Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!uunet!rsiatl!jgd From: jgd@rsiatl.UUCP (John G. DeArmond) Newsgroups: sci.electronics Subject: Re: Hot Wire Anemometers... Message-ID: <4183@rsiatl.UUCP> Date: 5 Oct 90 20:30:05 GMT References: <1990Oct4.181312.27021@xn.ll.mit.edu> <4082@kitty.UUCP> Organization: Radiation Systems, Inc. (a thinktank, motorcycle, car and gun works facility) Lines: 115 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**