Deadweight_tester Knowpia

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.

Formula edit

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 edit

p=

m.g_l.

æ
è

1-

r_a

r_m

ö
ø

+P.d.t

A_(20,0).(1+(a_p+a_c).(t-20)).(1+p_nom.l_p)

-(r_N2-r_a).g_l.h+p_vac

Gauge pressure edit

p_e=

m.g_l.

æ
è

1-

r_a

r_m

ö
ø

+P.d.t

A_(20,0).(1+(a_p+a_c).(t-20)).(1+p_nom.l_p)

-(r_N2-r_a).g_l.h

Nomenclature edit

p	absolute pressure at reference level	[Pa]
p_e	gauge pressure at reference level	[Pa]
m	total true mass load	[kg]
g_l	local acceleration due to gravity	[N/kg]
r_a	ambient air density	[kg/m³]
r_m	average density mass load	[kg/m³]
P.d.t	surface tension effect	[N]
r_N2	density of pressure medium	[kg/m³]
h	reference level offset DWT .. DUT	[m]
p_vac	vacuum residual pressure	[Pa]
A_20,0	PCU effective area at p=0 and 20^oC	[m²]
a_p	linear thermal expansion coefficient piston	[^oC^-1]
a_c	linear thermal expansion coefficient cylinder	[^oC^-1]
t	temperature of PCU	[^oC]
l_p	elastic deformation coefficient PCU	[Pa^-1]
p_nom	nominal line pressure	[Pa]
p_e	gauge pressure applied to PCU	[Pa]
p	absolute pressure applied to PCU	[Pa]
PCU	Piston Cylinder Unit^[1]

Piston cylinder design edit

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 :

gas operated gas lubricated PCU's
gas operated oil lubricated PCU's
oil operated oil lubricated PCU's

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

Gas-gas edit

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 edit

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 edit

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.