A Review of Constant Temperature Anemometer

Abstract

Over the years many researchers have done research on constant temperature anemometer. Many have introduced new variation in the basic design of constant temperature anemometer like including current boosting stages, more differential amplifier stage, temperature compensation etc.

The techniques to analyze these designs like by finding the frequency response have been implemented by many researchers in both similar and different manner.

This review paper will include the design of constant temperature anemometer, the determination of frequency response done by many researchers.

This review paper will include 15 research papers listed in the reference of this paper.

This review paper will mostly focus on two parts design of constant temperature anemometer and finding the frequency response of constant temperature anemometer.

Introduction

A hot wire steady temperature anemometer (CTA) is a generally utilized instrument for the examination of turbulent flows. It takes a shot at the rule of cooling impact of a flow on a heated body. It tends to be utilized for an assortment of direction including estimating mean and instantaneous speed, the turbulence intensity, and so forth.

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On account of its simplicity of dealing with and its adaptability for estimating high velocity fluctuations, the constant temperature anemometer is favored over constant current anemometer in many applications.

A typical constant temperature anemometer consists of a bridge circuit. In the one leg of the bridge sensor is attached, the resistors are attached in the rest of the three legs. The two resistances are fixed and at the third leg a potentiometer is used which allows the adjustment of the resistance ratio.

Then an amplifier called servo amplifier is connected in a feedback.

The servo amplifier keeps the bridge balanced by controlling the current to the sensor with the goal that the resistance - and henceforth temperature - is kept constant, independent of the cooling forced by the fluid. The bridge voltage is a measure of the velocity as it represents the heat transfer.

In designing the constant temperature anemometer there are many different techniques that the researchers have adopted. Some researchers have used typical circuit of constant current anemometer with little to no modifications. (Ossofsky 1948, Freymuth 1967, WOOD 1975, Freymuth 1977, Weiss, Knaussetal. 2001). While some have used current booster stages (G.L.Morrison 1970, A 1983, Watmuff 1995). Some have used modified or on bridge designs (Ligeza 2000, Stornelli, Ferrietal. 2017). Some have use temperature compensation design (Rômulo PiresCoelhoFerreira 2001, Sosna, Buchneretal. 2010). We will discuss these variation latter.

The technique for measuring the frequency response is majorly done by two techniques square wave method and the sine wave method.

Design of Constant Temperature Anemometer:

At first (Ossofsky 1948) discusses the design of a constant temperature anemometer. He specifies in his paper that steady temperature operation of the hot-wire anemometer offers focal points in the investigation of huge velocity fluctuations, of recurrence as high as 30,000 cycles for per second and the feedback amplifier of the CTA provides the high frequency operation most advantage over other modes of anemometer (WOOD 1975) has also used basic CTA circuit in order to find the frequency response has utilized constant temperature circuit indicating toggle operation for setting the nominal overheat in his research paper have used a variable capacitor in addition to the basic CTA circuit.

This capacitor can be used to vary the time constant of the bridge in his research have used current boosting stage in addition to the basic CTA circuit. The current booster stage more often than not comprises of a couple of power transistors. It will be accepted that the current booster has unity gain and zero offset precisely. Therefore, it implies that the current booster doesn’t requires to be considered in the analysis.

(Miller 1985) has introduce a new idea a current source driven bridge. Which have two different current sources connected to the feedback of the amplifier instead of the two bridge-resistances. These different current sources supply the bridge circuit in two halves. This feature disposes of the requirement for dynamic stability compensation which is fundamental for commercial constant temperature anemometers.

In (Watmuff 1995, Weiss, Knaussetal. 2001) have used offset voltage in the differential amplifier. This offset voltage is adjusted until the system response obtains the desired shape. Watmuff have also used current booster stage and multiple stage of differential amplifiers. These are adjusted and observed in real systems. Weiss and Knauss have also discuss technique to find signal to noise ratio. For a CTA bridge, the signal to noise ratio can only be calculated by measuring the signal in the potential flow of a wind tunnel and then measuring the electronic noise have used CTA circuit with temperature compensation. Sosna and Buchner have introduced new temperature compensation system 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 kept up at a constant temperature exactly several Kelvins above fluid temperature with the assistance of a Wheatstone bridge circuit. If there should arise an occurrence of an adjustment in media temperature, an alteration of the heater temperature is important; otherwise, the temperature difference falls/ascends according to the temperature change, and the sensor yield deviates from its calibration. Temperature compensation can be performed by the utilization of an extra resistive temperature sensor. The circuit configuration exhibited incorporates a potentiometer that is used for changing the resistance of the temperature sensor and its temperature coefficient of resistance (TCR) the adjustment of temperature compensation.

While Pomulo have performed temperature compensation using one sensor instead of using conventionally two sensors. In his research he has connected two resistors to the feedback and V+ of the amplifier. These resistors are alternative switched. He presumed that the hypothetical assessment of the proposed setup has shown great application potential for this design has proposed the idea of a four point non-bridge CTA circuit. The conventional circuit main disadvantage is that the probe overheat ratio depends on the resistance of the supply cable. To resolve this issue, he designed a four point non-bridge CTA circuit have performed signal conditioning on CTA.

The new compensating circuit being introduced is the two input logarithmic circuit which is done by the help of operational amplifier. Then PSpice simulation have been utilized to measure real-life air-flow velocity measurement data have developed a new technique an auto-balancing modified Wheatstone bridge. The condition of bridge auto-balancing is usually achieved through the help of an integrator circuit. This integrator circuit is connected in the feedback of an operation amplifier. This integrator circuit generates a signal between the bridge voltage and the exciting signal. Under balancing condition, the bridge voltage is zero. The implementation of VCR can be achieved by using attenuator.

Frequency Response:

There are basically two method that the researchers have used to found the frequency response of the CTA circuit. The first is method is known as square wave test. A step voltage is supplied on a resistor of the bridge, the CTA parameters such as gain of the amplifier, offset voltage, etc. are adjusted until the system response obtains desired shape. Then the cut-off frequency of the anemometer can be assessed by a particular measure.

The square wave test but has a few inadequacies. As described in (Weiss, Knaussetal. 2001) research.

1. The test doesn’t tell us about the shape of the frequency response.
2.  The cut-off frequency is determined by the assuming that the system is a second order system. Higher order effects can lead to unwanted results.
3.  The cut-off frequency is determined by careful adjustment which is extremely tedious and is nearly impossible to be achieved in short time.

Therefore, a new method sine wave test is used. In this test, a sine wave is supplied in the bridge circuit, transfer function then is obtained by changing the frequency of the supplied signal and measuring the amplitude and phase of the output signal. This allows post-anemometer compensation which is a huge advantage over square wave test. In spite of the fact that this strategy can be computerized, it is significantly more time consuming than the square wave test. (Browand 1975) in his research mentions the same points as above. (Freymuth 1967, Watmuff 1995) both have used square wave test method in their research. While (Browand 1975, Weiss, Knaussetal. 2001) have used sine wave method. Some like (WOOD 1975, Freymuth 1977) have used both method.

Conclusion

Mainly modification of CTA includes adding offset voltage, current booster stage, variable capacitor and multiple differential amplifier stage, these usually are adjusted according to the real time in order to ensure that system response reaches a well-defined shape. Other enhancing feature like non-bridge version, auto balancing bridge and temperature compensation are used in order to eliminate some drawback of the conventional amplifier. The determination of frequency response has two method both have advantage over one another.

Literature Cited

1. A, J. S., K. Hayakawa and K. C. Muck (1983). 'Constant Temperature Hot-wire Anemometer Practice in Supersonic Flows.' Experiments in Fluid 1.
2. Browand, P. D. W. a. F. K. (1975). 'Analysis of a simple circuit for constant temperature anemometry.'
3. Freymuth, P. (1967). 'Feedback Control Theory for Constant‐Temperature Hot‐Wire Anemometers.' Review of Scientific Instruments 38(5): 677-681.
4. Freymuth, P. (1977). 'Frequency response and electronic testing for constant-temperature hot-wire anemometers.'
5. G.L.Morrison, A. E. P. a. (1970). 'A study of the constant-temperature
6. hot-wire anemometer.' J. Fluid Mec 47.
7. Ligeza, P. (2000). 'Four-Point Non-Bridge Contant Temperature Anemometer Circuit.' Experiments in Fluid 29.
8. Miller, I. S. (1985). 'A constant temperature hot-wire anemometer.'
9. Ossofsky, E. (1948). 'Constant Temperature Operation of the Hot‐Wire Anemometer at High Frequency.' Review of Scientific Instruments 19(12): 881-889.