagnant air. This can be determined by recording the temperature difference between T 1  and T 2  when the case and electronics are cooled to room temperature. After powering the electronics, the two temperatures are immediately recorded and used to adjust both resistors to room temperature. The electronics is then allowed to heat the internal humidity sensor and thermistor to a maximum temperature in stagnant air, wherein the temperatures are recorded again. This difference may be used for DSP  205 as a maximum temperature difference caused by the electronic heating alone, which may assist in compensating the sensors disclosed herein. Dynamic air currents in an area can cause much larger temperature differentials which are not related to internal electronic heating. Accordingly, they can be ignored as temperature difference outside of the determined limits. The rate of change of the time-based filter above can be adjusted for the difference between the two temperature sensors such that small temperature differences react faster than large differences. 
         [0029]    Turning now to  FIG. 3 , an exemplary schematic illustration is provided for the sensor configuration of  FIG. 1 . Here, wall mount case  302  in contained in an area or room zone ( 101 ) which creates a boundary for the ambient room temperature T 2  and room humidity RH 2  at T 2 . Wall mount case  302  comprises a relative humidity sensor  307  with on-board temperature sensor  307 , where both sensors may be at elevated internal box temperature T 1  as heated by thermal energy from electronic components  304  and CO2 sensor  303 . A second temperature sensor  306  is extended out of a hole  305 in the wall mount case  302  to measure ambient temperature T 2  and may be further isolated from the case wall by a thermistor mounting insert  305  with 3 fingers to minimize thermal conduction from case  302 . Wires  308  connect sensor  306  to the electronics board  304 . 
         [0030]    In certain embodiments, temperature sensor  306  may be elevated above room temp by wall temperature  302 . In this case, temperate sensors  307  and  306  can be used to estimate room temperature  01 . The compensated temperature from  306  and  307  may then be used to determine the saturation pressure of room  101  and compensate internal humidity  104  to room humidity  101 . Temperature sensors  306  and  307  can be calibrated to room temperature on startup of sensor  102  before internal heating occurs.  FIG. 4  provides an exemplary graph illustrating compensation of sensors to a reference (room) temperature. Here, the sensors are normalized to each other and to room temperature, where the slope (dotted line) may be used to compensate T out  to room temp (T ref ) very accurately. Lines  401  to  405  represent temperature measurements between the difference in the two sensors and the difference between the T out  and room temperature (T ref ). This slope is determined experimentally for several sensors and the slope of the best fit line through the test sensors is used for all production sensors using this configuration. The estimated room temperature is determined by the established slope: m=(T out −T ref )/(T in −T out ). Next, Tin and Tout are measured, and room temperature is determined by: 
         [0000]        T   room   =T   out −( m *( T   in   −T   out ))
 
         [0031]    Thus, for example for m=1.1333 from the graph, 
         [0000]      if  T   in   −T   out =1 C  and  T   out =20 C,    
         [0000]        T   room =20 C− 1.1333*1 C= 18.86 C    
         [0032]    The T room  estimate may then be used to compensate RH sensor to high accuracy. Such a configuration may effectively compensate for RH, as the percentage of RH measurements is greatly affected by temperature as RH increases. For example a 0.5C error in temp would normally cause 2% error in RH at 60% RH. The compensated temperature may further be used to compensate relative humidity (RH 1 )  104  to the relative humidity (RH) of room  101 . 
         [0033]    While at least one example embodiment has been presented in the foregoing detailed description, it should be appreciated that a vast number of variations exist. it should also be appreciated that the example embodiment or embodiments described herein are not intended to limit the scope, applicability, or configuration of the invention in any way. Rather, the foregoing detailed description will provide those skilled in the art with a convenient and edifying road map for implementing the described embodiment or embodiments. It should be understood that various changes can be made in the function and arrangement of elements without departing from the scope of the invention and the legal equivalents thereof.