Calibrating your PCB shield

Once your bench is up and running you will need to calibrate it. Calibration is the process by which the software can determine whether the results measured by the MAF are accurate.

There are four calibration processes that need to be undertaken, in order, before you can use your flow bench.

1. Voltace Calibration
2. Sensor Calibration
3. Baseline Reference Leak Test calibration
4. Flow offset calibration

Voltage Calibration

Before plugging in your ESP32, you need to set the voltage of the buck PSU's on the shield. The reason you need to do this BEFORE you plug in your ESP, is that the ESP is a 3.3v device, and if your PSU's were shipped set to a higher voltage, then you could fry your ESP before you have even had a chance to use your flow bench.

To set the voltages you will need a suitable multimeter to measure the voltages with and a small flat screwdriver.

The DIYFB shield requires a 12v DC supply and can also be run from a car battery. Once plugged in you can set the PSU voltages as follows

• Unplug the ESP from the shield
• Plug in and switch on a 12v dc supply to your shield
• Measuring between the +out and -out terminals of U8 (bottom PSU)
• Adjust the output by turning the screw on top of the 'trim-pot' until the multimeter reads 3.3v
• (The 'trim-pot is the multi-turn potentiometer on the PSU Buck)
• Repeat the same process for U9, but this time set the voltage to 5v
• Once both the voltages are set, you can turn off the device and then plug in the ESP32.
• The ES32 will then take it's supply from the Shield.

Sensor Calibration

The sensor calibration is the initial setup that needs to be undertaken that ensure that all of your sensors are zeroed out correctly. The reason for doing this is that differences in manufacturing tolerances mean that not all components are made exactly the same and so there will be small differences between components that result in slightly different characteristics. When all put together, these differences in characteristics, along with differences in shield assembly, will affect the measured values and so it is important to take the differences into account and adjust the system so that each sensor circuit reads zero when at rest, or returns a value that is appropriate.

The method for doing this is fairly simple. Within configuration.h you will note that there are 'TRIM_POT' values for each of the main sensors. These trim pots are just like the physical trim pot that you used to set the voltages in the previous step, and allow you to trim the value of the sensor channel, except you trim these with your keyboard and not a screwdriver.

Simply adjust the values of the relevant TRIM_POT until the value you see displayed in the API or on the GUI is zero or matches the value of a calibrated test device. For example, if you are setting the value of a temperature sensor, adjust the TRIM_POT value until it matches a hand held temperature sensor. This should ideally be a calibrated device so that you know that the reading is accurate.

There are trim pot settings for discrete physical sensors such as the pressure sensors, MAF sensor and if used temp, Baro and RH sensors. For sensors that do not use discrete components, such as the BME, you will find that you can adjust the final value by modifying the 'FINE_ADJUST' offset if required.

Leak test calibration

There are actually two different types of leak test calibrations that you will need to carry out on your bench before you can use it. Each leak test has a specific purpose

Baseline Reference Calibration

The first type of leak test is to ensure that the bench construction is sound, and that there are no leaks in the system. This is carried out by sealing off the test surface and then running the bench. With no air able to enter the flow bench, the system should ideally read a flow value of zero. If there is a small amount of leakage, which for some bench deigns is inevitable, for example rotating orifice plate benches, we need to store this flow value and subtract it from all future flow tests. This ensures that the leakage amount does not inflate flow test figures.

It is also important to always retain this original test value, as we can also use it to help diagnose problems. For example periodically re checking the bench and comparing against this baseline value, will allow you to identify issues such as leaks or loose components. The baseline Reference Calibration value is essentially a snapshot of how the bench should perform in an ideal scenario, and provides you the ability to compare current and future performance against the baseline.

You can think of the baseline test as testing everything inside the bench up to the test surface.

Leak Test Offset

The second type of leak test is the calibration process you carry out each time you mount a job. The 'Leak Test Offset' relates to the test object itself. For example leaks relating to the spark plugs, valves, head seating surface etc, or maybe just by design. Whatever the reason, If you have a test object that has a small leak, but still want to go ahead with flow testing, you can store the leakage value. It will then be subtracted from all other flow tests carried out on the test object. Then when the job is finished, you can zero down the offset ready for the next job.

You can think of the Offset test as testing everything outside of the bench up to the test surface.

Conducting the Leak Test Offset calibration

• First ensure that you have already undertaken the Baseline Reference Calibration (see below)
• Install the test object on the Flow Bench test surface.
• Ensure that Spark plugs are installed and valves are closed.
• Block and seal the inlet port
• Start the bench
• Set the test depression
• Measure the flow value.
• To save the flow value as an offset
• Double click the status message to display the calibration dialog.
• Hit the 'Leak Test Calibration' button on the calibration dialog
• The current vacuum value will be stored in memory and also displayed on the configuration screen in the 'Leak Test Offset' field.

