Pneumatic Comparators or air gauge

A pneumatic comparator are utilizes a pneumatic circuit to measure dimensions individually or in combination.  Basically, comparators measures size by variation of flow or variations of pressure in its pneumatic circuit.

It compares a UN known dimensions of a part to a preset flow (master flow) or pressure value in the gauge and indicates whether the dimension is within tolerance or if it not, the amount by which it is out of tolerance.

Measuring head

The design of the air gauge head or the tooling is appropriate to the configuration of the part, condition, and dimension to be checked.

Pneumatic Comparators
Pneumatic Comparators

It can be an open – jet spindle to check a bore a without contact, a non – contact air snap or an engineered fixture using contact type air gauging cartridges and sensors as well as open jets.

The gauge head can be presented to the part or the part to the gauge head.

Method of amplification

Method of amplification essentially, an air gauge consists of an instrument that indicates the amplification, a pneumatic gauge head with one or more orifices through which air can escape, nozzles or air jets and setting masters.

It checks dimensional accuracy by determining the flow or pressure of air escaping between the gauging element and the part being gauged.

Pneumatic comparators uses

The pneumatic comparators are widely employed for the internal limit gauging and external limit gauging. The pneumatic comparators are used for measurement of diameter, taper, lobing, straightness, ovality etc…

Single diameter

The cylindrical plug gauge form is adopted which ensures constant air flow inside regardless of the gauge position the bore.

A reduction of flow in one jet is compensated by increased to avoid the instability and measure the diameter accurately even if the jets are flush. Usage of twin nozzle reduces the magnification and in order to restore magnification average separation is halved.


The three jets are arranged in parallel such that flow in two pipes remains equal and flow in third jet in third jet is sum of the flow in first two pipes. Results in high and low reading from relative plug and bore rotation.

Similarly, pneumatic gauging is applied for the external limit measurements such as ring gauges, height gauges and SMP gauges. Indirect and direct heads are used to measure cylinder bores and thickness of sheets respectively.

A leaf – jet plug is used in measurement of diameter of semi – blind hole at the bottom. Diameter of shallow holes can also be measured using them.