1、4757 An Experimental Evaluation of Part l9 Test and Eva1 u at i on Duct-Mounted Relative Humidity Sensors: Procedures Shailesh N. Joshi Student Member ASHRAE Michael B. Pate, PhD Member ASHRAE Ron M. Nelson, PhD, PE Member ASHRAE John M. House, PhD Member ASHRAE ABSTRACT Relative humidity sensors ar
2、e common components in building heating, ventilating, and air-conditioning (HVAC) systems, and their performance can significantly impact energy use in these systems. Therefore, a study was undertaken to test and evaluate the most commonly used relative humidity sensors in HVAC systems, namely, the
3、capacitive and resistive types. The procedures presented here provide a methodology to test and evaluate duct-mounted relative humidity sensors for accuracy, linearity, hysteresis, and repeatability. The test and evaluation procedurespresented in this paper are all inclusive in that they range from
4、procuring the humidity sensors to comparing the accuracy ofhumidity sensors. Specif ically, a procedure is presented to both procure humidity sensors from themanufacturers and to maintain quality control by controlling the storage, handling, and movement of the sensor while documenting time and date
5、 at each step. Furthel; it describes the apparatus and instrumentation, along with test conditions, used to pevform experiments on humidity sensors. Additionally, it outlines a detailed experimental procedure to evaluate the accuracy of humidity sensors. Finally, a discus- sion is presented on analy
6、zing and comparing the accuracy of humidity sensors by using test data. The results of the accuracy test and evaluation of the humidity sensors and the results of the linearity, repeatability, and hysteresis evaluation will be presented later: INTRODUCTION Relative humidity sensors are commonly used
7、 in building HVAC systems to monitor supply and return air conditions in air-handling units, to monitor conditions in occupied spaces, and to control humidification and dehumidification processes Curtis J. Klaassen, PE Member ASHRAE as well as economizer cycles. The performance of these sensors can
8、significantly impact comfort in occupied spaces and energy use in HVAC systems. In the latter case, relative humidity and temperature measurements of outdoor and return air conditions are used to compute the enthalpies of the two airstreams, with the result that the optimum amount of outdoor air ent
9、ering the building can be determined. If one or both of the computed enthalpies is erroneous, extreme energy penalties can result from the introduction of excess outdoor air. Because of their impact on comfort and energy use, a study was undertaken to test and evaluate duct-mounted relative humidity
10、 sensors used in typical building HVAC applications. Prior to conducting this performance study, an experimental procedure for testing and evaluating duct-mounted relative humidity sensors was developed and is presented here. This procedure provides a detailed description of the methodology to evalu
11、ate the performance of duct-mounted relative humidity sensors for accuracy, linearity, hysteresis, and repeatability. This paper presents an overview of the procedures avail- able in the open literature to evaluate relative humidity sensors used in HVAC environments. Further, the paper describes the
12、 commonly used relative humidity sensor types and methods for procuring humidity sensors from representative manufac- turers and for administering quality control, such as docu- menting storage and handling conditions. Furthermore, a description of the experimental test apparatus (i.e., humidity gen
13、erator) and instrumentation requirements (i.e., data acqui- sition system) is presented here. Additionally, steady-state criteria for recording data from the humidity generator and sensors are also discussed. Finally, a detailed description of data generation and an analysis for evaluating and compa
14、ring sensors are provided. Shailesh Joshi is a research assistant and Michael Pate and Ron Nelson are professors at Iowa State University, Ames, Iowa. John House is a research engineer and Curtis Klaassen is manager of the Energy Resource Station at the Iowa Energy Center, Ankeny, Iowa. 02005 ASHRAE
15、. 169 PREVIOUS STUDIES This section presents a review of previous research with an emphasize on describing methods and procedures used by past researchers to generate and measure relative humidities. These studies include procedures to test and evaluate relative humidity sensors in various HVAC envi
16、ronments, such as agriculture and shipboard environments; however, there has been no study to date that reports a detailed experimental procedure to evaluate accuracy, hysteresis, linearity, and repeatability of duct-mounted relative humidity sensors in building HVAC environments, where the emphasis
17、 is on human comfort and energy conservation. A briefreview of the past studies is presented below. Kitano et al. (1984) evaluated both linearity and hystere- sis of four different types of relative humidity sensors in a manufacturing plant environment. The authors use an environ- mental chamber to
