Calibrating the tyre pressure gauge

How accurate is your tyre pressure gauge? Between the different gauges I had, there were differences up to 1 Bar, which is considerable given the fact that you are supposed to have your tyre pressure accurate to about 0,1 Bar in order to keep maximum grip combined with minimal tyre wear.

So, how to check? One method is to inflate your tyres to correct pressure with a certified gauge and then test the pressure with your gauge at home. But how do you know that first gauge has a valid certificate, or how do you know for sure the reading is still correct after possible abuse?
The more reliable method would in my opinion be to find a cylinder and have it lift a known weight. The needed pressure can be calculated after measuring the cylinder internal diameter.
Do we have a cylinder at home, usable for the experiment? Yes, most of us have: A standard bicycle pump!

For calculating example, I used the values of my pump, but yours is likely to deviate. Use your own measurements if you try this.
1) Unscrew one of the ends of the pump and measure the inside diameter "D" of the cylinder. (Ø 36,2mm = Ø 3,62cm) (Not every cylinder is exact round. Therefore one should measure at multiple points and use the average value.)
2) Calculate the area inside the tube:
    Area "A" = Π (Pi) / 4 x D x D = 3,14 / 4 x 3,62 x 3,62 = 10,29 cm²
3) Take a test pressure close to the values you need:
    A pressure of 1 Bar = 1,02 Kg/cm²
    2,5 Bar = 2,5 x 1,02 = 2,55 Kg/cm²
4) Calculate the weight needed on the pump to get the pressure (2,5 Bar)
    Pressure x Area = Weight (force)
    2,55 Kg/cm² x 10,29 cm² = 26,24 Kg
5) How accurate does the weight on your pump have to be for a reliable measurement?
    For this we calculate the weight difference for 0,1 Bar:
    0,1 Bar difference would be the above calculated 26,24 Kg / 2,5 / 10 = 1,05 Kg
6) So, when we put a weight of 26,24 Kg on the pump and we fill it with a bit of air until the piston goes upwards (Not to the end), we have a pressure very close to 2,5 Bar
    (There will be some piston friction, but it should have almost no effect here.)
7) Performing the above and your gauge shows 2,6 Bar, whilst it is now certain that the load is 2,5 Bar, you will know the gauge deviation is +0,1 Bar.
    Sometimes it is possible to adjust the gauge, but likely not, in which case it is sufficient to mark the deviation on the gauge.

To get the air into the system, there are several options, varying from the use of a foot-pump to a compressor. In my case, I used the compressor, which conveniently has a pressure gauge on the tool for filling tyres.

To load the pump with 26,24 Kg (This weight needs to be calculated for your case), depending on how accurate you want to be:
- Take out the piston, measure its weight and calculate this in the grand total.
- One can use a known weight to get close.
- Easy to use and accurate is loading cans with water. Measure the weight of the cans first, then add water until the desired weight is reached.
(water is 0,998 Kg/dm³ which in our case may be calculated as 1 Kg/liter ).