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The transient response of capacitive thin-film polymer humidity sensors.

Article Excerpt
INTRODUCTION

Relative humidity (RH) measurement is important for many applications ranging from manufacturing environments to hospitals to commercial and domestic heating, ventilating, and air-conditioning (HVAC) processes. For HVAC applications, a commonly used type of humidity sensor is the thin-film polymer capacitive sensor. These sensors typically consist of a thin film of a hygroscopic organic polymer deposited onto a water-permeable substrate. They operate on the principle that the capacitance of the polymer film changes with absorption or desorption of water, and the measured capacitance can be accurately correlated to the RH of the air. Calibration of these sensors is achieved by exposing the sensing element to a known RH, typically with a saturated salt solution, and measuring the resulting output signal (Greenspan 1977; Bryant and O'Neal 1992). Integrated electronics usually provide temperature correction, and many sensors have a separate output signal for temperature measurement. These sensors are popular for use in HVAC due to their relatively low cost, high dependability, and acceptable accuracy for most applications.

Manufacturers of thin-film capacitive humidity sensors often report the accuracy of their instruments to be around [+ or -]2% or 3% at a single-reference temperature, typically between 20[degrees]C-25[degrees]C (68[degrees]F-77[degrees]F). Many manufacturers define the response time of their sensors in terms of a "90% response time," which is the amount of time it takes for their sensor to respond from one RH value to within 10% of a new RH value. Response times on the order of seconds are commonly reported for thin-film capacitive sensors, which are typically based on the transient response of those sensors in still (zero velocity), constant temperature air. Consequently, these types of RH sensors are often used in industrial applications where fast humidity measurement is essential. In addition, several researchers have used capacitive RH sensors to characterize the instantaneous latent performance of air conditioners during transient start-up and shutdown periods (Katipumula 1989; Kim and Bullard 2001; Dooley 2004). Therefore, knowing how the response time of these RH sensors are affected by air velocity and temperature changes would be beneficial to a broad audience. To date, there are no known studies that attempt to address these issues, and that has prompted the current investigation.

The objective of this study was to experimentally determine the effects of air velocity and step changes in RH and temperature on the transient response of thin-film capacitive humidity sensors commonly used in HVAC applications. Results were reported in terms of RH and temperature time constants.

LITERATURE REVIEW

Researchers have reported on the operating principles of capacitive thin-film humidity sensors (Yamazoe and Shimizu 1986; Brownawell 1989; Rittersma 2002) Yamazoe and Shimizu (1986) presented a review and discussion on the operating characteristics of various types of humidity sensors. They provided a detailed overview of capacitive thin-film sensors, showing that the measured capacitance of the sensing element is roughly proportional to the ambient RH. Another review of humidity sensing technology was presented by Brownawell (1989). His review focused mostly on sensors used specifically for HVAC applications and provided some discussion on the accuracy and dependability of capacitive sensors. Rittersma (2002) provided a review of transduction techniques in miniaturized humidity sensors. He pointed out that for capacitive sensors, the use of more porous ceramic dielectrics can enhance sensitivity, but response time can increase due to diffusion effects. His review also indicated that electrode geometry can play a crucial role in the response time of the sensor.

Several researchers have investigated the transient response of humidity sensors under fixed conditions (Marchgraber and Grote 1963; Pascal-Delannoy et al. 1997; Kuse and Takahashi 2000; Sorli et al. 2002; Wang 2005; NBCIP 2005). Pascal-Delannoy et al. (1997) investigated the use of a Peltier module in conjunction with a photodetector as a type of fast humidity sensor. Results of their work showed an average response time between 0.25 and 12 s in the range of 15%-70% RH. The response time of a similar device was investigated by Sorli et al. (2002). They characterized the response time for a humidity sensor consisting of a Peltier module and a reflective sensor. Their investigation focused on the high range of RH (80%-100% RH), and results from their study indicated that repeatable response times of less than 10 s were possible.

Marchgraber and Grote (1963) studied the response of a carbon humidity element exposed to a step change in RH. They reported that the transient response was characterized by a fast initial response followed by a much slower drift toward equilibrium at the new RH. Kuse and Takahashi (2000) investigated the transient response of a tin oxide semiconductor RH sensor exposed to step changes in RH. Their results showed that the response of the sensor could be characterized by an exponential curve with two time constants. Experimental work related to the transient behavior of a standard duct-mounted capacitive thin-film humidity sensor was conducted by Wang (2005). In his work, he found that the transient response of this sensor could also be characterized by an exponential function with two time constants. He reported that the first time constant was approximately 3 s, while the second time constant was around 100 s. A product testing report issued by NBCIP (2005) included response time data for six different commercially available humidity transmitters with sensing elements ranging from the resistive to capacitive type. Results of this study showed that the only capacitive thin-film polymer sensor tested in that study had an average response time of 7.2 s, which was the fastest of all sensors tested. For the response time tests, the sensors were exposed to both "forward" and "reverse" step changes in RH, but the affects of air velocity and temperature changes on the sensor response time were not considered.

Experimental Facility

A test section was designed and built that was capable of exposing duct-mount humidity sensors to a step change in RH and temperature at different duct air velocities. The test section consisted of two parallel ducts of rectangular cross section that shared a common wall. A schematic of this test section is shown in Figure 1.

[FIGURE 1 OMITTED]

Ducts were constructed of 0.25 in. (6.35 mm) thick acrylic glass. Two separate airstreams were fed into the test section from isolated sources, each with a different RH. A series of instrumentation ports allowed reference RH sensors and velocity probes to be inserted into each duct. The main insertion point for the test humidity sensor was on the outer wall of Duct A (Figure 1). The test sensor was exposed to the airstream in Duct B through an access point located in the common wall between...

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