Deadweight tester

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

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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.

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

• Blaise Pascal
  • Pascal (unit)
• Calibration
• Force gauge
• Piezometer
• Pressure measurement
• Pressure sensor
• Vacuum engineering

External links

• http://www.euramet.org/index.php?id=calibration-guides