Air gauging has rapidly increased during some past time due to the following important characteristics :

(a)   Very high amplifications are possible. It can be used to measure diameters, length, squareness, parallelism, concentricity, taper, centre disance between holes and other geometric conditions.

(b)    As no physical contact is made either with the setting gauge or the part being measured, there is no loss of accuracy because of gauge, wear. For this reason, air spindle and air snap gauges last very long. Also very soft parts which are easily scratched, can be gauged.

(c)    Internal dimensions can be readily measured not only with respect to tolerance boundaries but also geometric form. In other words, while measuring a bore it can reveal complete story of size, taper, straightness, camber and bell mouth etc.

(d)    It is independent of operator skill.

(e)    High pressure air gauging can be done with cleansing of the parts which helps to eliminate errors due to dirt and foreign matter.

(f) Gauging pressures can be kept sufficiently low to prevent part deflection.

(In general, high pressure gauges are suitable for those parts in which tolerances are relatively large and low pressure air gauges are preferable for highly precise work.)

(g)    Dimensional variations throughout the length of shaft or cylinder bore can be explored for out of roundness, taperness, concertricity, regularity and similar conditions.

(h)   Not only it measures the actual size, but it can also be used to salvage oversized pieces for rework or to sort out for selective assembly, i.e., it is suitable both for variable inspection (measurement of size) and attribute inspection (GO and NO GO) gauging and limits.

(i)    The total life cost of the gauging heads is much less.

(j) It is accurate, flexible, reliable, universal and speedy device for inspecting parts in mass production.

(k) It is best suited for checking multiple dimensions and conditions on a part simultaneously in least possible time. It can be used for parts from 0.5 mm to 900 mm diameter having tolerance of 0.5 mm or less. It can be easily used for on line measurement of parts as they are being machined and take corrective actions.

1. Systems of Pneumatic Gauges. Based on the physi­cal phenomena on which the operation of pneumatic gauges is based, these may be classified as :

(i) Flow or velocity type, (ii) Back pressure type.

Flow or velocity type pneumatic gauges operate by sensing and indicating the momentary rate of air flow. Flow could be sensed by a glass tube with tapered bore, mounted over a graduated scale. Inside the bore a float is lifted by the air flow.

Velocity of air in velocity type pneumatic gauges can also be sensed by sensing the velocity differential i.e., differential pressure across a venturi chamber. Such systems have quick response. These permit use of large clearance between nozzle and object surface, resulting in reduced wear of the gauging members. There is less air consumption. Magnification of the order of 500 to 5000 times is possible.

1.1. Free Flow Air Gauges (Flow or velocity type). In this case the compressed air after the filtering and pressure reducing unit flows through a tapered glass tube containing a small metal float and then through a plastic tube to the gauge head having two diametrically opposed orifices for air escapement into atmos­phere (Refer Fig. below). The position of the tube is dependent upon the amount of air flowing through the gauge head, which in turn is dependent upon the clearance between the bore to be measured and the gauge head. Fig. below shows a curve between the air flow and the clearance between the part and the orifice in gauge head.

The flow velocity type pneumatic comparator with zero adjust­ment and magnification adjustment is shown in Fig. below. Magni­fication can be changed by bypassing some of the air supply, using a screw at the inlet to the tapered glass tube.

The float can be zeroed by a bleed valve installed at the top of the tube. Size is measured by the velocity of air in a tapered glass tube which is measured by the height of the float in tube.

The straight portion of the curve is utilized for the measuring range. It provides high amplification (10 : 1) and thus within the linear range, it is possible to

read accurately upto microns depending upon the scale length, or classify the sizes quickly and accurately. The amplification can be changed by quick change of tube, float and scale. Air gauge amplification and range are based on the tooling and instrument standards of manufacturer. The amplification and in­strument are selected by considering the total tole­rance spread and choosing the instrument that covers the range. About 50 to 100 mm of column is usually allowed for the actual tolerance spread.

In the gauging head, the air escapement orifices are recessed below its cylindrical surface so that the orifices never contact the part being gauged. Thus the surface wear will not affect the accuracy till it is worn down to orifice level. Also the orientation of gauge or the way operator holds the gauge is of no consequence and same readings will be obtained for given diameter. On the gauge, knobs are also provided for adjusting float position and calibration. Air gauge is set by placing masters for maximum and minimum tolerances on spindle alternatively and adjusting the float position for each master by turning the knurled knobs at the base of the instrument.

