A pressure balance is simply an alternative name for a piston gauge (more frequently spelt piston gage). When fitted with a means of pressure control, additional pressure ports and masses etc, the complete system is sometimes known as a dead-weight tester(*). Not all practitioners differentiate consistently between these terms and dead-weight tester is often used as a multi-purpose default description.

(*) Pressure deadweight testers should not be confused with deadweight force machines for generating forces.

Additional information

Pressure balances/piston gauges are widely used for maintaining calculable pressures in a range extending from about 3 kPa (gas media, absolute- or gauge-mode) to 1 GPa (hydraulic, gauge-mode). Consisting essentially of finely machined pistons mounted vertically in very close-fitting cylinders, the internal pressure required to support the weight of the rotating piston and associated masses is calculated from the fundamental relationship between three quantities; mass, length and time:

Pressure = force/area = (m×g)/A

where m is the mass of the piston and associated masses, g is the local value of acceleration due to gravity and A is the effective area of the piston-cylinder combination, taken to be the area bounded by the neutral surface in the fluid between the piston and the cylinder.

Strictly the equation gives the pressure value above that of the air surrounding the top of the piston and masses. Thus if the apparatus is surrounded by the atmosphere the pressure value calculated is a gauge pressure; if mounted in a vacuum chamber it is an absolute pressure (see 'absolute', 'gauge' and 'differential' pressure modes?)

There is a small gap between the piston and the cylinder and when the piston rotates in the cylinder it is centralised by lateral forces in the pressure medium, thus avoiding contact between the piston and cylinder. If the gap between the piston and cylinder is too small the piston will not spin freely and frictional forces will introduce significant errors. If the gap is too large the pressure fluid will leak away rapidly causing the piston to 'fall' rapidly within the cylinder. Note that it is not necessary for the piston to rotate; in some designs it is the cylinder that rotates around the piston.

Masses are generally loaded either directly on top of the piston or via an overhanging weight carrier; the latter lowers the centre of gravity and can improve pressure stability. Non-magnetic stainless steel is the preferred material for masses and weight carriers as it is more stable than other materials, such as brass or cast iron.

Pressure balances are amongst the most reproducible of pressure instruments and are used for calibrating a wide range of mechanical and electrical pressure gauges. Strictly they maintain a calculable pressure rather than measure it and hence cannot be used for most on-line measurement applications. When loaded with specific masses, they maintain just one calculable pressure. At low pressures relatively large diameter pistons are used but as the pressure increases smaller diameter pistons are used to prevent the number of masses from becoming unmanageably large.