8 PRESSURE MEASUREMENT
Pressure measurements have an extremely wide range from micro-Pascal sound pressure
levels to determining the force exerted on a building as a result of wind to the control of a
process or reaction. Because of the extremely large range involved there is a wide range
of measurement equipment available. Fortunately most of the techniques used to
measure pressure are common across the ranges. You should investigate pressure
measurement techniques with the aim of understanding the underlying principals used.
8.1 MEASUREMENT OF BAROMETRIC PRESSURE
Typically, pressure is measured relative to the local barometric pressure. This can
change with elevation and local weather conditions. It is therefore important to measure
the local barometric pressure when making accurate pressure measurements.
A mercury barometer and tables for performing temperature and gravitational corrections
are used in this section of the lab. The barometer is located on one of the building pillars,
on the same side as the entrance door.
Measure the atmospheric pressure with the mercury barometer using the following
• Set the reservoir level to the reference pointer
• Read the height of the mercury column after aligning the bottom edge of vernier
slide with the top of the mercury meniscus.
• Note the temperature from the thermometer
• Record both temperature and gravity corrections from the tables.
• Correct atmospheric reading for temperature, gravity and elevation
• Record data in Table 8-1.
8.2 PRESSURE CALIBRATION WITH A DEAD WEIGHT TESTER
Pressure measurement devices should be regularly calibrated. The frequency of
calibration is a function of the measurement system used and could be a frequently as
daily but more typically is on an annual basis.
Two pressure measurement devices will be calibrated using a standard pressure
calibration device. The equipment to be used in the lab are:
• Hydraulic dead-weight pressure tester with the following two gauges attached:
o Bourdon tube pressure gauge, shown in Figure 8-1(a)
o Strain tube transducer with two active foil gauges and two dummy
compensation gauges, shown in Figure 8-1(b)
• 5 volt DC power supply for bridge circuit
• Digital voltmeter
Figure 8-1 A schematic of a (a) Bourdon tube pressure gauge and (b) a strain tube pressure
The dead weight tester should be used with the procedure designed for the unit. Overall
accuracy of the calibration will require strict adhesion to the test procedure as will as
avoiding potential damage the unit. The dead weight tester used in this lab is based on
hydraulic principles, however dead weight tester can also be based on pneumatics.
The test procedure is as follows:
• Open both valves A and B on the dead weight tester to vent the gauges to
atmospheric pressure. (See the instructions provided) MAKE SURE THE BLEED
VALVE WITH GREEN KNOB NEAR TEST GAUGES IS KEPT TIGHTLY
SHUT TO AVOID HYDRAULIC OIL LEAKAGE.
• Record the zero pressure reading on the Bourdon tube gauge and set the strain
gauge bridge output to zero volts.
• Calibrate the bourdon tube gauge and the strain tube transducer simultaneously
over a 1-300psi (0-2100 kPa) range using the dead weight tester.
• Always increase the pressure for each increment and avoid tapping or vibrating
the test gauges.
• After reaching 300psi (2100 kPa) check for hysteresis in the output by reducing
the pressure and collecting calibration data in reverse order back to zero.
Record the resolution of the bourdon gauge and the voltmeter used in Table 8-2 along
with the results of the calibration.
Figure 8-2 (a & b) Two schematic representations of a dead weight tester
8.2.3 Operating instructions for the dead-weight tester
The dead-weight tester, shown in Figure 8-2(a), is operated by drawing oil from a central
reservoir that surrounds the weight platform and pumping it into the weight cylinder.
The following operating instructions apply to the tester schematic, Figure 8-2(b).
• First fill the crank cylinder from the reservoir by opening valve A, closing valve
B and screwing the crank up.
• Then close valve A to the reservoir, open valve B and screw the crank down to
force oil into the weight cylinder and the test gauge volume.
• Repeat 1 and 2 in sequence until the stack of weights on the central platform
begin to rise. To prevent sticking of the weight platform piston, gently rotate the
weights while increasing the pressure with the screw crank. The weights should
finally float freely between the upper and lower stop points of the weight piston.
