Deadweight tester

Dead weight tester, Type PD23, Nagano Keiki Co., Ltd.

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.

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

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 :

  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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