Faculty of Engineering,
School of Petroleum & Chemical Engineering
Chemical Engineering Department
Aim of the experiment………………….4
Theory of the experiment……………...5
The description unit………………………6
The lay out unit …………………………….7
Performing the experiment …………10
The deadweight piston gauge (Bell and Howell) is used is to
measure pressure in terms of fundamental units - force and
area. A piston is inserted into a close fitting cylinder.
Weights are placed on one end of the piston and are
supported by fluid pressure applied
to the other end. For absolute pressure measurements the
assembly is placed inside an evacuated bell jar. Pressure
measurements take into account a number of parameters
affecting the instrument and its environment. they include
the uncertainty of the mass of the weights and the error in
the effective area measurement of the cylinder,
gravity measurement, air bouyancy and temperature.
Measurement units used are Kilopascals (Kpa) or Millibars
(Mb). The available range
is 20 Kpa. to 300Kpa. The accuracy we feel we have is better
than 0.1mb. throughout the full range.
Dead weight testers are named so because they utilize dead weights in
determination of pressures operating in a closed and compressed fluid system.
They are so called because of the reason that for each value of pressure to be
exerted/ applied in the closed hydraulic system, a predetermined / fixed amount
of weights are used to exert the force to counter balance the buoyancy forces. Each
weight is stamped with the equivalent amount of force that will be exerted on a
predetermined area of piston cylinder arrangement by using that weight. There
fore the device is called Dead Weight tester.
Working Principle: DWT (Dead Weight Tester) works based on the principle of
Pascal's law. The law states that in a closed system of incompressible fluid, the
pressure appliedwill exert equal amount of force in all the directions. In DWT
system, silicon oil is used within the closed boundaries of the Piston cylinder
arrangement, piping, pressurisation chamber and in the head on which the gauge
to be tested/ calibrated is fixed. The oil is taken in to the pressurisation chamber
from oil bowl and all the air entrapped is vented off. This is because of the reason
that air iscompressible and hence do not allow pressure to be exerted equally in all
the areas and there by create error in the test results. Once the system is full with
air free oil, pressure is gradually increased from the pressurization chamber. Oil
pressure starts increasing in all the areas including piston cylinder arrangement
over which the dead weights are mounted. As the force increases gradually and
equals the amount of down ward force being exerted by the dead weights, the total
system gains the state of equilibrium and just at that moment, the dead weights
starts getting lifted up. At this condition, the amount of force operating in the
entire system is same. The sum of pressure values
stamped on weights lifted is operating on the pressure gauge element also, which
is under test/ to be calibrated. Necessary corrections are made in the zero/ span
adjustments in gauges/ Pressure transmitters. Above sequence is repeated for all
the values at 0%, 25%,50%,75% and 100% to check for linearity and repeatability.
This is how the DWTs are working.
The device for calibrating pressure gauges essentially consists of two units:
1.the pressure gauge unit This is where the pressure gauge to be calibrated is screwed
in. for experimental purposes ,the unit comes supplied with a bourdon gauge already
2.the load unit
The load unit consists of several weights and a cylinder with a piston. An increase in
the load results in an increase in pressure . the load unit is connected to the pressure
gauge unit via an oil-filled line , enabling the bourdon gauge to display the increase in
The following sectional drawing shows how the load unit and
pressure gauge unit are connected. As you can see, both unit are
connected.by means of a pipeline.
When the support is loaded with weights, the oil pressure in the
system increases . the seal between the piston and the cylinder is
metallic, with on other sealing elements. The fit has been very
carefully almost entirely without friction, and with minimal oil
leakage. The weights are designed in such a way that pressure
increments of 0.5 bar are possible. Place the small weight on the
weight support first.A guide pin is provided for the purpose, the
other weight would lie askew on the plunger, and would corrupt the
measurements due to different levels of friction. The unit is also
equipped with a drain valve in order to drain the oil. In addition, an
overflow is incorporated, so that any leaked oil can be siphoned off
.the overflow can be emptied via a tap.
Open the overflow valve
_Remove the transport-cap
_If necessary topping up the hydraulic oil
_Inserting the piston
_To check the zero point of bourdon gauge, proceed as follows:
*press the piston out of the cylinder using the hand wheel.
*Remove the piston and weight support.
*adjust the oil level the open cylinder until the cylinder filled up the
The gauge pressure being tested should now indicate zero, as it is
only subject to ambient pressure.
_After the zero point of bourdon gauge has been checked, the weight
support is re-inserted into the cylinder of the pressure gauge unit
.the piston is then slowly lowered by unscrewing the counterbalance
cylinder until it is freely suspended.
_In order to avoid static friction . set the weight support in gentle
rotation. the mass of the support is 385 g .taking into account the
piston diameter of 12 mm, it is now possible to read the increase in
pressure from bourdon gauge .
_Other pressures occurring when the weight on the support is
increased can be calculated in exactly the same way .the display of
0.5 bar is achieved by placing a weight with a mass of 193 g on the
support .the other weights weigh 578 g, which is equivalent to a
pressure increase of 0.5 bar each time.
The calibration curve appears because of equality in the
ratio of Pacual &Pm ,it is a linear graph with equation of
So the slope is 1.0114 with the rule of
For measuring (dead weight piston gauge) we use many
types of bourdon gauge like