by Thomas Vaught » Fri Jan 06, 2006 9:46 pm
When I did the layout on one of my first orifice plates, I copied the basic ranges of a SF 1200 bench as I liked the design, (200, 400, 600 holes) with rubber stoppers.
A couple of years later I installed the first "port hole" in a SF 1200 bench, except on that bench I had also added a 100 cfm hole. The ranges were 100, 200, 400, 600 with none of the larger holes being able to read the smaller hole as a cross-check on the flow (assuming you wanted to be above the 70% range or higher). The 100 hole was 50% of 200 hole, 200 hole was 50% of the 400 hole. The 400 hole was better but still was only 66% on the scale.
A couple of years after that I made more plates and put in a "Dice" pattern 5 hole arrangement: 50 cfm, 100, cfm, 200 cfm, 400 cfm, 600 cfm. It worked well except I rarely used the 600 cfm hole or went as high as 1350 cfm.
I saw the value of cross-checking holes and played around with the hole sizes for cfm to try and get many ranges with a few holes and also a small cfm change per range. Now I had 50, 100, 200, 300, 400 holes where I could do 100 cfm steps and test using only one hole or still
do 1050 cfm if I needed it. I could also do some cross-checking: 100 + 200 holes = 300 cfm hole, 100 + 300 holes = 400 cfm hole, etc.
86rocco proposed to me a hole combo that was "different" for his bench:
I believe he had a 50, 75, 100, 150, and 250 sizes. Help me out here, Ed.
The point is he had looked at things differently:
Range 1 50 cfm
Range 2 75 cfm
Range 3 100 cfm
Now Range 4 was range number 1 plus Range number 2 so 125 cfm.
Etc, etc, etc. But basically he could do 25 cfm increments just by having the 75 cfm hole size as one of the holes. It also worked out that the combinations opened up a lot more for cross-checking. Like 9 possible
cross-checks over the whole range.
Simple is good but some people like my buddy want to be dead on with the number and want to work on the top part of the scale all the time.
The FP stuff with do that but we are talking non electronics here with my friend.
I think this is also one of the ways that SF is using their many orifices to calibrate their benches.
Last but not least.
I don't know if many of you have looked at the Orifice calc sheet in detail but it gives AREA of the hole.
If you plug in the normal .62 cd using a 2.5 sq in hole area you wind up being a few cfm over 50 cfm. Same deal for the 5 sq inch deal: a little
over 100 cfm. You do this for all of the SF 300 ranges: 50, 100, 200, 300, 400 and you find that they are all over the range value using the .62 value. You change the cd to .586 and guess what all of the holes match
the range perfectly.
What I think SF did was they knew it would be very hard to do a perfect .62 cd plate every time on every hole so they decided to make a square
edged orifice vs a sharp edged orifice. They possibly tried different material thicknesses and materials until they could consistantly machine a hole that was about a .586 cd on the SF 300 bench. The ones I have tested are pretty darn close to being on the number.
With the later SF 600 benches it seems they would just REPORT the calibration number vs trying to hit the number every time.
I reported in a earlier post about the .586 cd number. This is where that value came from.
Tom V.