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A dead weight tester apparatus uses unknown 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 [ citation needed ] 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] |
where :
p | : | reference pressure | [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.
p= |
| -(rN2-ra).gl.h+pvac |
pe= |
| -(rN2-ra).gl.h |
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] |
PCU | Piston Cylinder Unit [1] |
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 :
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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