philb@ptolemy.TEKTRONIX.COM (Phil Biehl;685-2122;60-850;684-2867;orca) (12/15/89)
Some one in sci.aeronautics mentioned using a device called a
"hotwire anemometer" to measure windspeed across a aircraft
wing. I've heard of these things before (I think the Voyager
Mars landers used one) but have never been able to find out
many details of how they work and from what/how they are made.
Anybody know anything about these things? Can one fairly accurately
measure wind direction as well as speed? I'd like to use this for
a home weather station. Any and all assistance is appreciated.
Thanks
Phil Biehl (503)685-2122
Tektronix Inc., VSG, MS 60-850, POB 1000, Wilsonville, OR 97070
{decvax,ucbvax,uw-beaver}!tektronix!orca!philb or philb@orca.WV.TEK.COMhenry@utzoo.uucp (Henry Spencer) (12/16/89)
In article <5653@orca.wv.tek.com> philb@ptolemy.TEKTRONIX.COM (Phil Biehl;685-2122;60-850;684-2867;orca) writes: > Some one in sci.aeronautics mentioned using a device called a > "hotwire anemometer" to measure windspeed across a aircraft > wing. I've heard of these things before (I think the Voyager > Mars landers used one) but have never been able to find out > many details of how they work and from what/how they are made. The idea is exceedingly simple: you run a current through a wire, enough to heat it up, and measure its temperature (e.g. by using a wire whose resistance changes in a known way with temperature). The stronger the air currents over the wire, the better the cooling and the lower the temperature will be. Of course, you have to compensate for variations in air temperature. In principle it's a trivial device, although I'd imagine that calibration could be quite a bit of work. > Anybody know anything about these things? Can one fairly accurately > measure wind direction as well as speed? ... You could measure direction by measuring X and Y components of velocity, if you can devise some sort of ducting system to get only one component flowing past each of two wires. Actually I guess it would have to be a bit more complicated than that, since the hot-wire anemometer won't tell you *which way* the air is moving through the duct. It should be practical, although again it would have to be calibrated. -- 1755 EST, Dec 14, 1972: human | Henry Spencer at U of Toronto Zoology exploration of space terminates| uunet!attcan!utzoo!henry henry@zoo.toronto.edu
dale@lamont.ldgo.columbia.edu (dale chayes) (12/18/89)
In article <5653@orca.wv.tek.com>, philb@ptolemy.TEKTRONIX.COM (Phil Biehl;685-2122;60-850;684-2867;orca) asked about "hot wire anemometors"
I don't know how they are used as anemometers for planes, but I am
aware (peripheraly) of their use as a replacement for gyrocompases in
moderately high dynamic envirnoments (remote controled vehicles).
In essence, you blow an air stream between two "hot wires" and the
differential cooling is a measure of the deflection of the air jet
and the rate of rotation of the platform.
--
Dale Chayes Lamont-Doherty Geological Observatory of Columbia University
Route 9W, Palisades, N.Y. 10964 dale@lamont.ldgo.columbia.edu
voice: (914) 359-2900 extension 434 fax: (914) 359-6817tomb@hplsla.HP.COM (Tom Bruhns) (12/20/89)
henry@utzoo.uucp (Henry Spencer) writes: >In article <5653@orca.wv.tek.com> philb@ptolemy.TEKTRONIX.COM (Phil Biehl;685-2122;60-850;684-2867;orca) writes: >> Anybody know anything about these things? Can one fairly accurately >> measure wind direction as well as speed? ... >You could measure direction by measuring X and Y components of velocity, >if you can devise some sort of ducting system to get only one component >flowing past each of two wires. Actually I guess it would have to be a >bit more complicated than that, since the hot-wire anemometer won't tell >you *which way* the air is moving through the duct. It should be practical, >although again it would have to be calibrated. One way to do this is to have a heater in a duct with sensors arranged in a differential manner, with one sensor up-stream and one down-stream (or, obviously, the other way around if the direction reverses...) The heater could, itself, be a hotwire anemometer, if you don't get enough sensitivity from the adjacent sensors. I have no idea if anyone actually makes anything like this; just a thought. (I envision the whole thing being two of these duct/heater/sensor arrangements set at pi/2 radians to eachother--or maybe even three of them set orthogonally if you want vertical component, too. Two might well be set in a vertical round cylinder; three in a sphere. Or even just have one heater, in the open, with six sensors around it...)
