Pressure balances operate over a pressure range extending from about 3 kPa (gas media, absolute- or gauge-mode) to 1 GPa (hydraulic, gauge-mode). Any one piston-cylinder, however, can only be used over a pressure range that typically varies from about 10:1 up to about 100:1. To cover a greater pressure range additional piston-cylinders are required.

From the equation in the section above, however, it would seem that a much larger range of pressures could be accommodated in a single instrument by using a large area piston-cylinder and a lot of masses. In practice this is not so; if the total mass is too large or too small the piston-cylinder does not remain concentric and its effective area can change significantly and unpredictably.

The lowest force/area quotient is, of course, obtained when using an unloaded piston - the downward force is just that due to the mass of the piston itself. Under these circumstances, however, the piston does not have appropriate inertia to both run concentrically or for a sufficient time to allow the pressure under the piston to stabilise. In practice a useful rule-of-thumb is that the lowest pressure that will produce repeatable and statistically sound measurements will be somewhere between 10 and 100 times smaller than the highest pressure at which the instrument was designed to operate.

In attempting to reduce the number of instruments required, users not unreasonably seek a certificate of calibration that covers the maximum possible operating range and it is usually the lower pressure limit where an extension is requested. Of course, it is possible that one particular instrument will operate better than nominally similar devices but obtaining the evidence takes considerably more than an extra cross-floating measurement or two at the lower pressure(s). The effective area values provided in a pressure balance certificate of calibration, together with their uncertainties, have to be established with adequate statistical confidence and this cannot be obtained with just a single or even a pair of measurements. At low pressures, piston-cylinders are particularly sensitive to, for example, their degree of verticality - substantially more than implied by a simple calculation from a measurement of verticality.

When attempting to extend a pressure range downwards, it is thus necessary to make a large number of measurements and, for example, dismantle and reassemble the apparatus in between. This can be done but the extra measurements represent a significant extra effort and hence cost that, from experience, usually end up confirming that an instrument's repeatability at the lower pressures is poor or erratic - even when, to the eye, the piston still appears to rotate freely. Where this is so, it is very difficult to ascribe a statistically defensible value of uncertainty and the subsequently issued certificate of calibration will have to contain correspondingly cautious text.