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A dead weight tester apparatus uses known traceable weights to apply pressure to a fluid for checking the accuracy of readings from a pressure gauge. A dead weight tester (DWT) is a calibration standard method that uses a piston cylinder on which a load is placed to make an equilibrium with an applied pressure underneath the piston. Deadweight testers are so called primary standards which means that the pressure measured by a deadweight tester is defined through other quantities: length, mass and time. Typically deadweight testers are used in calibration laboratories to calibrate pressure transfer standards like electronic pressure measuring devices.
The formula on which the design of a DWT is based basically is expressed as follows :
|p||=||F / A||[Pa]|
|F||:||force applied on piston||[N]|
|A||:||effective area PCU||[m2]|
To be able to do accurate measurements, this formula has to be refined.
Absolute pressure with vacuum reference
|p||absolute pressure at reference level||[Pa]|
|pe||gauge pressure at reference level||[Pa]|
|m||total true mass load||[kg]|
|gl||local acceleration due to gravity||[N/kg]|
|ra||ambient air density||[kg/m3]|
|rm||average density mass load||[kg/m3]|
|P.d.t||surface tension effect||[N]|
|rN2||density of pressure medium||[kg/m3]|
|h||reference level offset DWT .. DUT||[m]|
|pvac||vacuum residual pressure||[Pa]|
|A20,0||PCU effective area at p=0 and 20oC||[m2]|
|ap||linear thermal expansion coefficient piston||[oC-1]|
|ac||linear thermal expansion coefficient cylinder||[oC-1]|
|t||temperature of PCU||[oC]|
|lp||elastic deformation coefficient PCU||[Pa-1]|
|pnom||nominal line pressure||[Pa]|
|pe||gauge pressure applied to PCU||[Pa]|
|p||absolute pressure applied to PCU||[Pa]|
Piston cylinder design
In general there are three different kind of DWT's divided by the medium which is measured and the lubricant which is used for its measuring element :
- gas operated gas lubricated PCU's
- gas operated oil lubricated PCU's
- oil operated oil lubricated PCU's
All three systems have their own specific operational demands. Some points of attention :
Make sure that the PCU is clean. This is a very important issue as the PCU's operation is sensitive to contamination. Also when connecting a DUT, make sure that the DUT does not introduce contamination in the measuring system.
Lubricant of the PCU 'leaks' in the gas-circuit of the DWT. For this reason there is a small reservoir incorporated in the system. Before commencing a calibration it is a good practice to purge this reservoir. If the reservoir is full, oil will be introduced in critical tubing and will cause an uncontrollable oil-head.
When connecting an oil filled DUT on an oil DWT make sure that the DUT oil will not contaminate the DWT oil. If in doubt. Incorporate a small volume between DUT and DWT and manipulate pressure in such a matter that the oil flow is directed to the DUT.
For high accuracy measurement, friction can be lowered by rotation of the piston.
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- Blaise Pascal
- Force gauge
- Pressure measurement
- Pressure sensor
- Vacuum engineering