Harwood Engineering Company, Inc.

Harwood Controlled Clearance Free Piston Gages*

Primary Pressure Standard

The Basis of Pressure Standards in the United States is Controlled Clearance Gages ^**

Their characteristic extreme sensitivity, repeatability, and accuracy results from the minute clearance maintained between the piston and cylinder by means of a variable jacket pressure on the outside of the cylinder producing the close fit of a few millionths of an inch clearance - still with fully free sensitivity and exceptionally small leakage. The overall performance results in a simple evaluation of the effective area and determination of the magnitude of the measured pressure with great accuracy.

• Self-Calibrating
• Controllable long float
• Sensitivity better than 1 part in 3 million
• Computer control available

Mercury Columns and Free Piston Gages are Primary Standards

Secondary Standards depend upon the mechanical motion of spring devices that can be sensed mechanically (Bourdon Gages) or electrically by displacement devices using induction, capacitance, or resistance.

For establishing pressures above those of the mercury column there have been three types of piston gages devised.
 

Types of Piston Gages

Simple Blaze Pascal 1623-1662	Re-entrant P.W. Bridgman 1909-1961	Controlled Clearance D.P. Johnson & D.H. Newhall 1953

The measured pressure, P_m is established when the mass of the piston and its weights are in free floating, non frictional balance with the applied pressure. There has to be a small crevice between the piston and cylinder, so small that the effective area A_e is their arithmetic average.

As pressure ranges are increased in the simple gage in FIG. 1, its crevice increases because of the expansion of its cylinder and the contraction of its piston hence the leakage through the crevice and the fall rate of the piston increase creating an insufficient period of "floating time". The Opposite is true with the re-entrant type FIG. 2, since the applied pressure acts on both inside and outside of the cylinder's tip decreasing its crevice until finally at some pressure the cylinder's tip grips the piston. As implied by its name the Controlled Clearance Gage, FIG. 3, controls the crevice between the piston and cylinder at any measure pressure over its entire range at the operators will, by a simple adjustment of jacket pressure.