Internal Plate Hold-down fixture

[Brucepts]

New product in the works, internal orifice plate hold-down fixture. Features a Buna-N square seal, 1/4-20 "studs", brass thumb nuts.

The studs are 1/4-20 through bolts to hold the plate to the 3/4" thk MDF center board. Orifice plate is hand-tightened to the fixture with the brass thumb nuts.

Pricing will be $45.00 and comes with all hardware.

[Flash]

I seen that on you 2017 bench build and was going to ask you about that. When i get a little bit further on my bench build, i will be needing that. Vary Nice!!!!

[MAX_CFM]

JUST added this to my shopping list and updated my email to add this to my bill.

[Gianvito90]

hello guys, I bought Bruce's plans and I'm trying to understand how the bench works so that I can customize it or use for example Arduino and my software to match it to the bench.
I didn't understand what the aluminum plates are for, both internal and external.
I understood that they are used to regulate the flow, na if I use a bench capable of pulling 600 cfm at 28", what is the use of changing the plates? What does it mean when I find 28+16 =44 written? They are the measurements of the external plates and internal? I would also like to understand how does the bench measure the CFM? does it have a flow sensor similar to a MAF? where is the sensor located? I noticed the two differential sensors that must be positioned one in the upper chamber and one in the lower one, all height of the aluminum plate. Can you read the difference between the two chambers? isn't there a comparison with the atmospheric pressure? sorry for the many questions. Also, I can't access the discussions on the digital pressure gauge because you need a password... how do you get it?

[Tony]

Hi Gianvito90, welcome to the Forum.

First, I suggest you read through "Flowbench 101" that should answer many of your questions.
http://www.flowbenchtech.com/forum/viewtopic.php?t=5

Great job on the hold down fixture Bruce.

[86rocco]

Gianvito90,
The purpose of the orifice plate is not to regulate the air flow, an orifice type flow bench works by comparing the airflow through something with known flow characteristics, a calibrated orifice plate, to something with unknown airflow characteristics, i.e. the part being tested. Any time air flows through an object a pressure drop is induced across that object and the size of that pressure drop is directly related to the flow characteristics of the object and amount of air flowing and, because water filled manometers were originally used to measure these pressure drops, the traditional unit of measure for pressure in a flow bench is "inches of water column", for reference, 1 psi is approximately 28"WC.

When testing with an orifice type flow bench, we direct the same sample of air to flow through both the test piece and the orifice, we adjust the air flow to induce a specific pressure drop across our test piece, (this is referred to as the test pressure and often 28"WC is chosen) and then we measure the pressure drop this amount of air flow creates across the orifice (this pressure drop is referred to as the differential pressure) now, because we know the flow characteristics of the orifice, we can use the differential pressure reading to calculate the rate of air flow through the orifice and because it's the same sample of air, we now also know the rate of air flow through our test piece.

Prior to the widespread use of digital manometers, the differential pressure was most often measured using an inclined fluid filled manometer and for number of practical reasons such as space available for the manometer, the inclined manometer often had a pressure range of 0-16"WC and 16" has become the de facto standard and for this reason, orifices are frequently identified by the amount of air they will flow when subjected to a 16" differential pressure. So, when you see 28+16, that refers to 28" test pressure plus a maximum of 16" differential pressure and, whatever type of blower is being used to move air through the bench must be able to create a pressure difference of 44" at the desired maximum flow rate. The selection of 28" test pressure and 16" differential pressure as standard values is somewhat arbitrary and to some extent these values were chosen because it's fairly easy and practical to build a bench to meet that standard and because they give reliable, repeatable test results.

Different orifice sizes are used to make the optimal use of the scale on the inclined manometer. For reasons beyond the scope of this discussion, you get the best, most accurate results when your differential pressure readings are close to the maximum readable by your manometer so if you're testing something and your differential pressure reading are consistently in the bottom half of the inclined manometer scale, then switch to a smaller orifice or if your test piece maxes out the scale, switch to a bigger orifice. Generally it's a good idea to select the smallest available orifice that meets your testing requirements. Of course, when using digital manometers this is less of a concern and you have more flexibility with orifice size selection but it's still good practice to select the smallest size orifice possible within the limits of your manometer.

[Gianvito90]

hello, excellent explanation, but I can't understand why 28" + 16" must be able to pull the engines...isn't it enough to pull only 28? the sensor in the first chamber will read 28, and then the sensor in the second chamber will read the differential pressure, and based on the diameter of the orifice we can calculate the flow rate...right? by the way....is there a formula to calculate the flow rate?

[86rocco]

The test piece is in series with the orifice, there is a pressure drop when the air flows through the test piece and then there is an additional pressure drop across the orifice. Think of an electric circuit with two resistor in series, for a given circuit flow, there is a voltage drop across each resistor and the total voltage drop is the sum of the voltage drop across the individual resistors.

[Gianvito90]

my english doesn't help me....
ok, but the sensors practically read the pressure in the first chamber and the pressure in the second? the ecu then makes the difference and we get the delta P? if the pressure in the first chamber is constant (28") the delta P will change every time I open the valve of my test head, this is because I will increase the engine depression to remain constant at 28" in the first chamber, therefore I will have a greater depression in the second chamber because the diameter of the internal orifice remains unchanged right? now having the delta P, how do I get the flow rate? the ecu which calculation implements?

[86rocco]

I think we're understanding each other, in your example, yes the delta p across the orifice would increase as you open the valve while maintaining a constant 28" difference across the head.

Now, on to calculating the air flow. Air flow is proportional to the square root of the pressure difference.
So, Flow = C x √Δp where Δp is the pressure difference across the orifice and C is a constant that describes the air flow characteristics of your orifice.

For example: Let's say you have an orifice that's calibrated as 200cfm at 16"WC, so 200= C x √16, that would make C for that orifice 50
Now, if you're testing a cylinder head with a 28"WC pressure difference across the head and that air flow generates a 9"WC pressure difference across this orifice, the air flow through the head would be: C x √9 = 50 X 3 = 150 cfm @ 28"WC

Flowbench diagram 2.jpg

You might find the spreadsheet in THIS THREAD useful.