zeeff@b-tech.ann-arbor.mi.us (Jon Zeeff) (12/27/89)
>> "hotwire anemometer" to measure windspeed across a aircraft > >The idea is exceedingly simple: you run a current through a wire, enough >to heat it up, and measure its temperature (e.g. by using a wire whose >resistance changes in a known way with temperature). The stronger the >air currents over the wire, the better the cooling and the lower the >temperature will be. Of course, you have to compensate for variations in >air temperature. In principle it's a trivial device, although I'd imagine When I did on-board microprocessor programming for Ford, we used one to measure air flow into the engine. It was particularily nice because it measures the mass of the air (ie, denser air cools it more. Denser is not necessarily = cooler). -- Jon Zeeff zeeff@b-tech.ann-arbor.mi.us or b-tech!zeeff
tj@juno.ll.mit.edu (Thomas E. Jones) (10/05/90)
I'd like to find references on design criteria and the formulas related to hot wire anemometers. Also, any sources of ready made hot-wire anemometers would be appreciated. -- tj@xn.ll.mit.edu or tj@ll-xn.arpa (one of these should work) Thomas E. Jones, home (617) 924-8326 work (617) 981-5093
larry@kitty.UUCP (Larry Lippman) (10/05/90)
In article <1990Oct4.181312.27021@xn.ll.mit.edu>, tj@juno.ll.mit.edu (Thomas E. Jones) writes: > I'd like to find references on design criteria and the formulas > related to hot wire anemometers. Also, any sources of ready made > hot-wire anemometers would be appreciated. Offhand, I don't know of a specific reference to design criteria for hot-wire anemometers. Hot-wire anemometers are not difficult to design and build, provided that you have available to you a standard against which to calibrate and evaluate performance of your design. For maximum accuracy, and repeatability I would recommend a standard such as a rotary vane anemometer, as opposed to a pitot tube arrangement. You will learn a great deal after building your first circuit and comparing its performance to a standard under varying flow conditions. The simplest hot-wire anemometer employs a 4-arm bridge circuit, with excitation provided from a constant current supply, and with one arm of the bridge being the hot-wire element. The bridge error signal is proportional to the fluid velocity. An alternate version uses a hot-wire element that has intimate contact with a temperature sensor. A bridge circuit with an error amplifier fed back to the bridge excitation runs the hot-wire element at a constant *temperature*, with the adjusted excitation voltage being proportional to the fluid velocity. In simple terms, the convective heat loss of a hot-wire anemometer element is proportional to the square root of the fluid velocity. One can vastly simplify hardware design if linearization and calibration can be performed by a microprocessor. There are various vendors of commercial hot-wire and hot-film anemometers, such as DISA Electronics and Kurz Instruments. Larry Lippman @ Recognition Research Corp. "Have you hugged your cat today?" VOICE: 716/688-1231 {boulder, rutgers, watmath}!ub!kitty!larry FAX: 716/741-9635 {utzoo, uunet}!/ \aerion!larry
jgd@rsiatl.UUCP (John G. DeArmond) (10/06/90)
larry@kitty.UUCP (Larry Lippman) writes: >In article <1990Oct4.181312.27021@xn.ll.mit.edu>, tj@juno.ll.mit.edu (Thomas E. Jones) writes: >> I'd like to find references on design criteria and the formulas >> related to hot wire anemometers. Also, any sources of ready made >> hot-wire anemometers would be appreciated. > Offhand, I don't know of a specific reference to design criteria >for hot-wire anemometers. Look in the SAE "Sensors and Actuators" journals, specifically the 1990 edition (sp-805) and the 1989 edition (sp-771). You may order these from: Society of Automotive Engineers 400 Commonwealth Drive Warrendale, PA 15096. They accept phone orders and accept credit cards. The journals are about $40 each. In particular, check out paper # 890298 (sp-771), 890301 (sp-771), 890480 (sp-771), 900258 (sp-805), and 900259 (sp-805). There are articles in this journal dating back to 1982 that would also be of interest. In particular, you might want to look in the 1987 edition (sorry don't have the edition number handy) for an article on vortex shedding flowmeters that work by counting the vortices that shed from an airfoil. This mature technology from the process control world promises to be much more reliable and achieve greater sensitivity than hot wire flowmeters. Most