What is it about tyre pressure?

What do you measure at the gas station when you check your tyre pressure?

Tyre gauges at the gas station are measuring the pressure in the tyre relative to the surrounding pressure, i.e. the differnce by which the tyre pressure is exceeding the surrounding air pressure. (see also: Wikipedia-Cold Inflation Pressure)
Surrounding air pressure however depends on the location (height above sea level) and the atmospheric temperature and is decreasing in average by 100 hPa (equals approx. 1.4 psi) for every 1000 meters.
(The exact formula is quite complex. You can find details under Wikipedia-Atmospheric Pressure.)

Another important influence for tyre pressure is of course air temperature within the tyre or as first approximation the tyre temperature.
This correlation is expressed by the so called ideal Gas Law: when (absolute) temperature is increased by 10%, (absolute) pressure increases by 10% as long as the volume does not change – which can be assumed in a closed tyre.
As a rule of thumb this means for a 30 psi cold inflated tyre a temperature increase of 30 degrees (approx. 10%) will increase tyre pressure by 4.4 psi.

So if you want to measure your tyre pressure “exactly”, you have to measure:
at the same altitude level, at the same outside temperature and at identical tyre temperature; in order to eliminate differences between different instruments, you should also use the same gauge for each measurement.
The measured reading can be termed “exact” for this place, this time of day and this tyre state.

What measures a TireMoni sensor?

TireMoni sensors are measuring excess pressure in relation to a reference chamber within the sensor and are thus independent of environmental pressure. The sensors´ measurement values are calibrated for sea level.
This brings three crucial advantages for the user compared with measuring at the gas station:

  1. The measurement is taken regularly; one look at the display will do.
  2. The measurement is reliable, since it is always taken with the same instrument.
  3. Deviations from the desired state can be spotted early which leaves sufficient (>15 min) time for safely handling a problematic situation.

It is the latter which makes TireMoni invaluable for the motorist; in case of a puncture this much greater reaction time offers choices (change tyres on the spot or drive to a repair shop) and one can always find a safe spot to park the vehicle.

And what about measurement accuracy?

Actual measurement accuracy of TireMoni is not even relevant for the decisive advantages of the system. It will be of importance, if you want to make use of these other advantages:

  1. Less fuel consumption and
  2. increased tyre mileage

Note however: optimal tyre pressure in relatio to these aspects is a compromise which you have to find via experiment. There are too many parameters influencing the result (driving style, typical choice of route, load, choice of tyres, etc.).
The values given in the vehicle manual can be taken as useful initial values. Tyre manufacturers are generally recommending values that are higher by 2 – 3 psi.

What is important is that you compare like with like; it is a must that you always use the same instrument for the readings, i.e. best choice is to carry the instrument with you like you do if you own a TireMoni.
Equally as important is that you measure often, so that you can develop a sense for how your tyre pressure influences gasoline consumption and  tread wear. With TireMoni all you need to take a measurement is one look.

Due to its measurement principle at sea level TireMoni shows equal values as a calibrated pressure gauge; at 400 m height the instrument reads too low by 0,5 psi – which can be neglected for practical purposes.
At a height of 2500 m the deviation amounts to 3 psi, i.e. TireMoni shows 29 psi where you really have 31 psi. If you set the value recommended by the car manufacturer using TireMoni, then you are actually closer to the values that the tyre manufacturers are recommending. Values shown are closer to your pre-set warning thresholds, so that your driving safety is improved rather than impaired.

The TireMoni advantage is that you are always getting the same reading when driving from the valley up the mountain or the other way round. So one knows immediately: If the pressure value on the TireMoni display decreases, then the tyre actually loses pressure.

If you had an instrument instead which measures against environmental pressure, you would have to do some calculatios while driving downhill to find out if the pressure loss shown is related to the difference in altitude or if you really have a problem losing air from the tyre.

Fially for those who reall must have it spot on there is a way with TireMoni, too. Here is the procedure:

  1. With a calibrated barometer take a reading of current air pressure.
  2. With the sources and formulas given above, current air pressure and desired tyre pressure determine the absolute pressure for this tyre.
  3. Subtract 1 Bar from this absolute pressure – this is the pressure value that TireMoni should show after correctly filling the tyre.
  4. Measure tyre temperature – it should be between 17 and 20 degrees celsius; when necessary, cool or heat the tyre until temperature is within this range.
  5. Set the tyre pressure to the determined filling pressure (see 3).

Please note: if your environmental pressure changes or you are moving at a different altitude, you will of course have to adjust your tyres pressure; the same holds when you change the load of your vehicle.

Too complicated? Read again from the beginning…

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