Air flow metering can be a confusing subject to those who are not familiar with the subject. We are always more than willing to answer your questions or discuss our products with you. Call us at 415 892 8158 or email sales at flowperformance dot com.

We have a documents page on our web site that has information and instructions for our products. These documents are  in Adobe PDF format and can be viewed with Adobe Acrobat Reader. If you do not have the Adobe reader on your computer, you can download a free version from www.adobe.com. Please contact us for a link to our documents page.

A flow bench is an appliance that produces a difference in air pressure across a bounded path producing  air movement through the path. The volume of air that is moving through the bounded path is measured in relation to the differential pressure across the path.

An air source (shop vac) is used to produce a differential pressure across the test piece (cylinder head port, intake manifold port). The differential pressure is often referred to as "depression" or "test pressure" and is measured in inches of water column ("wc. 28"wc ~ 1 PSI). The test pressure is measured near the test piece (FR4 bench receptacle) and is used as a reference. It is important to know the test pressure because flow volume is dependant on test pressure.

       As the air source differential pressure moves air through the test piece, the air is also routed through a device that will measure the air volume.  This volume measuring device is often an orifice or a Pitot tube (flow element). Air volume is often measured in units of cubic feet per minute (CFM). These volume measuring devices create a differential pressure caused by the air moving across or through them. This differential pressure is measured and is used to calculate the air volume.

In a flow bench, a settling chamber is a box inline with air flow that allows the air to expand. When air flow is able to expand, the air movement slows down. This is useful for measuring static pressure and for introducing air into a measuring orifice.

In a flow bench, a discharge port is the opening, usually in the top of the flow bench, where the test piece is located for flow testing. Air is drawn through the dischage port to create a pressure differential across the test piece, causing air flow through the test piece.

For a flow bench, a cylinder bore adaptor simulates the cylinder of an internal combustion engine while flow testing cylinder heads. The cylinder effects the flow of a cylinder head, so it is useful to simulate the effects of the cylinder in relation to the cylinder head while flow testing. Some heads may experience more or less flow with different sized cylinders. You should never assume a larger cylinder will increase flow in a cylinder head, and that a smaller bore will restrict flow.

For a flow bench, an air source is the mechanism that creates a pressure differential that will cause air to move from one point to another. This is what creates the air flow through a flow bench and through the test piece. A shop vac is a good example of a small air source, using a vacuum motor to create the pressure differential. Many flow benches use multiple vacuum motors to create large amounts of air flow. Other air sources are industrial blowers and centrifugal blowers. Most air sources are bi-directional for flow benches, meaning they can move air in both directions.

A test piece is the object that is being flow tested. In most cases this will be an automotive cylinder head, intake manifold or exhaust. Other items that often find their way to a flow bench air air cleaners, filters, carburetors, velocity stacks, nozzles, couplers, adaptors of various types, HVAC items such as grills and directional outlets, and even model airplanes and cars. Yes, you can make a miniature wind tunnel from a flow bench.

It appears that the largest users of flow benches are performance automotive people. However, flow benches are also used by most all internal combustion engine manufactures.

CFM stands for Cubic Feet per Minute. It is a measure of volume over a period of time (1 minute).

A manometer is a device for measuring pressure. When you get your blood pressure checked, the glass tube with the mercury in it is a manometer, and it measures units of inches of mercury ("Hg"). Flow benches often use water or oil filled manometers, and measure pressure in inches of water column ("wc). 28 inches of water column is about 1 PSI pressure (Pounds per Square Inch). Flow benches also use electronic manometers instead of water or oil manometers. There are several types of water or oil manometers. A U-Tube manometer can measure large amounts of pressure but with limited resolution. A well Manometer has better resolution and can also measure large amounts of pressure. An inclined well manometer has very high resolution but limited range.

Inches of Water Column is a unit of measure for measuring pressure. 1 inch of water column is the amount of pressure that is required to move a column of water 1 inch. For instance, if you place a drinking straw into a glass of water and apply enough vacuum to the straw to raise the water up the straw 1 inch from the surface of the water in the glass, that would be about 1 inch of water pressure. Your glass and straw are a simple "well manometer" where the glass is the "well".

Yes, you no longer need your water or oil manometers when using the Flow Performance FP1. You also use the same connections as your old manometers, so you do not need to make any changes to your flow bench.

You can use the FP1 along side of your manometers, but you should not T into the same lines as your manometers. You should run separate lines from your flow bench to the FP1 when using your manometers. The reason is because any movement in the manometer columns of water, even microscopic movement, will cause pressure pulses in the lines that will affect the FP1 sensors.

A flow element is usually a self contained section of tubing that is equipped with sensors that can measure velocity of gasses passing through it.

