Manometer Scale & Orifice Calculations

[86rocco1]

No not quite, ratiometric means that the flow bench is comparing the flow through whatever you're testing to a calibrated orifice, the manometer scale is graduated in % so if you test piece flows 90% and the orifice is calibrated to flow 150 scfm then you test piece flows 135 scfm. Now since it's a comparative measurement rather than the an absolute measurement and since changes in air density will have the same effect on your test piece as they do on the calibrated orifice, your flowbench should give you the same readings whatever the air density might happen to be. IOW, once you've got your orifices calibrated, you never need to worry about air density at all.

[Ted B]

I think i'm following your point but it leads me to another question.

Say i go ahead and do the following:
1. i get a calibration plate that is 150 scfm at 28 inches drop,
2. i measure the pressure drop across the orifice plate in my bench,
3. i set that measurement at 100% then set up my scale from there.

Now, that i've set up my scale do i have a calibration that's always independent of the air density for the instrument every day? Or do i have a scale that has to be reset with my calibration plate every time i use the bench?

[coulterracn]

I drew the Incline Manometer using information from Ed's Excel docuement then saved it as a PDF file. This one is for 21" long scale with 6" rise. You can email the PDF file to your local printer and there is your scale ready to glue on or be laminated then attached.

[jfholm]

How did you go about making that scale? I need one for 36" length and 12" rise. I was going to machine all the marks in my aluminum bar I am making but would rather print it out like you have done in case I want to change it.

John

[coulterracn]

Using Ed's Excel spreadsheet you should be able to put in your length and rise to calculate the scale. Then draw your scale using that information.
Ray

[coulterracn]

John

Try this, use the dimensions in the middle column to draw your scale. You may need to change the value of the fluid density to match the fluid you're using in your guage.

Ray

[jfholm]

Ray,
I have that spreadsheet. Did you just draw by hand from those dimensions? Or did you have some way to have the program automatically draw it our for you? I was looking for a quick way of course.

John

[coulterracn]

I used a computer aided drafting program called AutoCad version 2008. There are other drafting programs out there that are inexpensive and easy to use. I also have a copy of AutoSketch 9 that is compatible with all AutoCad programs. It cost me about $30usd. www.autodesk.com is a good source for autocad and related software. They have trial versions available for free download.

If I get a chance later tonight I'll draw one up for the 36" and post it.

[jfholm]

Ray,
I have figured out how to do it in Excel by just making each row the proper width I need and just having the borders on the top and bottom only. Seems to be working great, but I would not have even thought of it if you had not done what you did.

Thanks a big thanks!
John

[jfholm]

Ray and whoever is reading this thread,
I have failed to do it in Excel. I thought I could do the row height to exactly what I wanted but it always rounds off so I cannot get the precision. So much for that idea.

I will have to try AutoCad.

John

[jfholm]

Well I whipped my scale drawing right out in AutoCad. I always seem to have a knack of going the long way around to get to the best way. If any of you are wanting to use a 36" long scale with a 12" rise for your inclined manometer I have attached the AutoCad file for download.

I had to print it out and tape it on my inclined in multiple pieces as I only have an inkjet printer.

What I was hoping to do is find someone who can print it out in one single piece for my new flow bench when I get it built. Maybe a local company can print one long decal for me.

John

[coulterracn]

I had a local print shop print mine at a cost of $1.25. You'll have to convert to a PDF file or JPG file for some print shops.

To convert a cad drawing (.dwg) to a .pdf you will need a PDF creator. Cnet download has a couple free programs that convert the .dwg to a .pdf through the Plot/Print function.

Ray

[jfholm]

I tried to convert it to a PDF file and it did but was not scaled properly then. It was either too short or too long. When I printed it right out of AutoCad it was right on the money.

John

[coulterracn]

Try printing this and check it for accuracy. If it works I'll finish it and convert for re-post

Ray

[coulterracn]

Try yours again. I did a convert to PDF on your autocad dwg.

Ray