hot wire flowmeters now use thin film resistors instead of wires. The films are much more stable and vastly more rugged than the thin wires formerly used. > Hot-wire anemometers are not difficult to design and build, >provided that you have available to you a standard against which to >calibrate and evaluate performance of your design. For maximum accuracy, >and repeatability I would recommend a standard such as a rotary vane >anemometer, as opposed to a pitot tube arrangement. Your best source of single quantity flowmeters is your local junk yard. For 5 to 30 bux, you can take one off a wrecked car. This flowmeter will have the signal conditioning electronics already built in and will produce a high level (typically 0-5 volts) output signal that needs no further conditioning. These are also already temperature compensated. One of the articles above includes the necessary equations to do a math calibration on a flowmeter given its diameter, the air flow, the temperature and humidity of the air involved and the thermal conductivity of the air. The other low cost method of calibration, and one used on some spirometers I have, is a calibrated cylinder. The cylinder is discharged through the flowmeter and some fairly constant rate. The volume under the cylinder is known and can be compared to the area under the output curve which represents total flow. IF the cylinder is discharged at a constant rate, the flow rate can be determined. My cylinder was supplied by the spirometer and consists of a fiberglass tube about 8 inched in diameter and about 3 feet long. An O-ringed piston fits the inside and is hooked to an operating rod with a handle on the other end. The far end is capped and is fitted with a hose fitting. Each end is equipped with positive travel stops and the cylinder is marked with its measured capacity. You could easily make one of these from plexiglas tubing and sheet plastic. Yet another method is the liquid displacement method which is used by some metrology labs as a transfer standard. The principle is simple. If you fill a cylinder with a liquid at a known rate, the same quantity of air will be displaced. If you arrange the fill liquid to come from a short, fat container raised at a high elevation relative to the container's height, you can achieve a remarkably constant flow. If you dump one gallon of water into a container in a minute, one GPM of air will leave the outlet. This technique using manometer (low vapor pressure, precise specific gravity) oil was used in a lab I used to work in for low flow calibrations. > The simplest hot-wire anemometer employs a 4-arm bridge circuit, >with excitation provided from a constant current supply, and with >one arm of the bridge being the hot-wire element. The bridge error >signal is proportional to the fluid velocity. > An alternate version uses a hot-wire element that has intimate >contact with a temperature sensor. A bridge circuit with an error >amplifier fed back to the bridge excitation runs the hot-wire element >at a constant *temperature*, with the adjusted excitation voltage >being proportional to the fluid velocity. Actually most industrial/automotive sensors are arranged in a power bridge arrangement whereby the sensor is operated at a constant temperature as measured by its resistance. A power op-amp senses the bridge error and applies whatever power is needed to maintain constant temperature (resistance) in the bridge. A similiar sensor run at zero power dissipation on the other leg of the bridge but co-located performs air temperature compensation. The output is proportional to the power input to the bridge. Most hot wire research in the automotive field is currently concentrated on pulse, PWM techniques to reduce the power draw by these sensors. See SAE paper # 890298. > There are various vendors of commercial hot-wire and hot-film >anemometers, such as DISA Electronics and Kurz Instruments. Kurz is probably the best known supplier. They can supply you with instruments that are nuclear qualified if you so desire and can afford :-) They are in Monterey, CA and can be contacted at 800 424 7356. You can get yourself a radon detector from them while you're at it :-) John -- John De Armond, WD4OQC | We can no more blame our loss of freedom on congress Radiation Systems, Inc. | than we can prostitution on pimps. Both simply Atlanta, Ga | provide broker services for their customers. {emory,uunet}!rsiatl!jgd| - Dr. W Williams | **I am the NRA**