The RINGBALANCE MEASURING PRINCIPLE

Fig. 1:
A hollow ring, free to rotate on ball-bearings, and half filled with fluid, is divided by partition T into two chambers.

Positive, and negative or differential pressures are applied to the ringbody chambers via flexible tubes S.

The pressure differential across the dividing wall T causes the ringbody to rotate until an equilibrium is reached with counterweightG.

The Ringbalance "Formula"

p	differential pressure [Pa]
s	counterweight moment arm [m]
r	verage Ringbody radius [m]
A	rea of partition T [m²]
G	counterweight [N]

Since the differential pressure is exclusively balanced by the counterweight G, it follows that neither the quantity nor the density (specific gravity) of the filling fluid play a role in the actual measurement or calibration.

Filling fluids
Nevertheless the "ideal" oil has to meet certain criteria:
thin-fluid, free of evaporation, non aggressive, and of high density, because a too light oil would make a large level change and overflow into the flexible tubes - see: Overload Protection

We use the following filling fluids

Mineral Oil 
density 0.8 kg/l, for ranges up to 700 Pa (3" W.C.)
Synth. Oil
density 1.9 kg/l, for ranges up to 1.8 kPa (7" W.C.)

Every Ringbalance instrument is filled in factory.