Post by thread:[EE]: Thermal anemometer sensor

I have to monitor wind flow from a cave entrance. Air flow (in-out) is usually fast in Winter & Summer (10-30 ft/sec) but is often very "light" in Spring & Autumn (less than 3 ft/sec) and probably a "cup" anemometer isn't a good choice for this low values, so I'm thinking to a kind of "thermal anemometer" circuit (not necessary an "hot-wire" type), simple to build and not too expensive in power requirements (max. 1W) and in accuracy (+/- 10% accuracy) I want use my 16F84 logger to do this job, so the simplest way that I know is to measure a frequency with TMR0 (I already have a TMR0 routine to read a conductivity sensor). The "ideal circuit" that I dream is a kind of "overheating" sensor (a thermistor?) that outputs a frequency, or that I can easily convert in a freq. I have searched on the web a sample circuit, but at the moment I haven't found anithing (enough simple..). Have anyone any suggestion for build this circuit? Thanks in advance Marco -- http://www.piclist.com#nomail Going offline? Don't AutoReply us! email spam_OUTlistservTakeThisOuTmitvma.mit.edu with SET PICList DIGEST in the body

I have a circuit that I have for that but haven't tried yet. It's for measuring the airflow for my foundry casting furnace. I'll PDF the schematic tomorrow and post a link where you can find it. An alternative, which was done in Circuit Cellar Inc a long time ago was to use Ultrasonic transducers and measure the speed of the 40KHz ping across sensors separated about 2'. Bedtime in Victoria BC CANADA Cheers, John > {Original Message removed}

I suggest you a differential pressure transducer it is used to measure the breathing frequency and flow ciao Abele ---- Original Message ----- From: "marco genovesi" <.....marco.genovesiKILLspam@spam@LIBERO.IT> To: <PICLISTKILLspamMITVMA.MIT.EDU> Sent: Friday, April 05, 2002 11:19 AM Subject: [EE]: Thermal anemometer sensor > I have to monitor wind flow from a cave entrance. Air flow (in-out) is > usually fast in Winter & Summer (10-30 ft/sec) but is often very "light" in > Spring & Autumn (less than 3 ft/sec) and probably a "cup" anemometer isn't a > good choice for this low values, so I'm thinking to a kind of "thermal > anemometer" circuit (not necessary an "hot-wire" type), simple to build and {Quote hidden}> not too expensive in power requirements (max. 1W) and in accuracy (+/- 10% > accuracy) > > I want use my 16F84 logger to do this job, so the simplest way that I know > is to measure a frequency with TMR0 (I already have a TMR0 routine to read a > conductivity sensor). > The "ideal circuit" that I dream is a kind of "overheating" sensor (a > thermistor?) that outputs a frequency, or that I can easily convert in a > freq. I have searched on the web a sample circuit, but at the moment I > haven't found anithing (enough simple..). > > Have anyone any suggestion for build this circuit? > > Thanks in advance > Marco > > -- > http://www.piclist.com#nomail Going offline? Don't AutoReply us! > email .....listservKILLspam.....mitvma.mit.edu with SET PICList DIGEST in the body > > -- http://www.piclist.com#nomail Going offline? Don't AutoReply us! email EraseMElistservspam_OUTTakeThisOuTmitvma.mit.edu with SET PICList DIGEST in the body

Somewhere I saw a datasheet or databook, I think National Semiconductor, using two LM34 or LM35 temperature sensors (about $2 ea). Side by side, one measuring ambient temperature and the other measuring the conduction (or radiation) of heat from the first. As air flows between the two, the heat transfer between the two would be interrupted and an accurate rate of flow could be calculated at any ambient temperature. Obviously this is an analog voltage so you will have to either A/D it or V/F it to get it into an F84. I would recommend using an 16F87x because of its on board A/D. This would keep your parts count down. Or if price isn't a factor, a 16C711 will work. Rick marco genovesi wrote: {Quote hidden}> I have to monitor wind flow from a cave entrance. Air flow (in-out) is > usually fast in Winter & Summer (10-30 ft/sec) but is often very "light" in > Spring & Autumn (less than 3 ft/sec) and probably a "cup" anemometer isn't a > good choice for this low values, so I'm thinking to a kind of "thermal > anemometer" circuit (not necessary an "hot-wire" type), simple to build and > not too expensive in power requirements (max. 1W) and in accuracy (+/- 10% > accuracy) > > I want use my 16F84 logger to do this job, so the simplest way that I know > is to measure a frequency with TMR0 (I already have a TMR0 routine to read a > conductivity sensor). > The "ideal circuit" that I dream is a kind of "overheating" sensor (a > thermistor?) that outputs a frequency, or that I can easily convert in a > freq. I have searched on the web a sample circuit, but at the moment I > haven't found anithing (enough simple..). > > Have anyone any suggestion for build this circuit? > > Thanks in advance > Marco > > -- > http://www.piclist.com#nomail Going offline? Don't AutoReply us! > email listservspam_OUTmitvma.mit.edu with SET PICList DIGEST in the body -- http://www.piclist.com#nomail Going offline? Don't AutoReply us! email @spam@listservKILLspammitvma.mit.edu with SET PICList DIGEST in the body

> I have to monitor wind flow from a cave entrance. Air flow (in-out) is > usually fast in Winter & Summer (10-30 ft/sec) but is often very "light" in > Spring & Autumn (less than 3 ft/sec) and probably a "cup" anemometer isn't a > good choice for this low values, so I'm thinking to a kind of "thermal > anemometer" circuit (not necessary an "hot-wire" type), simple to build and > not too expensive in power requirements (max. 1W) and in accuracy (+/- 10% > accuracy) You should be able to build an impeller that works fine at low velocities. I have a fan that spins just when I walk around with it. Imagine how well it would work if it didn't have to turn the motor too and had a bearing designed for that purpose. ***************************************************************** Embed Inc, embedded system specialists in Littleton Massachusetts (978) 742-9014, http://www.embedinc.com -- http://www.piclist.com#nomail Going offline? Don't AutoReply us! email KILLspamlistservKILLspammitvma.mit.edu with SET PICList DIGEST in the body