Free-flow column type gauges are usually assembled together side by side and thus multiple inter-related readings can be seen at a glance. This is the big advantage of air gauging that the multiple dimensions and conditions can be inspected with great ease, accuracy and speed.

Pneumatic circuits can be arranged to determine dimensional differences like taper (comprising the diameter of bore at different points along a part), bore centre distance and also to select parts to assemble to predetermined clearances or interference fits.

1.2. Back Pressure Gauges. The basic principle and the theory of pneumatic gauging in the back pressure gauges is described below. (Refer Fig. below).

Air from a constant pressure source flows to the atmosphere through two orifices Oc and Om in series.

P is the pressure upstream of the first orifice and p is the pressure between the two orifices, both measured with reference to the atmospheric pressure as datum.

The relationship between p and P will depend upon the rela­tive sizes of the two orifices : p being equal to P when 0m is blocked and tends to zero as Om is increased indefinitely. Let C be the geometrical area of Oc and M that of Om.Then if p and C are kept constant while M is varied, the relationship between the dimensionless quantities p\P and M/C is of the type shown in Fig. below.(The general form of this curve is quite well predicted by an analysis employing Bernouli’s equation for flow of a compressible fluid.)We are interested in linear portion of this curve.For design purposes we follow an empirical approach which is based on an experimental study at N.P.L. (London) of the rela­tionships between pressures and orifices areas.

The characteristics of p/P and M/C are determined experimen­tally for pressure P varying form 2 to 75 pounds per sq inch (0.13 to 5 kg/cm2) and inspection of any one of these shows that within

the range 0.6 p/P to 0.8 p/P, the curve approximates to a straight line, the equation for which may be written as

p/P= A-B*(M/C)

Examination of the family of curves shows that constant A, the intercept on the p/P axis is closely constant over the range of pressures investigated and for practical purposes, the value of A=1.10 can be adopted for any value of P likely to be used.

The slope b of straight portion characteristics is however not independent of P, its numerical value decreases as P increases and the limiting values found in the investigation are as under :

b=0.6 when P=2 lbf/in.2 (0.13 kg/cm2) b=0.4, when P=75 lbf/in.2 (5 kg/cm2)

1.3 Area of escape orifice. When the clearance between the surface and the nozzle face is zero, no air escapes from the nozzle and the area of the escape orifice is zero.

When the clearance between the surface and the nozzle face is very large, the area of the escape orifice is π/4 d2, where d is the diameter of the nozzle.

Between these extremes, especially where the clearance is small and where air gauging can be employed, the area of the escape orifice is π dl, that is, the area of the curved surface of the cylinder shown in Fig. below.

1.4. Range of linear measurement. The condition 0.6<p/P<0.8 defines a section of the characteristics which experi­mental investigation has shown to be linear to within 1 %. i.e. to say, Mn is the minimum value of M for p/P=0.8 and Mx is the maximum value of M for p/P=0.6.

If a lower order of linearity is acceptable, the limits for p/P may be extended. Thus for 0.55<p/P<0.85, the linearity is still, within 2%.

From Fig. below for linear range, we have

B(Mn/C)- 1.10-0.8=0.3

B(Mx/C)= 1.10-0.6=0.5

Mx/ Mn =5/3 and Mx - Mn/ Mx+ Mn= 1/4

Let [Ma = 1/2(Mx+ Mn)]

Then (Mx- Mn )/2 Mn = ¼ and  Mx- Mn = Mn /2

1.5. Pneumatic Sensitivity, i.e. how p varies by variation

of M.

By differentiating equation(1)

[p/P = A-b.(M/C)]

We have dp/dM = – (b/C)P

This expression may be changed into more useful form. Since p will have a value 0.7 P when M has its average value


0.7=1.10 – b*(Ma/C)

b/C= 0.40/ Ma

Hence dp/dM = – b/C * P = -0.40 *P/ Ma

It is observed that sensitivity is directly dependent upon operating pressure P and inversely proportional to Ma.


1.6. Changing sensitivity (Magnification). A plot of pressure against escape orifice area for a number of different sizes of control orifice will show that sensitivity increases as the diameter of the control orifice decreases, i.e., for small control orifices the change in pressure is greater for a given change in escape orifice area. Utilising this property, it is possible to set precisely the sensitivity (magnifica­tion) by incorporating a variable control orifice.

