Deadweight tester

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Dead weight tester, Type PD23, Nagano Keiki Co., Ltd. Dead weight tester, Type PD23, Nagano Keiki Co., Ltd. - National Museum of Nature and Science, Tokyo - DSC07787.JPG
Dead weight tester, Type PD23, Nagano Keiki Co., Ltd.

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.

Contents

Formula

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.

Absolute pressure with vacuum reference

p=

m.gl.

æ
è

1-

ra

rm

ö
ø

+P.d.t


A(20,0).(1+(ap+ac).(t-20)).(1+pnom.lp)
-(rN2-ra).gl.h+pvac

Gauge pressure

pe=

m.gl.

æ
è

1-

ra

rm

ö
ø

+P.d.t


A(20,0).(1+(ap+ac).(t-20)).(1+pnom.lp)
-(rN2-ra).gl.h

Nomenclature

pabsolute pressure at reference level[Pa]
pegauge pressure at reference level[Pa]
mtotal true mass load[kg]
gllocal acceleration due to gravity[N/kg]
raambient air density[kg/m3]
rmaverage density mass load[kg/m3]
P.d.tsurface tension effect[N]
rN2density of pressure medium[kg/m3]
hreference level offset DWT .. DUT[m]
pvacvacuum residual pressure[Pa]
A20,0PCU effective area at p=0 and 20oC[m2]
aplinear thermal expansion coefficient piston[oC-1]
aclinear thermal expansion coefficient cylinder[oC-1]
ttemperature of PCU[oC]
lpelastic deformation coefficient PCU[Pa-1]
pnomnominal line pressure[Pa]
pegauge pressure applied to PCU[Pa]
pabsolute pressure applied to PCU[Pa]
PCUPiston Cylinder Unit [1]

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 :

  1. gas operated gas lubricated PCU's
  2. gas operated oil lubricated PCU's
  3. oil operated oil lubricated PCU's

All three systems have their own specific operational demands. Some points of attention :

Gas-gas

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.

Gas-oil

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.

Oil-oil

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.

See also

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References

  1. Fluke Corporation. "Pressurements PressCal Deadweight Tester Software" (PDF). Retrieved 21 November 2019.{{cite web}}: CS1 maint: url-status (link)