Close valve B to hold a setting.
• To reduce pressure gradually to a lower setting, first remove the necessary
weights from the platform and then, with valve B open, screw the supply piston
up to reduce pressure until the weights again float freely. Close valve B and pump
the excess oil back to the reservoir by opening valve A and screwing the supply
8.3 DYNAMIC PRESSURE MEASUREMENT
Situations regularly exist where the pressure within a system is changing in time.
Consider the daily barometric pressure which will change between night and day and as
weather systems pass. The dynamic response of the pressure measurement device will
need to be sufficient to measure fast pressure fluctuation. However, often only the mean
pressure of the system is required to be monitored. The requirements of the measurement
need to be taken in to account when specifying pressure measurement device. In this
section of the lab, a dynamic pressure is monitored with different devices highlighting
their dynamic response.
The following equipment is used in this section of the lab:
• A reciprocating diaphragm air compressor with pressure regulated outlet
connected to the following three transducers:
o Bourdon tube pressure gauge with adjustable needle valve fluctuation
o Oil filled bourdon tube gauge with internal orifice damper.
o Variable inductance diaphragm pressure transducer (Validyne) with
capillary tube damper.
• Carrier - demodulator oscillator for inductance transducer.
• Digital storage oscilloscope.
Calibrate the output of the variable inductance transducer (Validyne) by first connecting
the output to the oscilloscope input. Set the scope input control to GND and note the zero
voltage position of the oscilloscope trace. With the air compressor off, open the toggle
valve at the end of the outlet pipe to vent the gauges to the room. Make sure that the
toggle valve on the inlet to the Validyne transducer is open. Set the scope input to DC
and adjust the ZERO control on the Validyne carrier-demodulator to produce zero volts.
Close the toggle valve and turn on the air compressor. Using the pressure reading of the
oil filled bourdon gauge as the standard, adjust the SPAN control on the carrier-
demodulator so that the average output voltage on the scope corresponds to a sensitivity
of 2.0 psi/volt.
Store the Validyne transducer output on the oscilloscope with the time base set to show
several cycles. Record the maximum and minimum pressures, and the time period of the
pressure fluctuations on a sketch of the wave form. Also record the compressor rated
speed from its nameplate and document all data in Table 8-3.
Close the toggle valve at the base of the Validyne so that the only pressure path from the
compressor to the Validyne is through the capillary tubing. Again store a pressure trace
showing several cycles, record maximum and minimum pressures and sketch the wave
form in Table 8-3.
With the needle valve on the un-damped Bourdon gauge fully open, record the range of
pointer oscillations. Slowly close the needle valve until the oscillations are reduced to a
level where the mean pressure can be read. Record this mean pressure and the pressure
indicated by the oil filled gauge in Table 8-4. Determine the time constant, τ, of the
needle valve damped gauge by assuming the response of the gauge to a sudden (step)
change in pressure is exponential as described by:
⎛ −t ⎞
P − Pf ⎜
= e⎝ (1)
Pi − Pf
• P is the pressure at any time t
• Pi is the initial pressure
• Pf is the final pressure
• t is time in seconds
• τ is the time constant in seconds
Knowing that a first order process is approximately 63% complete in one time constant
an engineering approximation can be made without having to take a number of data
points and plotting a graph. The mean pressure when the toggle valve is closed is
approximately 15 psig and 0 psig when the valve is open. Two thirds (approx. 63%) of
15 is 10 so that we need only estimate the time required for the pressure indicated by the
gauge to change either from 5 to 15 or from 15 to 5 in response to a rapid closing or
opening of the toggle valve (Approximating a step change in pressure). Is the fixed
needle valve setting effective in damping pressure fluctuations?
8.4 MEASUREMENT OF LOW PRESSURE
Low pressure measurement requires sensitive equipment capable of measuring low
pressure levels. Several low pressure measurement devices will be calibrated against a
Several different low pressure measurement devices are connected to a common low
pressure calibration source. The first device listed below, the micro-manometer is used
as the calibration standard. The following equipment is used in this lab:
• Micro-manometer with electrical contact indicating micrometer point gauge.
