# Hot wire anemometer

Discussion in 'Homework Help' started by nikomas, Mar 31, 2010.

1. ### nikomas Thread Starter New Member

Mar 31, 2010
11
0
Hi everyone.

I am trying to make a hot-wire anemometer with this Pt100 thin film:

http://uk.farnell.com/labfacility/dm-334/sensor-pt100-thin-film-2x10mm-cl/dp/8598525?Ntt=8598525

I want to heat it with constant current, and to measure wind speeds in range from 0 to 20 m/s.
I need to simulate it without measurement first, and I'm stuck. How much current do i need to give it for this kind of measurement? It must me method with constant current because that is the point of my project..

Can you give me some ideas?

Thanks!

2. ### someonesdad Senior Member

Jul 7, 2009
1,585
142
I assume you plan on measuring the voltage drop across the sensor, right? With a constant current, this would then reflect the resistance of the device, which in turn would be related to the cooling of the air flow across it, which determines the heat transfer to the air, which affects the temperature of the heated wire.

Thus, your fundamental measurement is a voltage. Thus, pick a voltage that's convenient to measure. You'd want to pick a voltage that was where your voltmeter was most sensitive and accurate. If it's 1 volt, then you need a 10 mA constant current.

To make an accurate device, I'll assume you've read some papers and know about the corrections and allowances that likely have to be done. If not, get thee to a library.

3. ### lja New Member

Aug 27, 2008
1
0
Don't forget to measure ambient temperature as well. 20 degree air moving 5 MPH will cool the sensor more than 50 degree air at the same speed. You'll need to compare the sensor voltage with the ambient temp readings.

4. ### nikomas Thread Starter New Member

Mar 31, 2010
11
0
For hot wire anemometers is characteristic that the electrical power input is equal to the power lost to convective heat transfer:

I^2*Rw = h*Aw*(Tw-Tf)

where I is the input current, Rw is the resistance of the wire, Tw and Tf are the temperatures of the wire and fluid respectively, Aw is the projected wire surface area, and h is the heat transfer coefficient of the wire.
The wire resistance Rw is also a function of temperature.
The heat transfer coefficient h is a function of fluid velocity vf according to King's law:
h=a+b*vf^c

where a, b, and c are coefficients obtained from calibration (c ~ 0.5).
If i use constant current hot wire anemometer, current will heat wire and resistance of wire will change.
If i know that i will use pt100 thin film

http://uk.farnell.com/labfacility/dm...25?Ntt=8598525

and if i assume that fluid temperature is 20 degree can i know how much current i need to heat wire enough to measure wind speeds of 20m/s so i can make electric circuit with that current?

5. ### beenthere Retired Moderator

Apr 20, 2004
15,815
293
You could use a temperature sensor like an LM34 for the ambient temperature input, as suggested in post #4.

6. ### someonesdad Senior Member

Jul 7, 2009
1,585
142
Since you're apparently doing a homework problem, I'd also suggest making estimates of heat loss due to conduction and radiation too, just to make sure you've accounted for everything. Radiation is likely negligible compared to the other losses, as long as the wire isn't getting e.g. red hot. But since you say this is a thin film device, it's deposited on a substrate and I would imagine conductive losses would be at least as significant as convective losses.