1.7. Overall Magnification. In a practical pneumatic measuring apparatus the area M will be associated with the measuring head and change in M will be the result of a change in the dimension which is being measured e.g. a change in the separation L between nozzle and surface (Fig. below).

The overall magnification of the apparatus, i.e. 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, will be proportion to the pneumatic sensitivity but will depend also on other two factors, viz.

(i) The way in which the area M changes as the dimension L changes, i.e. dM/dL.

(ii) The sensitivity of the pressure measuring instrument, i.e. (The rate of change of reading R of this instrument with respect to the varying pressure p).

Thus the overall magnification is given by ;

dR/dL = dR/dp * dp/dM * dM/dL

where               dR/dL =Overall magnification,

dR/dp =Sensitivity of pressure measuring device.

dp/dM =Pneumatic sensitivity.

dM/dL= Rate of change of M with L.

From equation (3), dp/dM is constant over linear range and

thus overall magnification will be constant, provided :

(i) dR/dp is constant, i.e. the pressure measuring device is linear which is usually the case ;

(ii) dM/dL  is constant, i.e. M changes proportionately with L.

This condition requires the measuring head to be correctly designed. The final escapement of the air from the nozzle to the atmosphere is taken as being through an area of the curved surface of the cylinder of length L and diameter D, where L is the separa­tion between the nozzle surface and the surface to be gauged and D is the internal diameter of the nozzle.


and             dM/dL = π D=constant

If La is average separation, i.e. corresponding to p=0.7 P. If         Ma=πDLa.

Now in equation

p/P = 1.10 – b *(M/C)

p/P = 0.7, M= Ma

b = (1.10 – 0.7)* C/ Ma = 0.4 * C/ Ma

Substituting the value of b in the following equation,

Substituting the values of M and Ma,

p/P = 1.10 – b * (M/C)

We have, p/P=1.10 – 0.4 * C/ Ma * M/C

                            =1.10 – 0.4 * M/ Ma

Substituting the values of M and Ma,

p/P =1.10 – 0.4 *(πDL/πDLa)

p/P =1.10 – 0.4 *(L/La)


This is the equation of the linear portion of the characteristics

showing the relationship between p/P and ratio of L/La. This

equation shows that for given values of P and La, p varies directly with L.

Now equation (2) for the linear range may be written in terms of L, i.e. displacement of the surface from the nozzle.

• •                                                   Lx – Ln = (1/2)La

i.e. the linear range of L is equal to half the average value of La.

Let λ be the length of scale of pressure measuring device corresponding to the pressure change from 0 to P (p may vary from 0 to P).

Then     (dR/dp)=(λ/P)

And       (dp/dM)=0.40 *(P/Ma)  ……….From Equation (3)

dM/dL= πD

dR/dL= dR/dp* dp/dM * dM/dL

= λ/P * 0.40 * P/Ma * πD

= λ/P * 0.40 * P/( πD La)* πD = 0.40 * (λ/La)


Therefore, for design purposes, the equations needed to suffice the quantitative estimation of the performance of pneumatic systems are (for 1 % linearity).

dR/dL= 0.40 * (λ/La)  …..(5)

Lα – Ln = (1/2) La ……(6)

p=0.7 P when L=La

and                                          λ=Length of scale of pressure measuring

device corresponding to the pressure change from 0 to P.

 Lα =Maximum separation between nozzle and surface and Ln  is the minimum separation.


The main disadvantage of air plug gauges is their limited measuring range which is usually restricted to 0.10 mm.

1.8. Response Speed. For a back pressure system the speed of response is not as fast as for free-flow type, because some time is required, for the pressure to build up. The speed of response becomes of concern when the gauging head is separated from indicating instrument by long distance.

A pneumatic measuring system will not correctly measure displacements of frequency greater than about 2 cycles/second, because of its slow speed of response. The response is consider­ably slower compared to the electrical system because of the following reasons. Between the control orifice and the measuring head, there exists a closed volume associated with the measuring instrument used to measure the pressure p. A dimensional change, e.g., a displacement of the surface alters the flow so changing the pressure p. The time needed to establish the new value of p depends on the total volume and on the rate of air flow into and out of it. The latter in turn depends upon the operating pressure P and the size of the control orifice and orifice in the measuring head. These orifices are related in size and determine the pneumatic sensitivity, the smaller orifices (corresponding to the higher magnification), having a more restrictive effect on the air flow and so slowing the response.

