1. Title: CALIBRATION OF PRESSURE GAUGES
BACKGROUND:
In order to ensure correct readings on the gauges they must be calibrated first.
Calibration may be described as a comparison of the readings of a gauge of an unknown accuracy
with those of a standard of known accuracy. To calibrate a gauge we make use of the dead weight
tester.
The figure below shows the schematic diagram of a deadweight tester. There are three
primary components of this device: a fluid that transmits the pressure, a weight used to apply
pressure and an attachment point for the gauge to be calibrated.
The fluid is oil that is essentially incompressible. The pressure at the piston face,
therefore, is equal to the pressure throughout the oil in the tester. Secondary components of the
dead weight tester are a reservoir and an adjusting piston. The reservoir accumulates oil displaced
by the vertical piston during tests when a large range of weights are used for a given gage. The
adjusting piston is used to make sure that the vertical piston is freely floating on the oil.
Three factors which may cause inaccurate readings in a Bourdon type gauge are: (1)
friction, (2) lost motion and (3) hysteresis.
Friction between links and pivots, shafts and bearings, or gear teeth may use some of the
force supplied by the Bourdon element.
Lost motion usually occurs as result of wear, such as enlarge pivot holes, worn out shafts
and bushing and worn out gear teeth.
Hysteresis is the failure of an elastic medium such as a Bourdon spring to return to the
same position for given pressure when this pressure is applied first in the direction of pressure
increase, then in the direction of the pressure decrease. For example, suppose we have a
frictionless gauge with 0 to 100 psi range, we increase the pressure from 0 to 50 psi and obtain a
reading of 49.5 psi. Then we increase the pressure further to 60 psi and gradually decrease it again
to 50 psi. This time the gauge reads 50.5 psi. Thus, we have a minus 1/2 pound reading on
increase of pressure and plus 1/2 pound on decrease of pressure making a total of 1 pound dead
spot. This one pound of dead spot is known as hysteresis.

2. Variation in a pressure sensor reading may be calibrated, using known pressures, to
give a gauge reading in engineering units. The pressure produced is:
Where: F = the force applied to the liquid in the calibrator cylinder
A = the area of the piston
d = piston diameter
P = pressure produced
When using SI units, the units of pressure are Newtons per square meter (N/m²,
also known as Pascals).
EQUIPMENTS:
1. Dead weight tester
2. Set of weights
3. SAE 30 oil
4. Caliper
The dead weight tester: