A reliable and economically feasible remote sensing system for temperature and relative humidity measurement

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Abstract

Sensing and recording of relative humidity and temperature plays an important role in many industries. In the past analogue devices were used for measurement of relative humidity and temperature. A simple technique combining analogue and digital circuit theory together with programming techniques has been used in this paper to design a remote temperature and relative humidity sensing system. The sensors analogue signal is applied to a micro-controller based data logger for storage purposes. The data is then transferred to the computer through standard RS232 serial port using the user interface program. The program also allows the user to input some of the important parameters such as sampling interval as well as starting date and time for the logging operation. A back-up power supply is also included to enable the system to remain functional in the event of a power failure. The proposed system has added advantages compared with existing systems, namely, in its low cost and high memory capabilities and the ability to perform in both real-time as well as off-line.

Introduction

Temperature and humidity process controllers monitor and refine ambient environmental conditions. Relative humidity and temperature measurement are vital in many industries. In the automotive industry, researchers seek knowledge of combustion and exhaust temperatures while in museums, polished metals such as brasses and bronzes, must be kept at 15% relative humidity or less [1]. In hospitals, the temperature in infant incubators must be kept in the proper range and in the food industry, temperature and relative humidity time cycles are monitored to ensure the high food quality. In pharmaceutical industries, relative humidity control is critical in the manufacturing of certain formulations, which are moisture-sensitive [2].

In the past, hygrothermographs were used to record the relative humidity over a long period of time. However, at present, electronic sensing and data acquisition techniques proved to be better approach. The sensed analogue signal will be converted to digital signal to enable it to be stored in electronic devices such as EEPROM and RAM. These make the electronic systems smaller, less expensive, more accurate and more reliable than chart recorders. The stored data is then downloaded to a PC. Data in PC format is preferable because it enables a more convenient approach for data analysis, presentation and storage. Furthermore, it does not require costly and inconvenient supply of charts because data can be viewed on a PC or laptop. Hardcopy can also be printed out using the printer when required.

Matko [3] used sensitive capacitive dependent crystals for high air humidity measurements. The sensor probe used is a comb-like humidity sensitive open capacitor whose output is used to change the frequency of a crystal. A micro-system has been developed by Yazdi et al. [4] that utilizes generic interfacing with capacitive sensors and then using micro-controller based wireless system, temperature pressure and humidity acceleration are measured. However, the system is quite expensive and complex in nature. Buff et al. [5] and Schmidt et al. [6] introduced the usage of surface acoustic waves (SAW) devices as temperature sensors while Cheeke et al. [7] used SAW devices as humidity sensors. Various techniques have also been proposed using different sensor systems [8], [9], [10] for measurement of humidity. In this paper, a simple and economically feasible technique for measuring relative humidity and temperature is proposed. The technique uses widely available components and would also be viable for commercial usage.

Section snippets

The sensor circuit

A humidity sensor, which consists of a plastic film, both sides of which are coated with gold to form a capacitor element, was used to sense relative humidity. For sensing the temperature, a thermistor bridge is developed. The sensing circuit has to be able to convert the sensed signals into 0–5 V voltage range to enable the ADC to convert the analogue signal to digital signal. For this reason, some linearization techniques were used.

Sensing of relative humidity. The capacitor used in sensing

Test results

Several experiments were carried out to test the accuracy of relative humidity and temperature measurements. The timing accuracy of the micro-controller was also tested. These experiments were done after the machine code had been programmed into the internal ROM of the micro-controller to avoid local heating from the in-circuit emulator.

Sensor accuracy test. Several experiments were conducted for different values of relative humidity. The results obtained from the Sling Psychrometer (SP) and

Discussion

A comparative study has been carried out with several industrial-standard relative humidity and temperature measuring devices that are available in the market. The results are summarized in Table 2. An analogue data recorder (Temperature/humidity/dew point recorder from SUPCO) is included in order to compare the advantages of digital data logger. Analogue recorder is not favourable as it is bulky, has limited functions and is expensive.

It is observed from Table 2 that proposed system has extra

Conclusion

A relative humidity and temperature measurement instrument with built-in sensing circuitry and data logger has been successfully developed and constructed. It is a low-cost product but has a large memory capacity. This is an added advantage compared to the other data loggers of the same class. A back-up power supply enables the system to keep functioning when power failure occurs. Various functions of remote sensing have been incorporated into the system such as delayed start through software

References (10)

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  • W. Buff, et al., Remote sensor system using passive SAW sensors, in: IEEE Conference Proceedings, 1994, pp....
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