single orfice with large scale - 0-600 cfm bench

[highway]

could you build a bench with a single orfice that flows 600CFM @28" and a inclined manometer with a long scale say 60"@X incline? It seems that you change the orfice to keep the incline mamometer from going out of scale. Thanks

[86rocco]

I can but you probably wouldn't be happy with the results.The nature of the scale is that the divisions aren't evenly spaced, they're much closer together at the low end of the scale than they are at the high end. To illustrate my point, here's a couple pictures of my scale, notice that at the low end, the difference from 0 to 10% is about 7 or 8mm but at the high end, the difference from 99 to 100% is about twice that, IOW, the scale is 20 times finer at the top end. My scale is 28" long but the same relationship is still there regardless of the scale's length. Play around with the spreadsheet found in and you'll get a feeling for what's going on.

[cboggs]

I have a 417 cfm range on my bench, .. ..

I do a bunch of heads that fall just under this range, .. small block mopar,
ford and chebby heads that have a peak flow of 350 to 400 cfm @ 28"

I just got the bench calibrated, but what I have been doing is just using
this one range to flow these heads from .100" lift at around 70cfm all the
way to .900" lift @ 400 cfm, .. .. and it's been dead nuts accurate.

The key I think is using electronics like the FP1 or in my case the
performance Trends Black Box. I don't think it's posable with a standard
scale, .. can't read it.

Curtis

[Tony]

If you can keep the flow up in the top three quarters of each scale, it will be a lot easier to read and still be fairly accurate. The divisions down in the first quarter quickly become very cramped and are not really usable.

Reading from 25% to 100 % of the scale length is a 4:1 pressure range, but only a 2:1 flow range, and if you want to read flow at low valve lifts accurately, several ranges are definitely required.

Something like:

Range 1 16 - 32 CFM
Range 2 32 - 64 CFM
Range 3 64 - 128 CFM
Range 4 128 - 256 CFM
Range 5 256 - 512 CFM

Five ranges might be the bare practical minimum. Six or eight ranges would be much better.

Two to one measurement ranges are also convenient, because the measurement orifice sizes will follow the following simple square root of two (x 1.4142) step sequence.

For example :

Range A 1.0"
Range B 1.4142"
Range C 2.0"
Range D 2.8284"
Range E 4.0"

[gofaster]

[color=#000000]I am ready to make my orifice plate and these are the figures I was using. My intention is to keep the inclined manometer above roughly 75% of flow on a square root scale when changing ranges. I am boring holes in a 1/8" thick stainless sheet.

My original assumption was a cd of 0.62.

Based on info from Larry C. about having a possible cd of 0.45, I think I should add a 1.500

[Scott]

I have a question, why not figure out yor orifice sizes assuming all is perfect (.62Cd) , but adjust your dimensions so they calculate to even flow numbers, like 400 is 400 not 393 or whatever, this way if it actually works out, the calulations from the percent of flow scale can pretty much be done in you head. You are boring the holes anyway and I would rather the flow numbers be nominal than the hole sizes.

RRBD

[larrycavan]

I think it's important to note that the actual Cd of the flowdisk ranges is an estimate that is not proven until final calibration of the bench has been performed.

Factors that you cannot predict can affect the final Cd value of the ranges. My bench happened to work out to a slightly lower Cd value than .62. Initially it was about .45. After a modification that value increased to .5xx. You'll want to carry out that value to the third decimal when you calibrate for best accuracy.

As Curtis mentioned, if you're considering a single range system, think seriously about electronics for your guage system. I would suggest to focus on what you'll actually be working with in terms of CFM. Select your ranges to correspond to peak CFM values of ranges you'll actually be using. The larger the hole, the more difficult it can be to nail the calibration.....

JMO
Larry C

[Tony]

I agree with both Scott and Larry, the best laid plans of mice and men...... !

But start off with a sensible sequence of orifice sizes (based on theory) to cover the ranges most useful to you. As the final exact Cd will be unknown, the manometer slope can then be adjusted to calibrate the 100 % flow figure. If the largest orifice size is calibrated correctly, the lower ranges should all then fall into line.

[Tony]

GoFaster, As a first attempt at building an orifice disc, why not make it easy on yourself and use aluminium ? It is a lot more friendly to work with, and if you get to repeat the whole exercise later (highly likely), you will be glad you did.

Using such a small part of the manometer rise means a huge number of ranges, which in practice will keep you very busy changing ranges all the time. Another aspect is that an overpressure event will blow the fluid out of the manometer and it will take time to drain back completely. Overpressure is far more likely when you are operating right up near 100% flow all the time and there is less margin for error.

[gofaster]

Tony,
The reason I want to work in the 75% + range is that it is the most legible half of the square root scale on my inclined manometer. I am already accustomed to working there with my SF-110, and it's no problem. Early on, I had a few miscues and blew out some fluid, but with experience it has ceased to be an issue. The big thing is that I can easily read the scale at the meniscus at that range.

The choice to make the orifice plate out of stainless is economy and durability. Since I will be using stoppers, I want to minimize wear. Originally I was going to use aluminum and have it coated with Ni-Tuff, but the coating costs were prohibitive. I thought I could have it coated along with the parts I regularly send to the anodizer, but it turns out that because of its size, the orifice plate would require a separate vat for coating and would cost about $250. As far as machining stainless is concerned, it is not a big deal with the eqiupment we have here. Analog and CNC machines.

Once the bench is running and calibrated, I'll probably try running lower on the scale and see if I'm comfortable with the results. I'm using Audie Flow Pro electronics and depression control on this new bench, and I have to see where it takes me.

I spent years using the manometers on the SF-110 and doing the math in the evenings at my kitchen table. I began using Performance Trends software with SF's FlowCom and Depression Control a few years back. It really speeded things up! The PT people told me I don't have to be as concerned with changing ranges with the electronics doing the reading and calculating for me, but it's my habit to do it at 75% +.

[larrycavan]

One more tip I'd consider worthwhile if you're going with a single range bench [or even a multi range] with electronic guages would be to add more motors.

I can't speak for any other system but the FP1 but since it's capable of 40", the potential for more measurable flow is built in. Add 2 more motors than you think you'll need and you won't be sorry...

Larry C

[highway]

Why not have a single orfice bench, and build a curved scale to accomadate for the changes in differental as the flow increases (keeping the scale evenly spaced) and have it read in CFM instead of % of flow? One scale no calculations simple right?

[Scott]

We're listening, keep going........
It would be great if it'll work.
I've thought of two different scales at different angles(wont work), but never one to do it all...

RRBD

[Thomas Vaught]

Percentages from day to day would not be an issue with a ratiometric bench as you would be comparing the test part against the orifice each time vs the "standard air day" deal you have with CFM.

Calling out CFM would be suspect as the air would change daily. Typical issue of a laminar device, constant flow corrections required.

Course, after you did the corrections all would be well.

JMO

Tom V.

[Scott]

An orifice bench with only one range might as well have the inclined gradated in cfm instead of percent, theres no other orifice's to worry about.......

RRBD