A calibration orifice is an orifice that has a known flow rate at a certain differential pressure. It is used to calibrate a flow metering system like a flow bench. A typical flow bench calibration orifice is a flat material about 6" x 6" square with an orifice in the middle. This calibration orifice is placed over the flow bench discharge port and air is drawn through the orifice at a precise test pressure to calibrate the flow bench.

In an orifice type flow bench, a measuring orifice is the orifice in the flow bench that is being used to measure the flow rate through the test piece. The air that is moved through the test piece also passes through the measuring orifice. As the air moves through the measuring orifice, a differential pressure is created across the measuring orifice. By measuring this differential pressure, the CFM can be calculated from the differential pressure and the area of the orifice.

Test pressure is the amount of pressure differential that is created across the test piece to cause movement of air through the test piece, and is often termed in Inches of Water Column ("wc). In most cases, the volume of flow through a test piece will increase as the test pressure increases.

Depression is a term used to describe the differential pressure across the test piece. The term Test Pressure is often used.

A velocity probe is a Pitot tube device used to measure local gas flow velocities inside a bounded path. A velocity probe creates a differential pressure in relation to the local velocity that can be measured with a manometer or the Flow Performance FP1. By probing a port with a velocity probe, areas of high velocity can be located, and are sometimes good areas to explore for improved flow. Pitot tubes are used on air planes and race cars to measure air speed.

Invented by Henry Pitot, pronounced Pea-Toe, it is a device that creates a differential pressure when inserted into a liquid flow. By measuring the differential pressure created by the Pitot tube, the velocity of the gas can be determined.

Velocity is the speed of an object. For flow bench work, air flow velocity is measured in units of Feet per Second or Feet per Minute. Typical air speeds for a typical automotive cylinder head port with a test pressure of 28"wc is around 200 to 400 Feet per Second. Feet per Minute = 60 * Feet per Second.

Just as molasses becomes very thick and hard to pour in January, air density also increases, or becomes "thicker" when cold. Air density is often measured in units of Pounds per Cubic Foot (lb/ft^3). Air density is affected most by temperature, and also by atmospheric pressure. It is also affected by humidity, but this is so minute that it is often ignored for most cases. Typically, air has a density of about .0750 lb/ft^3 at sea level. Many people use .0750 lb/ft^3 as a Standard air density.

The FP1 requires a serial port to operate from a PC. Many newer PC's, especially lap tops, no longer have a serial port but rather a USB port. You can buy USB to serial port converters that will make a USB port work just like a serial port. These converters cost from $29.00 to $12.00. Be careful not to buy a converter that is specifically made to connect PDA's or cell phones to a PC, these type will probably not work.

This often happens when using a USB to serial converter that is made specifically for connecting a PDA or cell phone to a PC. Contact Flow Performance for help in resolving this matter.

A 5 HP or higher shop vac is recommended, but not required. You will want a shop vac that uses 2" hose and has a blower port. A blower port is used to connect your 2" hose to the shop vac to use as a leaf blower, and is useful to use for exhaust port testing. Not all shop vacs have a blower port. You will find some shop vacs offer very high velocities on their blower ports. These higher velocity blower port models will probably give you the highest test pressures and flow volumes for flow testing.

Shop vacs will give you the best all around performance for cylinder head flow testing. But if you are only interested in very high flow rates at low test pressures (large carbs, intercoolers or intercooler plumbing, air filter testing or exhaust manifolds for instance), a blower may give you a bit more flow volume, but only at low test pressures. Upgrading the FE2.0 to a FE2.5 or FE3.0 may be recommended too. For instance, a blower may give you 250cfm @ 6"wc, but only 8"wc @ 50cfm, while a shop vac may produce 35"wc or more at 50cfm. A shop vac will work better than a blower for low valve lifts on cylinder heads. A leaf or dust collector blower may only be able to produce a maximum pressure of 8"wc or less. An industrial blower will give you the test pressures you need and the flow volume, but these blowers are very expensive and require large amounts of power.

The Flow Performance Basic Electronic Flow Bench Kit with the FE2.0 flow element is good for about 360 to 380 real cfm. While we have been able to get 425 cfm through our FE2.0 flow element on a special test setup and very careful calibration, you should not expect much more than 380 cfm from a typical flow bench setup.

What else will I need to operate the FP Basic Electronic Flow Bench Kit

You will need a 3/4" thick bench surface with a 4.5" hole cut into it to mount the FR4.

A Windows 95 or later PC with one free serial port (or USB port and a USB/Serial port converter).

An air source. A shop vac (5hp or more with blower port) is a great way to get started flow testing.