Keep looking for a hot wire anemometer circuit. These guys are very accurate at low flow rates. They are devilishly simple. You have a hot length of wire, with a known current oflowing through it. The temperature of that wire will be X in still air, and there is a direct relationship between air velocity over it and it's temperature. The resistance of the wire changes with temperature, so you just measure the voltage across the wire while pumping in a constant current, and do some math on the ourput. Now, about that math............. That's something I don't know. Hot wire anemometers are not very efficient. If this is an application running on batteries, you'd better look elsewhere. OTOH, they don't have to run all the time. We had a portable hot wire anemometer instrument that was battery powered 20 years ago. You could turn it on and take a measurement in 15 seconds, then leave it off for ten minutes and take another measurement. --Lawrence {Original Message removed}

How about using a mass-air sensor from an EFI vehicle? Most of these are the hotwire-type system and actually has a small sub-path thru which the air is measured. You would use the whole mass-air sensor as a sub-path for the wind passing thru the whole cave entrance. Output signal is a varying voltage. If you go with a stock sensor, you can pick these up for next to nothing. Also, if you decide you don't want to pick out values from a lookup table, or linear-ize (yes I made that word up) the input signal data with a formula, you can have a company like Pro-Flow calibrate the meter so that its output is linear. Cheers, -Neil. {Original Message removed}

Hi Marco, This is the easiest one I've ever seen. There is also a vector anemometer design by the same fellow, if you do a search for his name and anemometer or airflow you'll get it. This anemometer uses two transistors to form the "heated wire" approach, but uses a current mirror that depends on the base-emitter voltage. Two comparators give you a pulse output for your 16F84. It's all here: http://archives.e-insite.net/archives/ednmag/reg/1996/031496/06di3.htm Best regards, Bob -- http://www.piclist.com#nomail Going offline? Don't AutoReply us! email RemoveMElistservTakeThisOuTmitvma.mit.edu with SET PICList DIGEST in the body

Marco, Linear Technology in a circa 1985 Application Note #5 on Page 7 has a pretty good circuit for a hot wire anometer. The circuit could do with some newer technology parts but the principle still holds up. Don't forget to take the envelope off of the light bulb and it should work pretty well. The Ap. Note is at: http://www.linear-tech.com/pdf/an5.pdf Cheers, Phil -- "There are three principal ways to lose money. Wine, women, and engineers. While the first two are more pleasant, the third is by far more certain." -- Baron Rothschild ca. 1860 -- http://www.piclist.com#nomail Going offline? Don't AutoReply us! email spamBeGonelistservspamBeGonemitvma.mit.edu with SET PICList DIGEST in the body

Hi all, Here's the fairly simple Hot wire (so to speak) air velocity sensor. It came out of a Linear Technologies data sheet I think. Don't remember for sure. The configuration of the circuit is such that the op amp is configured as a comparator since I'm only concerned with air being there or not for combustion. I may eventually upgrade to create an analogue output so that I can maintain air velocity regardless of furnace load when the lid is open or closed. http://www.pacificsun.ca/john/www/pdf-files/Furnace.pdf You can check out the furnace burner and electronics at http://www.autoartisans.com/newburne.htm And see the entire works at: http://www.autoartisans.com/foundry.htm The photos are old and need updating but there is never enough time. John > {Original Message removed}

I don't know how much current you have available, but you might look at MAF's. Mass Airflow Sensors are (used to be?) used in automobiles to measure the temperature/density/velocity of air entering the intake manifold. They may not be cheap at the parts store, but the junkyard should have them for very reasonable prices. An anemometer may be a better choice that's easier to calibrate. Michael Brown Instant Net Solutions http://www.KillerPCs.net "In the land of the blind, he who has one eye is king" {Original Message removed}

Well, I have received a lot of useful suggestions for the anemometer... First, thanks to John Dammeyer for his schematic: the "airflow" section seems quite simple. Also the Linear Technology Hot-wire circuit suggested by Philip Starbuck seems to have a good sensitivity, especially on low flow rate (the problems that I see are voltage to pulses conv. and the 15V supply). A mass-air sensor from an EFI vehicle can be another idea but where I live isn't too easy find cheap samples of these sensors. Olin, I don't know if impellers work fine at these low velocities. Yes, at a first time I thinked to a self-made and very "light" cup anemometer (the sensor was a simple led+phototransistor as frequency counter: very cheap and easy to interface wit 16F84). But i think that isn't so easy to make an unit (really, i will need 4-5 of these ) sensible to 0,5 ft/sec and so strong to endure at 10 mt/sec or more. In fact, the cave that I'm studying have a very variable airspeed at the entrance: also in the mid-seasons (Autumn, Spring) when the air in/out from cave entrance is usually low, a deep low in barometric pressure can generate speeds over 30 mph for a lot of hours. Probably I have to use, together, the two kinds of flowmeters.. At the moment, however, I'm starting to build the circuit suggested by Bob Blick ("heated" transistor): it seems cheap and simple, 5V supply (as my logger) and with frequency output. I hope it will have enough sensitivity for my use. Thanks to all! Marco -- http://www.piclist.com hint: The list server can filter out subtopics (like ads or off topics) for you. See http://www.piclist.com/#topics