18、test the sensors, but they do not describe the procedures that were used to either set or measure the actual Lower electrode ig,. 1 Schematic of u cupacitive-Sipe humidi (adapted from Yumatuke 2004). humidity in the environmental chamber. Erdebil and Leonard (1992) report a test procedure to evaluat
19、e both accuracy and hysteresis of two capacitive-type sensors and an aluminum-oxide sensor in an unspecified animal environment. In their study, the exact relative humidity values, which ranged from 33% to 85% RH at a temperature of 2OoC, were produced using an environmental chamber. This environmen
20、tal chamber had a continuous influx and exhaust of moist or dry air in order to generate an atmosphere that simulated an animal environment. In the calibrationlaccuracy tests, the sensors were stabilized and then the air inlet to the chamber was switched to use air flowing over a saturated salt solu
21、tion. In this way, air of a different humidity was drawn into and then mixed with the air in the chamber. The details on experimental procedures for setting and measuring the actual relative humidity are not provided. Slayzak and Ryan (2000) present a test procedure to eval- uate and compare dew poi
22、nt and relative humidity sensors, such as the capacitive type. In their study, the test procedure consisted of maintaining a constant humidity ratio of 17h0.3 gkg and then increasing the dry-bulb temperature to vary rela- tive humidity. The authors, however, did not describe the method used to measu
23、re actual relative humidity for the purposes of comparing dew point and relative humidity sensors. In summary, the past studies report on procedures to eval- uate the performance of relative humidity sensors in HVAC environments, such as in manufacturing, shipboard, and animal environments; however,
24、 none of the studies provides a complete description of the test procedures used to generate and measure exact relative humidity values. HVAC RELATIVE HUMIDITY SENSORS The most widely used humidity sensors in HVAC appli- cations are the capacitive and resistive types. These relative humidity sensors
25、 consist of an integrated sensor and transmit- ter assembly. The sensor provides a measure of the relative humidity, while the transmitter generates an electronic output signal that is representative of the sensed relative humidity. Capacitive-Type Humidity Sensors The main components of a capacitiv
26、e humidity sensor are shown in Figure 1. A capacitor is formed by depositing a poly- mer or metal oxide film between a conductive material (lower electrode) and a porous conductive material (upper electrode) onto a glass, ceramic, or silicon substrate. The polymer layer absorbs water molecules as th
27、ey permeate through the porous upper electrode. The dielectric constant of the polymer layer changes as it absorbs moisture, causing the capacitance of the two electrodes to increase. The change in capacitance is directly proportional to the relative humidity. Capacitive-type humidity sensors are ty
28、pically used in home appliances, the paper manufacturing industry, combus- tion and heat-treating environments, high-temperature drying equipment, and HVAC climate control. The advantages of capacitive-type humidity sensors include accuracy (Le., the sensor meets the manufacturer-specified accuracy)
29、 in both the low relative humidity range (45% RH) and high ambient temperatures. The disadvantages of capacitive humidity sensors include sensitivity to contaminants and chemicals, inaccuracy above 95% RH, and the need for periodic recali- bration. Resistive-Type Humidity Sensors The main components
30、 of a resistive humidity sensor are shown in Figure 2. Resistive humidity sensors are composed of interlocked metal electrodes that are deposited on a substrate. The substrate is then coated with a moisture-sensi- tive material, such as a conductive polymer or a salt. As the polymer coating absorbs
31、moisture, ions are released, causing the electrical resistance of the polymer to change. The resis- tance, which is measured by the sensor, decreases as the humidity increases. Resistive-type humidity sensors are typically used in home appliances, HVAC climate control, refrigerators, and micro- wave
32、 ovens. The advantages of resistive-type humidity sensors 1 70 ASHRAE Transactions: Research / Conductive polymer film Figure 2 Schematic of a resistive-type relative humidity sensor (adapted from Yamatake 2004). include being accurate (i.e., the sensor meets the manufacturer- specified accuracy) in
33、 the high relative humidity range (95% RH). The disadvantages of resistive-type humidity sensors include reduced accuracy at low humidity (typically less than 15% RH), sensitivity to contaminants and chemicals, and the need for periodic recalibration of the sensor. SENSOR PROCUREMENT AND QUALITY CON