Some silicon adhesive (to mount FR4 to bench surface) and some contact cement (to glue rubber gasket to bench top).

To flow test cylinder heads you will need:

A cylinder bore adaptor. We can make just about any size you need.

A method to locate valves and measure their height. A dial gauge and stand can be bought from Harbor Freight or Grizzly.com for about $20.00.

In some cases, bore diameter can be critical to cylinder head flow rates at higher valve lifts. But in many other cases bore diameter does not seem to effect flow at all. Canted or angled  valve heads and hemi heads are much less effected by bore diameter if at all. On heads that are sensitive to bore diameter, the critical dimensions are the distance from the bore wall to the edge of the valve head. This can be adjusted by offsetting the head on the bore adaptor, making a small bore have the same effect as a larger bore by offsetting the head. It is a good practice to document the bore diameter for your flow test results, but a more important statistic is the distance from the valve head edge to bore wall since it is very unlikely that a head will be located on two different bore adaptors of the same diameter in exactly the same location.

If you are using a shop vac for an air source, you do not really need to adjust the test pressure in most cases, the FP1 electronic flow rate processor will estimate the flow rates to any test pressure you desire, in real time. When you use a higher power air source, you will find the need to control the test pressure to keep the FP1 from being over pressurized.

Each has it's advantages and disadvantages, but most will agree the accuracy is about the same, both are capable of +/- 1% accuracy. We like Pitot tube technology because we find the calibration does not change with use, it is fairly robust, and the portability and flexibility of installation.

Again each has it's advantages and disadvantages and it really depends on what you are measuring. We calibrate our FP1 electronic flow rate processor with lab quality Meriam water manometers in a climate controlled room. But here is what you need to think about....when you are measuring the pressure of air that is moving (dynamic), there is usually some degree of turbulence in the moving air. This turbulence may cause the water level of a manometer to move or "bounce" during the test procedure. This movement of water in a manometer can make it very difficult to determine what the reading should actually be. Many devices are available to reduce or "snub" this movement in the water level, but often these devices only reduce the movement, not eliminate it. You can see the problem with "dynamic" pressure measurement using manometers.

       With electronic pressure transducers, the "dynamic" pressure can be examined and analyzed by digital circuits to produce a single value that is a good representation of the actual pressure . We think this ability alone makes the electronic pressure sensor superior for repeatable measurement of dynamic pressures found in most flow benches and HVAC systems. Repeatability is essential to accuracy. Now, add to this the ability to calculate flow rates and velocities, record and display them in real time as you test - while manometers require you to write down the readings on paper or punch the numbers into a calculator, or both, and perform multiple calculations to determine your flow rate data.

       Resolution is another important factor. While manometers are available with .001"wc or higher resolution, the range of these manometers is not enough to be of any practical use in any specific application except table top lab applications. On the other hand the FP1 electronic flow rate processor has a resolution of .001"wc with a range of 40"wc. If you were to buy a manometer with this specification, it would probably be 40 feet long.

       So, to summarize, we think that while manometers are an excellent calibration tool and excellent for measuring "static" pressures, electronic sensors are superior for "dynamic" pressure measurements and real time pressure data and analysis.

We believe the Flow Performance FP1 electronic flow rate processor is the most accurate, repeatable, flexable and powerful electronic manometer available for the money. Not only is the electronics upgradable, but the hardware is also upgradable, you do not have to worry about your FP1 becoming outdated or obsolete.

If your flow bench uses a pressure differential producing device to measure flow (orifices, Pitot tube, nozzle, LFE) you can use the FP1 with your bench. The FP1 will handle orifices and Pitot tubes directly, using MS Excel will allow you to use any other pressure differential producing devices with the FP1. We have helped numerous people get their flow bench projects up and running where manometers just did not produce the expected results or were not allowing correct calibration. We are very experienced with most bench plans and know how to get you up and running and calibrated quickly using the FP1.

There seem to be 2 popular vacuum motors being used for flow bench air sources. The Ametek 115923 dual stage vacuum motor is used in many commercial flow benches and is available from Grainger for less than $100.00. Also very popular is a GS Electric single stage vacuum motor available from surpluscenter.com (#16-1234) for about $20.00. We tested both motors and while the Ametek is able to produce more sealed vacuum pressure, the GS Electric motor can produce a bit more cfm. Both pulled 101 cfm at 28" of water in our testing. Both motors are thru-motor, meaning the air pulled in from the vacuum motor is exhausted over the motor to cool the motor. Another type of vacuum motor is bypass, which means the exhaust air is not exhausted through the motor, the motor has a fan on it for cooling with bypass designs. Bypass designs have cooler exhaust air while thru-motor designs heat the exhaust air quite a bit.