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National Bureau of Standard: Ubraty, £-01 Admin. Bldg.

JAN 6 WO

NBS TECHNICAL NOTE 512

The Accuracy of

Air Tower Pressure Gages

In Suburban Washington, D.C.
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NBS TECHNICAL NOTE 512

ISSUED DECEMBER 1969

Nat. Bur. Stand. (U.S.), Tech. Note 512, 10 pages (Dec. 1969)

CODEN: NBTNA

The Accuracy of Air Tower Pressure Gages

In Suburban Washington, D.C.

B. G. Simson and R. W. Radlinski

Office of Vehicle Systems Research

Institute for Applied Technology

National Bureau of Standards

Washington, D.C. 20234

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(Order by SD Catalog No. C 13.46: 512). Price 25 cents

The Accuracy of Air Tower Pressure Gages in Suburban Washington, D. C*

B. G. Simson and R. W. Radlinski

A survey of 50 air tower pressure gages in

service stations of suburban Washington, D. C,

was performed. Results showed that a motorist

using these towers has only a 20 percent chance

of inflating his tires within ±1 psi of the pressure indicated by the tower's gage. It is shown

that a calibration of the tower gages would reduce

the standard deviation of the obtained pressure

to 0.5 psi.

Key words: Air towers; tire pressure.

1. Introduction

A recent tire usage survey by the Davidson Laboratory of the Stevens

Institute of Technology [l]t included an inspection of 72 air tower pressure gages at service stations in the Eastern United States. Results

showed that "usually, the air tower delivers a pressure slightly less than

the indicated setting." In another study of air towers [2], Consumers

Union reports that "about 40 percent of the equipment was off between one

and three pounds ..."

The work described below deals with air towers at service stations in

the suburbs of Washington, D. C. The close proximity of the test stations

was chosen to keep environmental conditions, such as temperature and baromentric pressure, as constant as possible. All measurements were completed

in about one week.

2. Test Procedure

A population of 200 service stations was chosen from the following

suburbs of the Washington, D. C. , metropolitan area: Chevy Chase, Silver

Spring, Rockville, Wheaton, and Gaithersburg. From this population, a random sample of 50 service stations was selected for the survey.

*This work was carried out at the National Bureau of Standards under the

sponsorship of the Department of Transportation, National Highway Safety

Bureau (FH-1 1-6090). The opinions, findings, and conclusions expressed

in this publication are those of the authors and not necessarily those

of the National Highway Safety Bureau.

tFigures in brackets indicate the literature references at the end of this

paper.

1
The apparatus used to measure the air tower pressure is shown in

figure 1. It consists of a 0-60 psi test gage (specified by the manufacture!

to have an accuracy of ±0.15 psi) connected to the spare tire of the survey

team's vehicle. The air tower gage was set to the desired test pressure

and the spare tire (previously deflated to low pressure) was "pumped up"

until the tower bell stopped ringing. Then the test gage pressure was recorded. The test pressures* were 20, 24, 28, 32, and 36 psi and the

sequence of pressure settings was randomized for each tower. The test gage

was checked at the onset and conclusion of each test day against a master

gage (specified by its manufacturer to have an accuracy of ±0.10 psi).

AIR TOWER

SHUT OFF VALVE

CHECK

VALVE

SPARE TIRE IN VEHICLE TRUNK

Figure 1. Apparatus for measuring air tower accuracy.

3. Data

The data collected from 50 service stations are shown in table 1.

Reported are the test gage readings corresponding to air tower gage settings of 20, 24, 28, 32, and 36 psi, respectively.

*A11 pressures are "gage" pressure, not absolute pressure,
Table 1. Test gage readings (psi) for various air tower settings