34、TROL Relative humidity sensors were procured from six differ- ent humidity sensor manufacturers. Out of the many sensor manufacturers, only sensors from six manufacturers were selected for testing based on the following two reasons: 1. These manufacturers occupy a major market share in commercial pr
35、oduction and distribution of WAC-grade duct-mounted humidity sensors. The project timeline allocated to complete the testing of humidity sensors was limited. A sensor procurement procedure was important to increase the likelihood that the sensors would be taken from different manufacturing lots, the
36、reby providing a random sample of humidity sensors from a particular manufacturer. In addition, quality control procedures were important to ensure that all humidity sensors were exposed to similar environmen- tal conditions throughout the study. The procedures for sensor procurement and quality con
37、trol are discussed below. Sensor Procurement The first step in procuring the humidity sensors was to identi major manufacturers of both resistive and capacitive humidity sensors for HVAC applications. The sensors were ordered in three separate batches over a period of several weeks. Initially, one h
38、umidity sensor unit from each of the manufacturers was ordered. After two weeks, the next batch of sensor units was procured in a similar fashion. The final batch of sensor units was ordered two weeks after receiving the second batch of sensors. All of the above test sensors were ordered from either
39、 the sensor manufacturer or an authorized 2. distributor. As noted before, half the sensors ordered were of the resistive type and the other half were of the capacitive type; the total purchase was 18 sensors. Sensors with manufacturer- stated accuracies of %3% FW were tested because they are common
40、ly used in HVAC applications. For consistency, all of the sensors selected for test provided an output voltage of 0- 10 v. Sensor Quality Control After receiving a batch of sensors, a continuous record of location and ambient conditions was maintained to ensure that all the sensors were subjected to
41、 similar environmental condi- tions. For each sensor, information was recorded and appro- priate precautions administered as follows: 1. All the humidity sensors were labeled for easy identifica- tion. 2. The sensors were stored in a uniform environment similar to that existing in a laboratory, clas
42、sroom, or office building. To prevent damage, the sensors were kept in their shipping boxes or in an equivalent storage box. Care was taken to ensure that no extraneous matter (e.g., dirt, chemicals, etc.) that might influence the sensor operation and accuracy was in the vicinity of the humidity sen
43、sors. A preliminary check of the sensors was performed to ensure that they were working properly in order to prevent testing delays that might occur if a particular sensor was found to be malfunctioning later. The following steps, which did not involve actual testing of the sensor, were taken to ens
44、ure that each sensor was working properly: 3. Each sensor was subjected to the manufacturer- stated voltage input, and then a multimeter was used to check if the sensor read the applied voltage correctly, The voltage output signal from the sensor (0-10 V) was checked using a multimeter. Upon passing
45、 the above tests, the sensor was returned to the storage box and saved for further testing. 4. A continuous record of the location, time, and date of each sensor was maintained on a log sheet, including the transfer from and to storage for testing. Manufacturers written instructions regarding instal
46、lation and operation of the sensor were followed at all times. 5. EXP E RI M ENTAL AP PARAT U S The humidity sensors were tested in this study by using a known standard that was traceable to the National Institute of Standards and Technology (NIST). A NIST-traceable humid- ity instrument produces kn
47、own values of humidity accurately by using NIST principles developed for humidity calibration. Specifically, the humidity sensor experiments performed in this study used a humidity generator consisting of a self- ASHRAE Transactions: Research 171 Table i. Technical Specifications of the Humidity Gen
48、erator (TS 2000) Specification Description Relative humidity operating range (To RH) Resolution (% RH) Accuracv (YO RH I I 1 Value or Type 10 - 98 0.02 * 0.5 Chamber temperature range (“C) O - 70 Chamber temperature resolution (“C) If 0.02 I Chamber temperature accuracy (“C) Chamber pressure range (
49、mia) Chamber temuerature uniformitv (OC) If 0.1 I f 0.06 Ambient Gas type Calibration standard I Gas flow rate (slum)* 15-20 I Air NIST (two-pressure humidity generator) AIRCUPPLY WURATOR oQpN9al * Note: splm-specific liter per minute TEsTctlAMBER- UMWST Figure 3 Two-pressure humidity generator principle (Thunder Scientijk 2000). contained apparatus capable of producing known humidity values using the fundamental principle of the “two-pressure” generator developed by NIST. This system, which was acquired from a commercial vendor, has the capability of supplying accurate and
