In order for a thermodynamic parameter or physical quantity to be truly defined, a technique for its measurement must be specified.
A meter does not perform its task accurately if it behaves like a reservoir of the state variable it is trying to measure.
Ideal meters have no effect on the state variables of the system they are measuring.
In some cases, the thermodynamic parameter is actually defined in terms of an idealized measuring instrument.
Although pressure is defined mechanically, a pressure-measuring device called a barometer may also be constructed from a sample of an ideal gas held at a constant temperature.
The behavior of an ideal gas is given by the ideal gas law: where P is pressure, V is volume, N is the number of particles (total mass divided by mass per particle), k is the Boltzmann constant, and T is temperature.
It is simultaneously dynamically connected to an external pressure reservoir, from which it is materially and thermally insulated.