General Piston Pressure Gauge Considerations | 2 |
Measurement of Pressure with the Piston Pressure Gauge |
Although the trend is swinging toward the use of true mass in favor of apparent mass, there is a small advantage in the use of the latter. When making calculations for air buoyancy from values of apparent mass, it is unnecessary to know the density of the mass. If objects of different densities are included in the calculation, it is not necessary to distinguish the difference in the calculations. This advantage is obtained at a small sacrifice in accuracy and is probably not justified when considering the confusion that is likely to occur if it becomes necessary to alternate in the use of the two systems.
A satisfactory approximation of the force on a piston that is produced by the load is given by:
⎛ |
|
| ⎞ |
⎜ |
| pAIR ⎟ | |
F = M A ⎜1 | − |
| ⎟g |
| |||
⎝ |
| pBRASS ⎠ | |
Where:
Fis the force on the piston
M A | is the mass of the load, reported as "apparent mass vs. brass |
pAIR | standards" |
Is the density of the air | |
pBRASS | Is the density of brass (8.4 g/cm³) |
gis the acceleration due to local gravity
Temperature
Piston pressure gauges are temperature sensitive and must, therefore, be corrected to a common temperature datum.
Variations in the indicated pressure resulting from changes in temperature arise from the change in effective area of the piston due to expansion or contractions caused by temperature changes. The solution is a straightforward application of the thermal coefficients of the materials of the piston and cylinder. The area corresponding to the new temperature may be found by substituting the difference in working temperature from the reference temperature and the thermal coefficient of area expansion in the relation as follows:
A0(t ) = A0(r ) [ 1 + c (t − r) ]
Where:
A0(t ) | is the effective area at temperature, t |
A0(r ) | is the effective area at zero pressure and reference temperature, r |
cis the coefficient of thermal expansion
Reference Plane of Measurements
The measurement of pressure is linked to gravitational effects on the pressure medium. Whether in a system containing a gas or a liquid, gravitational forces produce vertical pressure gradients that are significant and must be evaluated. Fluid pressure gradients and buoyant forces on the piston of a pressure balance require the assignment of a definite position at which the relation P = F / A exists.