It has been examined theoretically and experimentally that response is slowed by using a large operating pressure P and a large volume, but by these high sensitivity (magnification) is obtained. In any practical pneumatic measuring system, the overall response will be influenced by dynamic characteristics of the pressure measuring device. Thus it follows that the use of a low operating pressure will not improve the overall response if a low pressure measuring device of slow response is used to measure the pressure changes. High sensitivity will inevitably be associated with slow response and the only factor left to the designer is the volume, which should be made as small as possible for quick response.

Since gauge is always located at some distance from the control unit the effect of variations in the gauging position does not reach the control unit instantaneously, though the size variations of the object will promptly affect the air flow at nozzles. The time gap between the sensing and indication is known as response time which depends upon : Length of air line between the nozzels and indicator, (ii) type of gauge system, aid (iii) the design of control unit. Response in the case of flow type pneumatic gauges is relatively quick. Response in case of back pressure gauges is slow, the compressibility of air also contributing to the delayed transmission of the variations sensed at the nozzles.

Response time of back pressure type pneumatic gauges can be improved by utilising following devices :

(i)      Using filled system pressure gauge, thereby reducing volume of air.

(ii)    Restricting the unimpeded escape of air through the orifices when the gauging head is not in operation, by using a spring-charged cover sleeve around the gauge head.

(iii)  Counteracting the unrestricted air escape by an auxiliary air supply relay whose operation automatically discontinues as soon as a Specific back pressure develops during the actual gauging process.

(iv)  Using a high speed relay to compensate for additions to the volume of the instrument system.

1.9. Zero Setting. It is accomplished by means of a bleed valve and consists in adjusting the indicating element of the gauge to that marking on the scale which was selected to signel coincident with the nominal limit size represented by a setting master.

1.10. Datum Control. If means be provided to change the pressure in the cavity (between control orifice and measuring orifice) using a variable bleed to atmosphere, a datum or zero can be provided which varies the pressure ‘p’ when the escape orifice area remains fixed.

This addition to a circuit provides means of accommodating small differences which inevitably occur in the manufacture of gauge heads. Limited use of a datum control in the form of a bleed to atmosphere has an insignificant effect on linearity.

However, this system depends highly upon the pressure regulator to maintain the supply pressure within very close pressure limits. Thus the pressure regulator is a critical component in this circuit. This problem is overcome in differential back pressure circuit in which accuracy is maintained regardless of some variation in the regulator performance which controls supply pressure. Fig. below shows a practical flow-responsive system    which is in common use.This system employs variable orifices for sensitivity and datum control.

1.11. Amplification Adjustment. This permits different range of gauge indications on same scale length and is carried out with a precision valve of the control unit. Both zero setting and amplification adjustment should be checked from time to time depending upon experience.

1.12. Jet Recession. It has already been seen that when the surface being measured is very close to the nozzle face, equal increments of change in clearance do not produce equal increments of pressure change. The system is not linear under these conditions.

Because of this the faces of the jets on air plug gauges are ground below the body diameter of the plug as shown in Fig. given. This grinding back is called jet recession and it is the means by which the non-linear portion at the high pressure (low flow) end of the pressure/clearance curves is avoided.

1.13. Measurement of Bore. [Refer Fig. 5.41 (a) and (b)]

In the above discussion we have assumed a jet separation of the nozzle from the flat surface. This is also applicable to measure­ment of bore, where L is modified as :

L=L1+L2, L1=Radial gap for one jet and    L2=Radial gap for other jet.

Theoretically L1 should be equal to L2, but even if L1≠L2 the back pressure would be closely equal to that corresponding to the condition L1=L2=L/2. This is a big advantage as operator need not be very meticulous about orientation of the measuring head in the bore, and thus the readings from operator to operator will be uniform as they do not depend upon a high degree of operator skill or sense of feel.

1.14. Measuring Heads

Measuring heads fall under two categories, viz. direct head [Fig. (a) and (b)], and indirect or contact head [Fig.- (c) and (d)]. Tapered nose type direct head [Fig. 5.42(a)] is quite popular as it permits easy access in constricted measuring conditions. The ratio of land diameter (overall diameter at tapered small end) compared to jet diameter is twice in size. Bigger ratio would affect the escapement of air and the characteristics of the system.

