Development of an Automatic Quality Inspection System for Capacitive Polymer Dew Point Meters 

Jui-Ching Chiu¹, Chen-Ting Wu¹, Ming-Chung Tsai¹, Jen-Cheng Chu²

¹ Graduate Institute of Automation and Control, National Taiwan University of Science and Technology

² eyc-tech

Abstract

The objective of this study is to develop an automatic quality inspection system for capacitive polymer dew point meters under controlled low-humidity environments. The system consists of a PC controller (with a human–machine interface), a test chamber, a dual flow valve control module, a dew point sensor, a chilled mirror dew point hygrometer (MBW 973), a dew point meter microprocessor module, a data acquisition module, a communication system, and a linear compensation system.

In this study, a test chamber capable of controlling the dew point measurement range was first constructed. The dew point sensor was then installed inside the chamber for range testing. The PC outputs voltage signals via a DAQ card to control the dual flow valves, thereby regulating the chamber humidity from −60 °C to 20 °C. A chilled mirror dew point hygrometer (MBW 973) is used as the reference standard.

Servo feedback control is applied to the dry and humid air flow control valves to achieve mixed-flow measurements covering low to high dew point ranges. Based on the IEC 61298-2 standard, the system controls output signals at different humidity levels to complete the automatic quality inspection process. The system allows configuration of HMI parameters, quality inspection modes, and reference standard parameters, and automatically records data and generates quality control reports.

Through automated range testing design, this study establishes output matching and feedback mechanisms for dry and humid control valves, completing the flow control parameters of the dew point test chamber. Servo feedback control enables the chamber to achieve target dew point values within a tolerance of ±1 °C across all measurement ranges.

Keywords

Humidity, dew point, capacitive polymer dew point meter, data acquisition, quality inspection.

1. Introduction

Humidity is a critical environmental physical parameter in many industries, such as the paper, food, and electronics industries. Neglecting moisture generation can result in equipment damage or pipeline corrosion. Therefore, humidity measurement and monitoring have become increasingly important across various industries. In recent years, research topics and measurement technology developments related to temperature, humidity, and dew point have gradually increased [1–8].

Currently, the most common electronic relative humidity sensors are resistive and capacitive types. These two types are classified based on changes in the electrical properties of the sensing element: resistive sensors use resistance changes as signals, while capacitive sensors use capacitance changes. Differences in manufacturing processes result in different characteristics, each with its own advantages and disadvantages, depending on user requirements [6,9].

Resistive relative humidity sensors use polymer materials combined with electrodes. Changes in relative humidity cause variations in ionic conductivity, resulting in resistance changes. These sensors are based on stable polymer materials that produce stable signal variations; however, they are still susceptible to damage in highly corrosive environments. In general, resistive relative humidity sensors perform poorly in environments below 15% RH, although most humidity sensors also struggle in such low-humidity conditions. Near 100% RH, resistive relative humidity sensors exhibit excellent performance [4], which is difficult for capacitive relative humidity sensors to match.

Capacitive polymer sensors are commonly used for relative humidity measurement and can be combined with simple temperature measurements in the device to enable dew point detection. Capacitive polymer relative humidity sensors consist of a polymer thin film combined with electrodes placed between upper and lower electrodes. When water vapor is absorbed by the thin film, changes in relative humidity alter ionic conductivity and thus change the dielectric constant.

Materials used in this type of humidity sensor typically require a low dielectric constant (approximately 3–4 Fm⁻¹) and high resistance. Since water has a dielectric constant of approximately 80 Fm⁻¹, as moisture absorption increases, the dielectric constant of the material increases with ambient humidity. This phenomenon results in a proportional linear relationship between capacitance variation and relative humidity [9].

2. Capacitive Polymer Dew Point Meters and Stable Humidity Environment Control

2.1 Capacitive Polymer Relative Humidity Sensors and Dew Point Meters

The basic structure of a capacitive polymer relative humidity sensor consists of a polymer thin film combined with electrodes placed between upper and lower electrodes and fabricated on glass or ceramic substrates, as shown in Figure 1 [10]. When water vapor is absorbed by the thin film, changes in relative humidity alter ionic conductivity, resulting in changes in the dielectric constant. As relative humidity increases, capacitance increases accordingly, as shown in Figure 2 [11].

Like other types of relative humidity sensors, this type of sensor experiences reduced accuracy when exposed to contaminated or extreme environmental conditions. However, due to improvements in materials, capacitive relative humidity sensors can operate at high temperatures (up to 100 °C) and offer good linearity, which are their key advantages.

Several methods are available to detect capacitance changes. For example, LC oscillation circuits composed of inductors and capacitors can detect humidity through changes in oscillation frequency, and frequency variation measurement methods are also commonly used for humidity detection [11].

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