• Adjustable water column low pressure calibration source
• Vertical U-tube alcohol filled manometer.
• Inclined oil filled manometer.
• Bellows - flat spring dial gauge (Magnehelic)
• Electronic capacitive pressure transducer with digital voltmeter
Make sure that the bleed valve on the water column calibrator is open. Zero the inclined
manometer by sliding the metal scale card and if necessary adjust the Magnehelic gauge
pointer and scale on the U-tube manometer. Set the adjustable screw on the micrometer
of the micro-manometer to contact the water at a micrometer reading of about 2.5mm.
Record this zero reading. (This "live" zero is used so that pressures both above and below
atmospheric may be read)
Calibrate each of the pressure indicators against the micro-manometer by raising the level
of the water column, allowing the pressure to equalize, and recording the reading on each
device. Obtain about 10 points between zero and 15mm H2O and record the data in
Table 8-1 Barometric pressure measurements
Uncorrected Altitude Corrected Instrument
Reading correction pressure resolution
from table from table
(mmHg) (mmHg) (mmHg) (mmHg)
Table 8-2 Calibration using a deadweight pressure tester
Bourdon tube resolution Multi-meter resolution
(psi) Bourdon tube gauge Strain tube transducer
Table 8-3 Calibration of the Validyne pressure transducer. Make sure that the Validyne sensitivity
is set to 2.0psi/V.
Without capillary damping With capillary damping
Oscilloscope wave shape:
Sketch the wave forms to scale.
Minimum voltage (V)
Maximum voltage (V)
Minimum pressure (psi)
Maximum pressure (psi)
Compressor speed (rpm)
Validyne time constant (sec)
Mean pressure (psi)
Table 8-4 Dynamic pressure measurement measurements with a Bourdon tube
Un-damped Bourdon tube
Damped Bourdon tube gauge
Valve open Valve closed
Maximum pressure (psi)
Minimum pressure (psi)
Mean pressure (psi)
Table 8-5 Low pressure calibration data
Micro-manometer U-tube Magnehelic variable
(inH2O) (mmH2O) (inH2O) (mmH2O) (mmH2O) (mmH2O) (V)
8.5 THE REPORT
The report for this lab is a worksheet. Ensure that the following are included in the
8.5.1 Required plots
Please use Excel to provide these plots.
Pressure Calibration with the dead weight tester
1. Plot the Bourdon tube pressure versus the deadweight pressure.
2. Plot the Bourdon tube pressure error versus the deadweight pressure.
3. Plot the strain tube transducer output versus the deadweight pressure.
Measurement of low pressure
4. Plot the U-tube readings (mmH2O) versus the micro-manometer readings
5. Plot the inclined monometer reading versus the micro-manometer readings
6. Plot the magnehelic gauge readings versus the micro-manometer readings
7. Plot the Setra variable capacitance gauge readings versus the micro-manometer
8. Plot the error in pressure of the inclined manometer versus the micro-manometer
9. Plot the error in pressure of the magnehelic gauge versus the micro-manometer
Please re-type these questions with their answers on a separate sheet.
• Local radio stations typically report that the barometric pressure in Edmonton is
around 101kPa. Does your measurement (Table 8-1) support this? Why are the
• Based on plot 1, determine the hysteresis of the Bourdon tube gauge.
• Based on plot 2, determine the accuracy of the Bourdon tube as a percentage of
• Based on plot 3 determine the accuracy, sensitivity, and maximum deviation from
linearity for the strain tube transducer.
• Based on plots 4 to 7, determine the sensitivity and nonlinearity of each low
pressure measurement device.
• Based on plots 8 and 9, determine the accuracy of the inclined manometer and the
magnehelic pressure gauge as a percent of full scale.
• Is the frequency of pressure fluctuations in dynamic measurement (Table 8-3)
related to the compressor speed?