Conducting the Baseline Reference Calibration

The method used to conduct the baseline reference calibration is exactly the same as the leak test offset calibration with one caveat. You need to manually copy the value from the 'Leak Test Offset' Field, to the 'Leak Test Baseline' Field and hit the 'Update' button.

The reason for adding this manual step in the process rather than using a dedicated button is intentional. A button is very easy to accidentally press. By making the initial Baseline calibration a manual process, it helps to prevent the Baseline Reference Calibration value from being accidentally or inadvertently overwritten.

To carry out the Baseline Reference Calibration, do the following

• First ensure that the existing calibration values are all set to zero.
• You can do this by typing '0' in the field and hitting the 'Update' button or by deleting the cal.json file and restarting the system.
• Seal the Flow Bench test surface opening.
• Start the bench
• Set the test depression
• Measure the flow value.
• To save the flow value as an offset - Double click the status message to display the calibration dialog.
• Hit the 'Leak Test Calibration' button on the calibration dialog
• The current value will be stored in the "leak Test Offset' value field.
• Copy the value to the Leak Test 'Baseline' field and hit 'Update' to save it.
• Once complete you can then move on to the Flow calibration.

Undertaking a leak test

To compare the current bench operation with the stored calibration values you need to replicate the original test.

• Seal off the test surface so that it is air-tight
• Start the bench
• Set depression value to match that which was used for the original leak test
• View the Flow differential value. This is the difference between the stored value and the current value.
• You can access the Flow Differential values by clicking the Pitot Tile title until it changes to 'Flow Diff'
• You can further select the different type of Differential flows by clicking on the description below the displayed flow value
• This will cycle through the following

• User (a user set value)

• Baseline (the Raw flow value + Baseline)

• Offset (the Raw flow value + Baseline + Offset )

Flow calibration offset

The Flow offset calibration records the difference between what your MAF thinks it is flowing, and what the bench is actually flowing

Calibration involves using an orifice with a known flow rate and then comparing this to the measured value. If there is a difference, the system will record an offset that is then used to adjust all future measured values.

The calibration orifice needs to be very accurate and produce a known flow value at a known reference depression. Whilst calibrated orifice plates are the most accurate way to do this and are certainly the best way to calibrate a bench with a high reference pressure, for the basic shop vac style bench we suggest the use of a regular compact disc as this has a standardised holes size and is readily available worldwide. Using a CD negates the need for either buying a calibration orifice plate or accurately making one. For more powerful benches that are capable of pulling much higher vacuum, we recommend using a circular saw blade. Just like the CD, these are also available globally and follow standardised sizes. The saw blade will not deform like the CD at higher depression values.

The flow rate for a standard Compact Disc is: 14.4cfm @ 10"/wg The flow rate for a saw blade with 10mm bore is: The flow rate for a saw blade with 20mm bore is: The flow rate for a saw blade with 30mm bore is: The flow rate for a saw blade with 1/4" bore is: The flow rate for a saw blade with 1/2" bore is: The flow rate for a saw blade with 5/8" bore is:

To calibrate the bench use the following procedure

• Before calibrating you will need to save the 'Calibration Orifice Settings' into the configuration settings
• Turn on bench and perform a leak-test to make sure that there are no leaks
• Center the orifice on test surface, making sure that there is an airtight seal between the orifice and the bench
• Turn on the vacuum source and allow the bench to stabilise
• Set the reference depression to the same used to measure the calibration orifice
• Double click the status message to display the calibration dialog.
• Hit the 'Calibrate Flow' button to record the results.
• The calibration offset will be saved to the 'Calibration Offset' field.

A note on Calibration orifices

If you intend to operate your bench at higher test depressions, then we recommend that you obtain an orifice that has been calibrated at the reference depression you most commonly use.

If you find that when undertaking a leak test, your bench seems to pulse of fluctuate, especially if testing at higher depressions. You may find it useful to do the leak test using a small orifice instead. You can then manually subtract the orifice flow value from the resultant leak test.

Exporting Calibration Data.

The calibration data is stored in a file called 'cal.json'. You can download this file from the file management dialog by clicking on the title. We recommend that after you have undertaken the initial baseline and flow calibration, you save a copy of 'cal.json' to your local computer for safe keeping. This way should anything happen you will always have backup of the baseline calibration data. You can also write the baseline offset on the bench itself or in a log book or documentation that you keep with the bench.