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

piston cylinder

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.