Trouble with my new flow bench

[Machined]

I tested a cylinder head at .500 lift on the intake side against a 2 1/8" orifice at 28" of test pressure.The intake is supposed to flow 220-230 stock at that lift but the reading goes off the scale. From what I understand a 2 1/8" orifice is supposed to flow about 324cfm at 28" test pressure.I then flowed the 2 1/8" orifice against a 2 1/4" orifice and the same thing. I marked off on the inclined manometer about where the water should stop at 28" and it just kept climbing the more I opened the valve to bring it up to 28".In fact at about 24" it was reading about 100%.Anyone have any ideas as to what I did wrong?Any help would surely be appreciated. Thanks

[Rick360]

Unless your inclined manometer has a total (100%) range of 28" then that is your problem. That orifice would flow 324cfm (by your numbers) @ 28" differential across the orifice. The orifice range should be calibrated to match the water pressure range of your inclined manometer. Normally the range of these manometers would be < 10" water. If your manometer has an 8" range for example that would make a 324cfm @ 28" orifice into a 173cfm @ 8". I didn't calculate the flow for that diameter, but it sounds close. You need a much bigger orifice to get a 324cfm range for a normal inclined manometer.

Rick

[Machined]

The inclined manometer is 30" long.Whenever I test something that should be like 85-99% at 28" of the orifice size in the bench it goes beyond 100%.I also tested at 10" of test pressure and did the conversion to 28" and the same thing.It went beyond the supposed flow of the orifice I was testing against.You think maybe I need a bigger orifice or do you think there might be something constructed wrong internally?I did a leak test and pulled the pressure to 28" and the water in the inclined stayed at 0. Thanks

[bruce]

Is this a homemade bench? How did you machine the orifice holes? Have you ever had the orifice checked to see what it flowed on someone else's bench? Post some pics so we can get a better idea of what your setup is like?

If its homemade and you just machined a straight through hole for the orifice your Cd will be different than your calculated flow, the edge needs to be "dead sharp" to give accurate readings. The sharp edge also has to be the leading edge facing the airflow. The back angle after the sharp edge should be a 45* angle with a .125 thickness.



Edited By bruce on 1104595173

[Rick360]

[Machined]

Sorry I don't have a way to take pictures.Yes it is homemade similar to the MSD bench. The orifice holes were drilled through 26 gauge sheet metal with hole saws.I know it's a pretty crude way but I just wanted to test the bench before I got more exact. I figured even if the orifices didn't flow exactly as I figured at least the smaller of the two wouldn't flow more than 100%.The inclined manometer I made myself.It's 30"s long.The tubing is 1/8"ID.Sorry guys i'm not too flow bench smart so bear with me.Thanks

[Rick360]

Describe your manometer better. Does it have a well on one side or does the tubing go the entire distance from + to - pressure connections?

The VERTICAL RISE of the fluid is what counts, NOT THE LENGTH.

Put a level at the 100% mark over to directly under the 0% mark on inclined manometer,(make sure bubble is level) now measure from the 0% mark to the level. This will tell the vertical rise. With that info and well size I can help you determine the pressure diff. for your manometer.
A vertical manometer and a air pressure source could also be used to measure the 100% D.P.

Rick

[Machined]

I have a 5/8"ID syringe to hold the water at the bottom of the incline. The1/8"plastic tubing is attached to the bottom of the syringe and kind of u's up to the manometer and runs the entire length of the manometer and out the top of my flow bench and to the back where I have it attached to an 1/8"OD brass tube inserted into one of the plenums.The top of the syringe has a rubber stop pressed in with a short length of 1/8" brass tubing through it where one end is open to the inside of the syringe and the other end is where the other 1/8"plastic tubing is attached which runs out the top of the bench and to the other 1/8" brass tubing inserted in the other plenum.I just move the syringe up or down to zero the water at the bottom of the inclined manometer.I had the vertical rise at 12" but moved it to 13" to see if it made a difference.The rise is at 13" from the 0 to the 30" mark now.Does this sound about right?Thanks

[Rick360]

Accoring to the inside area's of the tube and the well, the fluid will move 1.2" in the well at 100% (30") on the inclined tube. If your fluid is water or has SG of 1.0 then the D.P. of your manometer at 100% would be 11.8"WC. (If your Spec. Gravity is different then multiply 11.8 * S.G. and use that number for further calculations) A sharp edged orifice will flow ~89cfm per sq. in. of area at 28" D.P. Since you need flow at 11.8" D.P. convert the 89@28" to 11.8" by
SquareRoot(11.8/28)*89 = 57.7766. This will be the cfm/sq.in of orifice area @ 11.8" D.P. multiplier. This will be specific to your manometer

If you want a 300cfm orifice.
300/57.7766 =5.1924 square inches of area
5.1924 sq. in circle = 2 * square root(5.1924/3.1416) = 2.5712" diameter orifice.

Your original orifice of 2.125 orifice = 204.9 cfm @ 100% of 11.8" D.P. manometer.

I hope your orifice thickness is a mistype ... 26 gauge is to thin for a orifice. It will bend under pressure and distort the readings.

Rick

[Machined]

Thanks Rick. I'll work with what you've given me.I'll see if I can get some more precise orifices to work with. I plan on building a much better bench with alot better precision in the future. Right now this is just something to play around with until I can better understand the science of airflow.