Deadweight tester

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 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.

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

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]
PCU Piston 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
Wikimedia Commons has media related to Deadweight testers.

References
  1. Fluke Corporation. "Pressurements PressCal Deadweight Tester Software" (PDF). Retrieved 21 November 2019.
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