1. Pneumatic comparators
Pneumatic comparators use air as a means of
measurement. The basic principle involved is
that changes in a calibrated flow respond to
changes in the part feature. This is achieved using
several methods and is referred to as pneumatic
gauging, air gauging, or pneumatic metrology.
It is possible to gauge length, diameter,
squareness, parallelism, taper, concentricity, etc.,
using a simple set-upFor instance, if one is
inspecting the bore of an engine cylinder, it is also
possible to assess its size, taper, camber, and
straightness in the same setting.
2. Advantages
Absence of metal-to metal contact, higher
amplification, and low cost. Absence of metal-to-
metal contact between the gauge and the
component being inspected greatly increases the
accuracy of measurement.
The gauge also has greater longevity because of
a total absence of wearable parts.
Amplification may be increased without much
reduction in range, unlike mechanical or electronic
instruments.
Pneumatic comparators are best suited for
inspecting multiple dimensions of a part in a single
setting ranging from 0.5 to 1000 mm
3. Pneumatic Gauges:
Based on the type of air gauge circuit,
pneumatic gauges can be classified as free
flow gauges and back pressure gauges.
4. Free Flow Air Gauge
This uses a simple pneumatic circuit.
Compressed air with a pressure in the
range 1.5–2 bar is passed through a
tapered glass column that contains a
small metal float.
The air then passes through a rubber or
plastic hose and exits to the atmosphere
through the orifice in the gauging head.
Since the gauging head is inserted inside
the work part that is being inspected,
there is a small clearance between the
gauging head and the work part.
This restricts the flow of air, thereby
changing the position of the float inside
the tapered glass column.
The set-up is illustrated in Figure
,Compressed air from the factory line is
filtered and reduced to the required
pressure.
A shut-off valve is provided to ensure
shut-off of air supply when not in use.
5. Flow Characteristic Curve
Flow–Clearance curve
Figure illustrates the relationship
between the clearance and the flow
rate.
It is clear from the graph that the
flow rate increases with the increase
in the clearance. The curve has a
linear portion, which is used for the
purpose of measurement.
This linearity in the gauging range
permits dimensional variation to be
accurately measured up to 1 μm.
A calibrated scale enables the
reading to be directly read from the
scale. Amplification of up to
100,000:1 has been built into these
gauges
this type of pneumatic gauge is relatively free from
operator error.
6. Air gaps in air gauging
A typical gauge head has two orifices diametrically opposite each
other
If the spindle of the gauge head is moved to one side, the air flow is
decreased; however, the air flow through the diametrically opposite
orifice increases by an equal amount.
7. Working Principle
. The float takes up a position in the tapered tube such that
the air velocity through the ‘annulus’ created by the float and
the tube is constant.
The air then escapes through the gauging orifice. In order to
use the gauge as a comparator, the user uses a master
gauge of known dimension and geometric form, and sets the
float to a reference value by adjusting the air flow rate.
In other words, the air gauge is set to a datum rate of air flow
through the system. Now, the master gauge is taken out and
the gauge head is inserted into the work part being inspected.
Any variation in the dimension of the work part will produce a
variation in the rate of flow through the system. This is
reflected in the change in height of the float in the glass
column, and the difference in dimension can be directly read
on the graduated scale.
8. Back pressure Gauge: Working
Principle
This system uses a two-orifice
arrangement, While the orifice O1 is
called the control orifice, the orifice O2 is
referred to as the measuring orifice.
The measuring head gets compressed
air supply at a constant pressure P, which
is called the source pressure.
It passes through the control orifice into
an intermediate chamber. Air exits the
measuring head through the measuring
orifice.
While the size of the control orifice
remains constant, the effective size of the
measuring orifice varies because of the
gap d between the measuring orifice and
the work surface. Depending on the gap
d, the back pressure Pb changes, thereby
9. Characteristic Curve Of A Back
Pressure Gauge
Assuming that the areas
of control orifice and
measuring orifice are A1
and A2 respectively, the
relationship between the
ratio of back pressure to
source pressure and the
ratio of the areas of
control orifice to
measuring orifice is
almost linear for Pb/P
values from 0.5 to 0.8.
This range is selected for
the design of the back
10. Construction: A Back Pressure
Gauge
Compressed air is filtered and passed through a pressure
regulator. The regulator reduces the pressure to about 2 bar. The
air at this reduced pressure passes through the control orifice
and escapes to the atmosphere through the orifice of the
measuring head.
11. Construction:
Depending on the clearance between the
measuring head and the work part surface, back
pressure is created in the circuit, which, as
already pointed out, has a direct relationship with
the effective area of the measuring orifice.
Various transducers are available to display the
linear gap between the measuring head and the
work part.
