9 | P a g e
Single internal battery - CR2032 coin cell battery. When replacing battery,
please ensure the “+” sign of the battery is facing up,
away from PCB.
Internal PCB - internal electronics circuit (The pressure sensor chip
is mounted within this PCBA, and it senses the tire
pressure against a built-in vacuum, resulting in an
Absolute Pressure reading. The internal firmware
will then subtract the sea-level pressure of 101.3
Kpa (14.7 psi) from this reading. This final reading
which will be shown in the app can be termed as i)
Tire Absolute Pressure minus sea level pressure,
or ii) Gage Pressure reference to sea-level altitude.
With this formulation, Fobo tire sensor will read the
same pressure value for any given tire, irrespective
of altitude (assuming a constant temperature). This
will ensure the correct tire footprint per vehicle
manufacturer‟s pressure recommendation, for all
altitude above sea-level, given a constant
temperature.
As a corollary, a tire with a Fobo reading of, say 40
psi at an altitude of 5000ft, will read the same 40
psi when driven to sea-level, given a constant
temperature. In reality, it will read higher due to the
warmer temperature at sea-level, and may need
some air release if the temperature difference is
great. Tire pressure generally will increase 1 psi
(~7kPa) for every 10 deg F (5.6 deg C) rise in
temperature.
Fobo wishes to highlight the above formulation is
for usage at sea-level and above, and will not be
accurate for use otherwise. In practice, this should
not be an issue as the lowest area on earth will
result in an insignificant error of 0.3 psi (~2 Kpa).
For users who wish to retain „Gage Pressure
Reading‟, the Fobo Tire App has a Gage Pressure,
adjusted to local altitude using cell towers or
individual phone barometer if available. This Gage
Pressure button can be accessed from the App
(Kindly refers to Section 5.10 of this user manual).
