How to Calibrate a Pressure Gauge With a Dead Weight Tester ~ Learning Instrumentation And Control Engineering Learning Instrumentation And Control Engineering

How to Calibrate a Pressure Gauge With a Dead Weight Tester

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Basic Operating Principle

Deadweight Testers (DWT) are the primary standard for pressure measurement. There are three main components of this device: a fluid (oil) that transmits the pressure, a weight and piston used to apply the pressure, and a connection port for the gauge to be calibrated.

The dead weight tester also contains an oil reservoir and an adjusting piston or screw pump. The reservoir accumulates oil displaced by the vertical piston during calibration tests when a large range of accurately calibrated weights are used for a given gauge. The adjusting piston is used to make sure that the vertical piston is freely floating on the oil. Please see How a Dead Weight Tester Works for a detailed description of the working principle of the device.

Calibration Basics

To carry out tests or calibrate a pressure gauge with the dead weigh tester(DWT), accurately calibrated weight masses (Force) are loaded on the piston (Area), which rises freely within its cylinder. These weights balance the upward force created by the pressure within the system:
PRESSURE = FORCE/AREA = W/A
So for each weight added, the pressure transmitted within the oil in the dead weight tester is calculated with the above formula because the area of the piston of the tester is accurately known.

Note:
if weights are in pounds (lbs) units and the area in inch square, then the calculated pressure unit is in Pounds per square inch(PSI).

If the weights are in kilogram (kg) units and the area of piston in meters square, then the calculated pressure [P = (W*G)/A, G = gravity in m/s2] unit is in N/m2 or pascal.

During calibration, the system is primed with liquid from the reservoir, and the system pressure is increased by means of the adjusting piston. As liquids are considered incompressible, the displaced liquid causes the piston to rise within the cylinder to balance the downward force of the weights.

Calibrating a Pressure Gauge with the Dead Weight Tester:

To calibrate a pressure gauge with a dead weight tester, set up the device on a level, stable workbench or similar surface as shown in the diagram below:
Proceed with the calibration according to the following steps:
Step 1:
Connect the pressure gauge to the test port on the dead weight tester as shown in the diagram above. Ensure that the test gauge is reading zero, if not correct the zero error and ensure that the gauge is reading zero before proceeding with the calibration exercise.

Step 2:
Select a weight and place it on the vertical piston

Step 3:
Turn the handle of the adjusting piston or screw pump to ensure that the weight and piston are supported freely by oil.

Step 4:
Spin the vertical piston and ensure that it is floating freely.

Step 5:
Allow a few moments for the system to stabilize before taking any readings. After system has stabilized, record the gauge reading and the weight.

Step 6:
Repeat steps 2 through 5 for increasing weights until the full range or maximum pressure is applied to the gauge and then decreasing weights until the gauge reads zero pressure. Calculate the error at each gauge reading and ensure that it is within the acceptable accuracy limits.

If you are doing a five point calibration, then increasing weights should be added corresponding to 0%, 25%, 50%, 75%, and 100% of the full range pressure of the pressure gauge. And for decreasing pressure you proceed in the order 100%, 75%, 50%, 25%, 0%.

For pressure gauges with less accuracy specifications, calibration at the points: 0%, 50% and 100% will suffice.

After calibration, your data can be recorded in a table in this manner:

Upscale Readings:

% Input Weights,W DWT Pressure (W/A)* Test Gauge Pressure Error
0



25



50



75



100




*DWT Pressure = W/A, if W is in lbs, and A in square inch  then DWT Pressure is in PSI(pounds per square inch). However, if W is in kg and A in square meters, then :
DWT Pressure = (W*G)/A, Where G = gravity in meters per seconds square(m/s2) and the DWT Pressure is in N/m2 or Pascal

Downscale Readings:

% Input Weights,W DWT Pressure (W/A)* Test Gauge Pressure Error
100



75



50



25



0




At each pressure reading, the absolute error is calculated thus:
Absolute Error = DWT Pressure – Test Gauge Pressure
The absolute error at each point should be within the acceptable accuracy limits of the gauge.

If the gauge error is in % span proceed as follows to calculate the error:
Span = Maximum pressure – minimum pressure
%Error = [(DWT Pressure – Test Gauge Pressure)/Span]*100  for each pressure gauge reading.
The error in % span should be within the acceptable accuracy limits otherwise the calibration will have to be repeated to correct the errors.

If the pressure gauge error is in % FSD(Full Scale Deflection), proceed as follows to calculate the error:
% Error = [(DWT Pressure - Test Gauge Pressure)/FSD]*100

Correction Factors:

The deadweight tester has been calibrated to the Gravity, Temperature, and Air Density stated on the calibration certificate right from the laboratory.
Equations and factors are given on the certificate to adjust for any variations in these environmental conditions.
Always refer to the documentation for the Dead Weight Tester to ensure that for maximum accuracy, the necessary calibration correction factors are applied to any reading from the device.

Gravity Correction

Gravity varies greatly with geographic location, and so will the deadweight tester
reading. Due to the significant change in gravity throughout the world (about 0.5%), ensure that the tester in your possession has been manufactured with the specification of your local gravity, otherwise you  may have to apply the correction for the calibrated gravity.

To correct for gravity use:
True Pressure = [(Gravity (CS))/(Gravity(LS))]*P(Indicated) 
Where:
P(Indicated)  = Pressure indicated by gauge being calibrated
Gravity(CS)  = Gravity at Calibration Site
Gravity (LS) = Gravity at Laboratory Site

Temperature Correction

Temperature and Air Density variations are less significant than gravity. Variations should be corrected for when maximum accuracy is required.
To correct for Temperature variation use:

True Pressure = P(Indicated) [1+ {T(DWTCT) – T(OT)}*{ΔP/100}] 
Where:
P(Indicated)    = Pressure indicated by gauge being calibrated
T(DWTCT)     = Dead Weight Tester calibrated temperature in the laboratory
T(OT)              = Operating temperature at calibration site
ΔP                    = Percentage pressure change per unit temperature change