Head at Fig. 5.42 (b) provides good protection to the nozzle due to incorporation of guard ring and escapement holes. In the case of indirect measuring heads, the jet is protected from accidental damage. The size of air escapement is controlled by a needle valve or flat plate [Fig. 5.42 (c) and (d) respectively] which move due to movement of measuring plunger. By changing the taper of needle valve, the range of measurement can be changed. Parabolic needle provides linear response. Fig. 5.42 (e) shows the plug gauge used for measurement of diameter, lobing, taper etc. The measuring side is made somewhat smaller than the bore so that it enters much more conveniently.

2. Solex Pneumatic Gauge. (Fig. below). This instrument is produced commercially by Solex Air Gauges Ltd. This is generally designed for internal measurement, but with suitable measuring head it can be used for external gauging also.

It is obvious from the equation for sensitivity that in order that sensitivity (magnification) remains constant, the supply pressure P must be constant.

Thus it is necessary to have a simple pressure regulator which may control the pressure of air from the normal supply line, and if necessary the pressure might be reduced also. The arrangement used in Solex gauge is to pass the high pressure air after filtering, through a flow valve. There is a tank is which water is filled upto a certain level and a dip tube is immersed into it upto a depth corresponding to air pressure required. In Fig. 5.43, it is represented by H. Since air is sent at higher pressure than required one, some air will leak out from the dig tube and bubble out of water and the air moving towards control orifice will be at desired constant pressure H.

Now-a-days diaphragm type pressure regulators are readily available in the market and they are better for regulating the pres­sure than the above device. The air at reduced pressure then passes through the control orifice and escapes from the measuring jets. The back pressure in the circuit is indicated by the head of water
displaced in the manometer tube. The tube is graduated linearly to show changes in pressure resulting from changes in internal diameter of the work measured. This instrument is capable of measuring to the accuracy of microns.

It is very obvious from Fig. 5.43 that the diameter being measured at any instant is corresponding to the portion against two jets. Now to find the concentricity (roundness of any job at any section), the workpiece may be revolved around measuring gauge. If no change in reading is there, then it is perfectly round hole Similarly the diameter can be noted down at several places along the length of bore and thus tapering of hole is determined. This method is, therefore, best suited for measuring roundness and taperness of cylinder bores and gun barrel bores.

By having suitable measuring head this can be used for exter­nal gauging, and head in this case will be as shown in Fig. 5.44.

This can best reveal any lobing effect also. It is also possible to have arrangement to measure the length of slip gauge by having the flat table and one jet at the top.

  • 2.1. Overall Magnifica­tion and Range. From equation dp/dM=0.40 P/Mc, the pneumatic sensitivity of a pneumatic measuring apparatus can be increased by increasing the operating pres­sure P, but from equation dR/dL=0.40 (λ/La) the overall magnification is controlled by the length of scale of pressure measuring instrument corresponding to pressure change prom 0 to P. If this scale length is to remain of convenient magnitude, increasing the operating pressure is not a suitable method for improving the overall magnification. The only effective method for obtaining the higher magnification is, therefore, to reduce the average separation between nozzle and surface, which at the same time, of course, reduces the range of linear measurement.
  • 2.2. Limitations of empirical approach. From the view­point of air flow, the effective area of an orifice is not usually identical with its geometrical area. If two orifices are made by producing holes-of identical geometrical area in two thin discs, their effective areas may be appreciably different as a result of edge effects on the air flow arising at the peripheries of the orifices. Again the relationship between effective area and geometrical area is unlikely to be the same for air flow through an orifice and the jet of air from a nozzle. In the experimental determination of the
    plP, M/C characteristics the value of M and C used were the geometrical areas of orifices Om and Oc. Therefore due to effective area being different from the geometrical area, the empirical equation obtained by analysing these characteristics would not be expected to provide a completely accurate  numerical forecast of performance.


Nevertheless, experience has shown that they do give a first approximation sufficiently reliable to permit the required performance to be obtained by a single-step corrective adjustment of the control orifice.

3. Differential Comparators. A later development brought out the balanced circuit type of air-gauge. In this equipment a differential pressure indicating mechanism connected across the two air-paths and a built-in gauge zeroing valve is provided. Such a balanced circuit is shown schematically in fig. 5.45. An air gauge based on this balanced circuit is called ‘Differential Comparator’.

Compressed air from a suitable source, after passing through air-drier and filter is regulated for constant pressure by a pressure regulator. Now the air flows into two channels, each of which has a control orifice Ocl And Oc2 (control orifices are also called master jets).

