Hello PIC.ers, This question came up a while back. My archives contain a bit of thread, with my own and other people's replies. Date: Sun, 8 Apr 2001 10:03:56 +0200 From: Kent Johansen Subject: Re: [PIC] Flow measurement: Was: [PIC]: Inductive sensor & Coins Hi, The best way to use a thermistor to measure flow is to keep it at a constant temperature = constant resistance. You can do this with a simple opamp. After that, it is pretty linear within Re=100-10000 Siemens had som very small PTC's (0.2 mm) once. Kent On 6 Apr 2001, at 21:50, John wrote: > Hello Javier & PIC.ers, > > > > >Date: Thu, 5 Apr 2001 02:32:15 -0300 > >From: Javier > >Subject: [PIC]: Inductive sensor & Coins > > Unrelatedly, but someone lately was looking for help with an air-flow > sensor. > The dandiest design I've seen uses a (ptc) thermistor in the flow > (I *know*, I *know* those things are very non-linear...) > Anyhows, this little matter was gotaround by configuring the part in a > simple feedback cct. that maintained the ugly thermistor at a constant > temperature.... >> ie constant PD. > > As flow changes, the current required to maintain the PD changes, and this > is measurable & linear. Well, I suppose it is if you assume the heat > transfer > rate is proportional to flow rate, maybe only true in laminar conditions, > or maybe only true in turbulence, or, ... what is this leading to... > best regards, John email from the desk of John Sanderson. JS Controls, PO Box 1887, Boksburg 1460, Rep. of S. Africa. Tel/Fax 011 893 4154, Cell 082 741 6275, web http://www.jscontrols.co.za Manufacturer & purveyor of laboratory force testing apparatus & related products & services. _______________________________________________ http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist