by 84-1074663779 » Tue Feb 10, 2004 10:39 pm
The problem I have with this, is that the output from the flowmeter does not correspond to anything you can really rely upon. It will need to be first calibrated on someone elses bench, to develop a voltage versus flow curve. Then assuming that absolutely nothing ever changes, you then can duplicate all the errors in the other guys bench.
So you set it all up and you find that at a certain cylinder head valve lift there is 1.034 volts or something. Then you either apply a complex multi order polynomial equation to convert it direct to CFM, or you eyeball a curve drawn on a piece of graph paper.
The whole thing is prone to errors creeping in, and not terribly convenient to use. The idea that you can just feed the electrical output into a digital display that then says 125.184 CFM, looks impressive but might be less so if you knew the flow was actually closer to 130 CFM, and yesterday it would have been closer to 120 CFM.
The beauty of an orifice plate, or pitot bench is that the output pressure readings follow a predictable square law curve. If you can prove the flow at one or more points, you can pretty well rely on the results everywhere over the whole flow range for that particular sized flow path.
A water manometer is simple, reliable, linear, and repeatable. As a retired electronics engineer, I am painfully aware of the errors that can creep into more seemingly sophisticated methods of measurement.
I use an orifice style bench with a ready excess of available blower pressure. The only really irritating feature is converting percentage flow to CFM. Soon, I hope to make up eight flow scales calibrated directly in CFM displayed behind my flow manometer.
For ease of use there will be eight scales that correspond to the eight orifice plates installed in my turret. The individual flow scales will be mounted on a drum that revolves behind the flow manometer. Changing ranges should then be pretty easy, and If I want exactly 300 CFM, I do not have to first work out how much flow percentage it should be on which orifice.
I can just wind up my blower speed to the 300 CFM line on orifice number #8 or whatever. It should be the next best thing to a digital readout. This has not been done yet, but will probably be my next flowbench upgrade.