From the control orifice Qcl, air flows to the measuring head where it meets further restriction of the workpiece or the master settings. The restriction of the workpiece builds up back pressure as explained earlier. At the same time, other half of the air is flowing through the other control orifice Oc2 to the reference jet Om. By closing or opening the valve of reference jet Om, the pressure in the space between Oc2 and Om is regulated (adjusted) to match the back pressure from the measuring jets, which is sensed by the pressure indicating device fitted across the two channels as shown.


At this adjustment of the reference jet, the pressure indicator would indicate equal pressure in the two channels and hence read zero on the scale. This zero setting (adjusting of reference jet Om) is done with master workpiece whose dimension is exact nominal size.

Now the variation of the dimension at the measuring head would cause change of back-pressure in channel A. This pressure would be different from the mean pressure which has been already set in the channel B (by reference jet). Now the difference of pressure of the two channels would be indicated by the pressure indicating device which can be directly calibrated in terms of variation of dimension from the mean dimensions. Hence the instrument based on the measurement of differential pressure is called Differential Comparator.

If the dimension causes a decrease in gap L as compared to La, this in turn decreases M and hence increasing back pressure in channel A and vice versa. In these cases the pressure indicator would show readings on both sides of zero corresponding to ± deviation from master setting.

3.1. Advantages of Differential Circuit over Single Channel Circuit. (i) Effect of change of operating pressure P. The operating pressure may vary slightly from the designed value. It can be shown that error due to change of pressure would be 0.6 to 0.8 times the change in pressure in single channel and in case of differential circuit the error would be 0.1 times the change in pressure.

(ii) Zero setting of master gauge is an extra advantage.

(iii) Rectification for control orifice. In a single channel system, the practical limitations may not give the perfectly correct and accurate dimension of the control orifice as designed. Some­times it may go out of the useful range of the design and it may have to be discarded.

Therefore, in order to avoid the error of manufacture and also to take into account the fact that the geometrical area is different from the effective area, we need a needle valve so that area may be adjusted accordingly.

But in the differential circuit which incorporates a needle zero adjusting valve, the off-set of the actual size of the control orifice from the designed value can be nullified by adjusting this valve.

4. Non contact and Contact Type Pneumatic Gauging Elements. Non contact tooling is best suited for automatic gauging applications because of the advantages of no contact, clearing of oil or foreign particles from gauging area, etc.

In the case of non-contact air gauge tooling, only the air coming out of the air escapement orifice touches the part to be measu­red, the air flow rate depending on the cross-sectional area of the jet and the clearance between the jet and the part to be measured. It may have a single jet, two diametrically opposite jets or more evenly spaced jets. Single open-jet tooling can be used for checking outside diameter, height, depth, straightness, squareness, etc. and Fig. 5.46 shows a few of such applications.

Dual jet technique can be applied for determining true diameter, Out of round, bell mouth, thickness, etc. The various gauges may be designed either for presenting the gauges to part of vice versa.

Many modern mechanical assemblies demand that holes should be closely controlled for straightness as well as diameter. An air plug gauge for gauging hole straightness is shown in Fig. 5.47.

In the contact tooling, a mechanical member is incorporated between the air escapement orifice and the part to be measured. The air flow from the jet changes due to displacement of this mechanical member when it contacts the part. The mechanical member could be a ball, lever, plunger or blade. A big advantage of contact type air gauge is that a mach bigger measuring range (upto 2.5 mm) is possible i.e. it is suitable for wide range of gauging. Another advantage is that it eliminates surface roughness from size. It may be mentioned that open jet type method would be subject to error for rough surfaces because it measures a combination of size and surface finish ; further its range of measurement is limited. Ball jet spindle gives a point reading rather than the average over a small area and is best suited for gauging inside diameter of soft or porous parts and for rough bores. Leaf jet spindle can be used for checking laminated bores, blind holes in which keyways etc. do not permit the use of open jet spindles at extreme bottom of blind holes etc. Blade jet spindles are used for inspecting gun bores in which oil grooves, or slots do not permit the use of ball jet or leaf jet spindles.

