Archive for May 20, 2011

While generally most testing is only good relative to the same test bench, it’s also nice if you can get some level of absolute accuracy for a sense of scale and to speak more of a common unit of measure. I do this testing all for hobby, so none of my meters are sent in for any sort of monthly calibration or certification.  While I thoroughly enjoy my little testing tools, they are not proven accurate and I simply don’t have a means or desire to do so while hobby testing. It’s very expensive and cost prohibitive at the recreational level I’m working within.

I figured the next best thing is to run my own checks using basic tools I have readily available.  Those basic tools include a  graduated container, a stop watch, and section of tubing to be used as a static pressure head water filled tube manometer.  In spirit of sharing information, I wanted to include this in a mini “how to testing” blog of sorts.

Some of these methods can also be used for casual testing.  If you think you might have a flow problem, but don’t have a meter…you can test your flow rate fairly accurately by the method described below.  Also if you think your pump is not up to par, you can (with a fair amount of work) also test pumps using tube manometers and bucket/stopwatch for flow rate.  I’ve done this before…long ago, but it’s very much a plausible method to extract a more scientific level of data or to test the accuracy of meters.

Testing Flow Meter Accuracy

Testing flow rate (Gallons Per Minute) is fairly simple.  Find a nice large known volume to fill and measure the time required to fill this known volume.  In my case I had an old 5 gallon race gas can that had gallon increments.  Mixing 2 stroke gas requires some level of ratio precision, so these cans are a bit more graduated than a normal gas can.  If such a graduated volume isn’t available, you could also create your own by using a smaller known volume and marking the container as you fill it up with the smaller known volume container. (A large bucket and an old empty one gallon milk jug would be one cheap and readily available option).

Moving onto the next variable…Time in Minutes. I took my DROID phone (Not the first time I’ve used it for science!), downloaded a stopwatch app, and proceeded to test in gallon increments.  I measured the time in minutes and seconds to fill the container from zero to the 5 gallon mark.  Here is the test setup and results:

Per the King Instruments Site, the larger King Instruments 7520 should read within 2% of full scale or .02X5 = 0.10 GPM.    I got around 1.7% of the measured scale or .06GPM maximum.  Flow rate calibration appears to be within specification…very good!

Testing Pressure Meter (Manometer) Accuracy

Testing lower levels of pressure is also fairly simple.  A manometer is nothing more than a pressure meter that measures the pressure  difference between two points.  If you disconnect the negative pressure terminal, it is now exposed to atmospheric pressure.  When you zero out to the atmosphere, you now have gauge pressure.  If you fill a column of water on the positive terminal, the net column height (Max level minus level in terminal tube) is the actual gauge pressure differential measured.   The tricky part is finding something plumb that you can measure a column of water.  Note that the tubing doesn’t need to be straight, but your measurement from water level at the top to lower water level needs to be plumb vertical.  Excuse the spider webs…:)  I just connected a funnel and clamped it to a column in our vaulted ceiling area.

Per the Dwyer Site, the Series 477-5 is capable of reading .5% of full-scale.  Full scale is 30.00PSI or 831 inches h2O.  So the possible error is .5% of 831 or 4.1 inches of water.  I measured an error of 1.2% of 100 inches or about 1.2 inches which is well within the 4.1 inch specification.  Looks like my manometer is also reading just fine within specs..:)

Manometer Pressure T fittings

In manometer testing, I think there is also some importance in the T line fitting used.  If it’s not a large smooth pass through fitting or the fittings are not perfectly identical, there could be some induced pressure differential from T fittings.  Like the venturi in a carburetor, if you reduce the sectional area, you will create a vacuum.  There is also the possibility with a large opening for the pressure junction that you could get odd flow momentum effects such similar to a pitot tube. I have seen some odd things in the past that makes me a bit careful about these fittings.  I think it’s good practice to spend some time on these fittings to minimize disturbance of flow and ensure equal pressure measurement conditions.  Water flow does change states and behavior as it increases in velocity, so keeping it down to nice slow speeds may help minimize measurement oddities.

I created my own T fittings from 1/2″ copper tubing and brass fittings soldered.  This leaves a clean and thin walled 1/2″ ID fitting with minimal turbulence and restriction.  They also have very small port openings leading to the manometer in an effort to minimize both restriction and any pitot tube like effects. I have used these custom manometer fittings for some time now and have been pleased with repeatability of pressure drop or pump tests.

Conclusion

While absolute accuracy is not really important at all in comparison related tests, it’s nice to know that my meters are within at least manufacturer specification.  That’s one way to do some checking on your flow and pressure meters if you’re interested and I’m pretty happy now that I finally checked mine.  There is also not much information on this, but I think it’s worth your time building your own T fittings when doing manometer related measurements.  These test methods can also be used by other reviewers that don’t have a manometer or flow meter.  I think it’s always good to do some of the more down to earth type tests like this now and then.  A number displayed on a magic box is worthless without the understanding of what it’s actually doing.  While time consuming, these forms of pressure and flow rate testing are also very educational.  I think they should be mandatory before anyone starts testing with a flow meter or digital manometer….:)

Looks OK from what I can tell..time for more pump testing..

Cheers!
Martin