Flow sensing in hostile environments is of increasing interest for applications in the automotive, aerospace, and chemical and resource industries. There are thermal and non-thermal approaches for high-temperature flow measurement. Compared to their non-thermal counterparts, thermal flow sensors have recently attracted a great deal of interest due to the ease of fabrication, lack of moving parts and higher sensitivity. In recent years, various thermal flow sensors have been developed to operate at temperatures above 500 °C. Microelectronic technologies such as silicon-on-insulator (SOI), and complementary metal-oxide semiconductor (CMOS) have been used to make thermal flow sensors. Thermal sensors with various heating and sensing materials such as metals, semiconductors, polymers and ceramics can be selected according to the targeted working temperature. The performance of these thermal flow sensors is evaluated based on parameters such as thermal response time, flow sensitivity. The data from thermal flow sensors reviewed in this paper indicate that the sensing principle is suitable for the operation under harsh environments. Finally, the paper discusses the packaging of the sensor, which is the most important aspect of any high-temperature sensing application. Other than the conventional wire-bonding, various novel packaging techniques have been developed for high-temperature application.

Thermal Flow Sensors for Harsh Environments.

This paper presents a new temperature compensation technique for thermal flow sensors that are operated in a constant-temperature-difference (CTD) mode by means of a simple analog circuit. The resistive heater of a thermal flow sensor is maintained at a constant temperature some tens of Kelvins above fluid temperature with the help of a Wheatstone bridge circuit. In case of a change in media temperature, an adjustment of the heater temperature is necessary; otherwise, the temperature difference falls/rises with respect to the temperature change, and the sensor output signal deviates from its calibration. Temperature compensation can be performed by the use of an additional resistive temperature sensor. The circuit design presented here includes a potentiometer that is capable of changing the resistance of the temperature sensor and its temperature coefficient of resistance (TCR) for an easy adjustment for temperature compensation. This gives the freedom to use any material such as platinum, aluminum, or, in our case, an alloy of tungsten and titanium (WTi) for the temperature sensor, regardless of its resistance value and TCR with respect to the heater of a thermal flow sensor.

A Temperature Compensation Circuit for Thermal Flow Sensors Operated in Constant-Temperature-Difference Mode

Due to scarcity of accurate information and available data of actual human breathing, this investigation focuses on characterizing the breathing dynamic process based on the measurement of healthy human subjects. The similarities and differences between one breathing thermal manikin and the human subjects, including geometry and breathing functions, were thoroughly studied. As expected, actual human breathing is more complicated than that of the manikin in terms of airflow fluctuations, individual differences, and exhaled flow directions. The simplification of manikin mouth structure could result in overestimated exhaled velocity and contaminant concentration. Furthermore, actual human breathing appears to be relatively stable and reproducible for an individual person in several conditions and is also accompanied by some uncertainties simultaneously. The averaged values are used to analyze the overall characteristics of actual human breathing. There are different characteristics of the exhaled breath between male and female subjects with or without wearing a nose clip. The experimental results obtained from the measurement of human subjects may be helpful for manikin specification or validation and accuracy assessment of CFD simulations.

Measuring the exhaled breath of a manikin and human subjects

A compensation method of the sensitivity drift of the Wheatstone bridge sensor is proposed. The technique was carried out by placing a temperature sensor and the bridge to be compensated inside a generalized impedance converter with input reference current. No internal modification of the bridge arms is required so that the circuit is capable to compensate practical industrial sensors. The method is based on the temperature modulation of the current supplied to the bridge, which improves previous solutions based on constant current compensation. Experimental results are shown using a commercial magnetoresistive bridge sensor.

Temperature compensation of Wheatstone bridge magnetoresistive sensors based on generalized impedance converter with input reference current

Constant temperature hot wire anemometers are widely used in measuring fluid velocity. In this work it is shown that such system can be used to measure fluid flow in the forced oscillations technique applied to the human respiratory system, in substitution to the pneumotachograph. Experimental results are shown to compare the proposed system with the original one.

Breathing flow measurement with constant temperature hot-wire anemometer for forced oscillations technique

In conventional constant temperature hot-wire anemometers, the compensation for the fluid temperature is done by using two sensors. In this paper, the analysis of an anemometer circuit using only one sensor, operating alternately at two different temperatures is presented. The effect of the amplifier input offset voltage is taken into account, and its influence on the measurement of fluid velocity is quantified. Preliminary experimental results are also presented.

J.S. de Rocha Neto

Papers

Hot-wire anemometer with temperature compensation using only one sensor