1.1.BACKGROUND OF THE STUDY
According to W. Stephen Woodward W. Stephen Woodward Jan 22, 2001, explicit airflow detection is essential in many applications. High powerdensity electronics are liable to overheat and selfdestruct when coolingfan failures go unnoticed. Heating and airconditioning systems often incorporate multipoint monitoring of ventilationduct flow. Cleanroom airhandling systems with undetected dirty, blocked air filters can ruin process yield. Laboratory fume hoods can contain volatile solvents or toxic reagents, making adequate air turnover critical to safety. In these and similar scenarios, the consequences of undetected airflow interruption can range from the merely expensive to the frankly dangerous. Therefore, it becomes necessary to use some reliable means for airflow detection. Usually, either a mechanical pressureactuated vane switch or one of the various
types of heattransferbased airflow sensors is employed.
An advantage of the method of air sensors used here is that they contain no moving parts. But they often require several watts of heating input to run hot enough to overcome ambient temperature variations. The detector described here is a powerthrifty member of the thermal genre. It employs an ambientcompensated airflowdetection scheme based on differential heating of a series
connected transistor pair. In operation, 200mV reference regulator A1 maintains a constant Q1/Q2 current drive equal to 40 mA i.e., 200 mV/R1. Since the two transistors pass the same current, their relative power dissipations are determined solely by their respective V voltages. For the circuit constants shown, these power levels work out to
4 V 40 mA 160 mW for Q1 and 0.75 V 40 mA
30 mW for Q2. The 130mW heatflow difference leads to a temperature difference determined by the heatdissipationversusairspeed characteristics of the 2N4401s plastic TO92 package. The TO92s thermal impedanceversusairspeed characteristic is well approximated by the simple equation shown below
Z Z 1/S K A
Where:
Z total immersion junctiontocase thermal
Impedance 44C/W
S stillair casetoambient conductivity 6.4 mW/C
K Kings Law thermal diffusion constant 750 W/Cfpm
A airspeed in ft. /min.
Therefore, the Q1/Q2 temperature differential ranges from 130 mW 200C/W 26C at 0 fpm zero flow, to 130 mW 75C/W 10C at 1200 fpm the 14mph breeze found at the output face of a typical 100cfm cooling fan. This flowdependent temperature differential gives rise to a flowdependent V differential via the 2N4401s typicaltransistor V temperature coefficient of 2 mV/C. Comparator A2 matches this Q1 /Q2 ratio to R2/R3. Under high airflow, Q1 is cool and Q1 /Q2 > R2/R3, which makes A2s output high i.e., flow OK. With a stagnant airflow as might connote fan failure, flue fouling, or filter fillup Q1 is allowed to heat up, driving Q1 /Q2 < R2/R3. This causes A2s output to slew low, asserting the lowflow faultalarm condition. For these circuit constants, the noflow alarm threshold is 100 fpm Fig. 1, again. But this line in the sand can be easily adjusted. Raising Q1s power dissipation by boosting collector current increases the threshold.
Setting R1 4 , for instance, would bump Q1s power input to 200 mW and quadruple the lowairspeed set point to 400 fpm. Increasing R1 allows the setpoint to be moved the other way toward a lower flow level.
For example, R1 6.4 would cool Q1 to a tepid 125mW and thereby quarter the nogo flow criteria to 25fpm.
Besides being adaptable to different flow rates, the circuit also can accept different supply voltages. In these cases, R1 must be multiplied by V 1/4 to hold
Q1s I V heating level constant.

1.2.STATEMENT OF PROBLEM:
Owing to the alarming rate of the ugly incidents caused by the unavailability of air detector, the following forms the statement of problem of this study