Fig 5.48 shows a small plunger type air gauging cartridge which is highly efficient size-sensing element for wide range of guaging, tooling, fixturing, and machine control applications. It essentially consists of a spring loaded plunger. The spring tries to keep the plunger outwards and when the part to be measured comes in contact with the part it moves in and at the end restricts the orifice, thereby increasing the back pressure. The maximum and minimum limlits of the plunger movement can be set with the help of masters. Such cartridges can be secured in gauging position on various types of fixtures and used for measurements like height, depth, flatness, concentricity, squareness, inside/outside diameter, etc.

Fig. 5.49, shows the same principle used in a test indicator which is very efficient for several applications. It has a tiny stylus capable of entering into small holes, keyways, slots etc. and its movement causes the tapered end to act as a precision valve stem to regulate the amount of air flowing through an orifice. It is free of hysteresis or lag or drag in indications when the stylus is moved in any direction across the workpiece.

The contact type adjustable spindle kits and adjustable air snap guages available in market are found to be very suitable for handling new designs, altered dimensions and various other varied applications.

5. Indirect Pneumatic Gauging Devices

The open jet has the disadvantage of small measuring range. It can be overcome by using indirect pneumatic gauging devices by using a gauging cartridge. Such cartridges employs a contacting stylus, the inner end of which is tapered and forms the restriction in an escape orifice. The position of the stylus and consequently the position of the taper in the orifice causes changes in the area of the orifice. Changes in the rate of taper change the measuring range of the cartridge. Measuring ranges upto 3 mm can be obtained with this type of cartridge.

6. Air gauging with electronic sensors. Air gauging systems operate on either low or high air pressure. While low pressure systems have greater sensitivity, quicker response time and minimal distortion when, measuring flexible components, the high pressure systems are self cleaning type and have a large measuring range. Basically air gauging comprises air jet gauges—such as ring or plug, and air operated liquid columns for multi-dimensional measure­ment.

Now-a-days electronic flowmeters fare used in place of air operated liquid columns. They have the advantage of measuring flow of air with the added benefit of electronic display. Such instruments can easily have 2 or  3 range selection to give an extra magnification factor. Tolerance limit lights can be incorporated to indicate whether parts are inside or outside manufacturing tolerances. Response time increases many folds.


The versatility of air gauging is enhanced by the wide range of measuring tools like 2 and 3 jet non-contact air plug gauges, setting rings and air jet ring gauges.

7. Multi-Gauging Systems. Multi-gauging systems are used to measure a number of dimensions simultaneously. Parts to be gauged are compared with a setting master which simulates the component. The features gauged could be external/internal dia­meters, lengths, straightness, squareness, ovality, run-out of faces, etc. The measuring head gauging fixture is specifically designed to suit the component to be measured and may be completely special or it may be built using a series of modular elements. It contains the means for sensing the dimensional difference between the components and the master which may take the form of mechanical or electronic probes or air jets connected to the means for amplifying the difference.

The amplifying and display of readings may take the form of dial gauges or some form of electronic or air/electronic system.

Display may be analogue, digital or graphic and may be augmented by out of tolerance indication.

Using electronic differential methods, the relationship between different features can be related to a common datum.

The choice of system depends on number of factors like initial cost, dimensional tolerances of the features to be measured, complexity of the component, complexity of the features to be measured, number of features to be measured, speed of measurement required, skill or otherwise of the user.

The reasons affecting the choice of displays are given below :

(i) Pointer displays—best where a rapid check of run-out or concentricity is required.

(ii) Columns—natural choice where a considerable number of dimensions are involved.

These are fastest and most convenient form of displaying the readings for every dimension.

(iii) Digital—provides high accuracy over a longer  measuring range and best for situations where it is required to work in drawing dimensions. Can be viewed without strain over longer distances

(iv) Graphic on VDU—the most sophisticated display essential

when statistical process control systems are employed.

Systems capable of dealing with very large number of inter­related dimensions can be developed.

Automatic inspection machines incorporate both automated loading hopper, magazine, pick and place robot and—automated segregation of inspected components. Automatic inspection is essential where the complexity of the component is such that manual methods can not achieve the desired levels of accuracy.

The results of inspection could be fed to electronic computer based system which may also control the machine operation. The use of such computer based processing also allows the results obtained to provide a wide range of control facilities including feedback for con­trol of the manufacturing process.

Coordinate measuring machines accommodate multidimensional inspection by using a single point contact to take successive measure­ments over the component profile. The contact movement. and processing of the dimensional information is under computer control, which can also provide similar facilities to those offered by multi­point gauging.