In this set up back pressure is let into a bourdon
tube, which undergoes deflection depending on
the magnitude of air pressure. This deflection of
the bourdon tube is amplified by a lever and gear
arrangement, and indicated on a dial
12. Construction:
Magnification of up to 7500:1 can be achievedby
employing the bourdon tube principle. Readings
up to 0.01 mm is common in most of the gauges.
The back pressure gauge is essentially a
comparator, and the initial setting is done by
means of reference gauges. It is important for
both the reference gauge and the workpiece being
inspected to have the same geometric form.
Therefore, slip gauges are used for flat
workpieces and ring gauges are preferred for
cylindrical workpieces.
13. Construction:
The master gauge is used and the instrument is
set to a reference value by varying the input
pressure of air as well as by means of variable
bleed to the atmosphere.
This can be done by operating the pressure
regulator. Air pressure is adjusted so that the
instrument is set to some datum value on the
scale.
Now, the reference gauge is taken out and the
workpiece is introduced with the measuring
gauge. The deviation in dimension can be directly
read on the scale.
14. Solex Pneumatic Gauge
This air gauge has been developed and marketed by Solex Air
Gauges Ltd, USA, and is one of the most popular pneumatic
comparators in the industry
Compressed air is drawn from the factory air supply line, filtered,
and regulated to 2 bar
15. Extra air, by virtue of a slightly higher supply air pressure, will
leak out of the water tank in the form of air bubbles and
escape into the atmosphere. This ensures that the air moving
towards the control orifice will be at a desired constant
pressure.
The air at a reduced pressure then passes through the
control orifice and escapes from the measuring orifice in the
measuring head.
Based on the clearance between the work part and the
measuring orifice, a back pressure is created, which results in
the head of water being displaced in the manometer tube.
The Solex comparator has a high degree of resolution, and
variation in dimension up to a micrometre can be determined
easily. Amplification of up to 50,000 is obtainable in this
Solex Pneumatic Gauge
18. P is the source pressure while p is the back
pressure or pressure between the control and
measuring orifice.
The relation between the p and P depends on
the relative sizes of the two orifices Oc and
Om.
p=P when Om is blocked and p= 0 when Om
is increased indefinitely.
Let C is orifice area of Oc and M is of Om
20. WE are interested in the linear form of the
curve.
Between the values of 0.6 to 0.8 the curve
approximates to linear curve. The equation of
which may be written as ,
Value of A= 1.1 for series of characteristics curves.
Slope b however is not constant ,
b=0.6 when P= 0.13 kg/cm2
b= 0.4 when P= 5 kg/cm2
21. Area of the escape orifice:
When the clearance between surface and
nozzle face is zero, no air escapes from the
nozzle and area of the escape orifice is zero.
When the clearance between surface and
nozzle face is large, no air escapes from the
nozzle and area of the escape orifice is
Between these two extreme values when
clearance is small and gauging is applied area
of the escape orifice is , πDL.
22. Range of the linear
measurement:
As we know that, 0.6<p/P<0.8, the
characyteristic curve is linear within 1%.
Mn= Minimum value of M for p/P of 0.8 and
Mx= Maximum Value of M for p/P of 0.6
Then if we take the ratio,
25. Overall sensitivity:
The overall magnification of the apparatus
is the ratio of the linear movement of the
pointer or index of the pressure measuring
instrument to the change in the dimension
which produces it.
It actuall depends on
1. Pneumatic sensitivity
2. The way in which area M change changes
as L chnges
3. The sensitivity of the pressure measuring
instrument
26. Overall sensitivity:
Overall sensitivity =
M=
Now if λ be the length of the scale of pressure measuring device
corresponding to the pressure change from 0 to P
28. Numerical
The operation of the pneumatic comparator is
represented by equation,
for 0.6>>0.8
Where , P= supply pressure p= pressure between
measuring and control orifices
b= constant =0.5 for P=2 kg/cm2
M=πDL
L= separation between nozzle surface and the
surface to be gauged
D= measurement orifice diameter
d= control orifice diameter= d2
29. The control orifice is 0.5 mm diameter and
measuring orifice is 1 mm diameter hole.
Calculate : a) The range of linear
measurement b) measuring head , pneumatic
gauge and overall sensitivities.
The back pressure gauge has a deflection of
20 mm for 100 kg/cm2 pressure change.
Air supply pressure is constant at 2 kg/cm2
30.
corrosponding to maximum separation ,
where Lx is the maximum
separation.
After solving for d=0.5 , D= 1 mm , we will get, Lx= 0.062 mm
corrosponding to minimum separation ,
where Ln is the minimum separation.
31. After solving for d=0.5, D= 1 mm , we will get,
Ln= 0.037 mm.
Therefore , Linear Range = Lx-Ln = 0.062-
0.037=0.025 mm
b) Measuring Head sensitivity =
=πD =3.142 x 1 mm=3.14 mm
Pnuematic
sensitivity=
Ma=πDLa=π . 0.05 mm2