Tower Setting

Sample 20 psi 24 psi 28 psi 32 psi 36 psi

1 17.4 21.5 25.5 29.5 33.7

2 14.0 18.2 22.0 26.7 29.6

3 18.4 22.3 26.0 30.0 34.2

4 21.5 26.0 30.4 34.2 38.4

5 19.6 22.4 26.2 31.8 35.2

6 19.0 24.6 27.8 31.7 36.4

7 18.8 23.7 27.3 31.5 35.6

8 19.8 23.8 27.9 31.8 35.2

9 18.2 21.0 24.7 29.4 33.0

10 18.6 21.8 26.1 30.3 33.3

11 21.2 23.8 27.8 31.0 35.7

12 19.3 23.2 27.8 31.7 35.4

13 18.8 22.6 26.0 30.2 34.4

14 14.6 19.3 23.6 27.0 30.7

15 16.0 20.0 24.2 27.5 31.8

16 19.7 24.3 29.6 32.4 36.2

17 18.7 22.0 26.0 30.0 34.3

18 21.8 25.9 30.4 34.4 37.7

19 18.0 22.4 25.6 30.0 33.6

20 22.2 26.5 29.7 33.2 37.4

21 21.1 24.5 28.5 32.4 37.7

22 19.7 23.4 28.6 32.6 * 23 22.0 25.5 29.4 32.7 37.0

24 16.8 21.3 26.3 29.1 32.9

25 21.2 25.9 28.1 32.7 36.2

26 21.6 25.6 28.8 33.6 36.8

27 20.2 24.5 28.4 32.2 35.9

28 12.6 14.6 19.4 23.0 26.6

29 22.0 25.8 29.4 33.4 37.5

30 20.2 24.2 28.5 32.6 36.8

31 17.0 21.5 25.0 28.0 32.0

32 18.2 21.8 26.5 29.4 33.2

33 18.0 21.4 25.2 29.6 34.5

34 15.0 18.8 21.6 25.4 29.2

35 16.9 21.0 25.1 28.8 33.0

36 19.8 23.7 28.2 32.0 35.4

37 18.5 22.2 26.2 29.7 34.8

38 20.0 24.6 29.3 32.8 35.2

39 13.6 17.4 21.3 25.1 28.7

40 18.0 22.0 25.5 29.2 33.4

41 19.3 23.3 27.3 31.5 35.3

42 20.2 23.7 30.2 31.3 36.0

43 27.5 31.0 35.0 38.5 42.7

44 33.6 37.6 43.7 48.0 52.3

45 32.4 37.6 42.5 46.4 51.1

46 24.5 28.6 32.6 36.4 40.6

47 18.8 23.7 26.8 31.0 34.0

48 22.4 26.8 31.0 34.8 39.8

49 23.2 27.0 30.2 35.2 38.8

50 23.3 28.2 32.2 35.3 39.6

*Tower would not stop pumping at this setting.
4. Statistical Analysis

An analysis was first made to separate systematic and random type

errors in the observed differences between the various towers. It is useful for this purpose to represent the data by an appropriate mathematical

model

The procedure is illustrated in figure 2. The abscissa represents the

average values for all air towers at each of the five settings. The value

denoted C is the average of these five averages. The ordinate represents

the values obtained by air tower 17. The five points thus obtained fall

close to a straight line, which is also shown on the graph. The height of

this line at the point C is denoted by A and its slope by B.

If x. is the abscissa for the jth point and y. the corresponding

ordinate, J the situation is represented by the equation

y . = A + B(x.-C) + random error [1]

J J The "random error" term is added to express the fact that the points do not

fall exactly on the straight line. This equation applied to air tower 17.

More generally, the same process can be applied to any air tower. For the

Hh air tower, the equation will be

y. . = A. + B. (x.-C) + random error [2]

The parameters A. and B. may vary from one air tower to another. The following general facts can be derived from the above [2] equation.-

(1) If A. is constant for all towers, and B. is unity for all towers,

then 1 1

y.. = constant + (x.-C) + random error

In that case, there would only be random differences between the data

obtained from different towers.

(2) If B. is unity for all towers, but A. varies from tower to tower,

then ^ 1

y-- = A. + (x.-C) + random error

In that case, A. represents a systematic shift for tower i, but this shift

would be the same for all pressures.

(3) Finally, if B. is not constant and not equal to unity for all

towers, then the systematic shift from tower to tower would vary from one pressure to another. Actually, A. and B. will never be rigorously constant

even if no systematic variations are present because of the effects of

random fluctuations. It is then necessary to ascertain whether the fluctuations observed in A. and B- exceed what may be expected as a result of

chance.
35 r20 25 C 30 AVERAGE AIR TOWER READOUT (psi)

Figure 2. Pressure reading of air tower 17 vs the average reading

of 49 air towers.

The values of A. and B. calculated for all 49 towers* are shown in

control chart fashion in figure 3, and the value of C = 27.81. Also

shown on the graphs are two sigma control limits. These limits indicate

the variations in A. and B. that would result from random fluctuations

alone , and they are such that, in the absence of systematic effects , approximately 95 percent of the plotted points should fall inside the

bands determined by these limits.t

*Tower 22 was omitted because of incomplete data.

tA more detailed discussion of this statistical procedure may be found

in reference [3]

.
50.00 1-

45.00 -

40.00 -

CL ~ 35.00

30.00

25.00 -

AVG. • ••• •

• • • • •

20 30 40 50

1.200 1- g 1.150

g? I 100 1— UJ2 1.050 o

co i.ooo UJ

,950 — — .900 CD

AVG. • • •• \r^fi

10 20 30 40 TOWER CODE NUMBER

50

Figure 3. Parameters characterizing systematic differences between

air towers.

It is obvious from these graphs that large systematic errors exist in

terms of A. between the towers since all but 5 points fell outside the two

sigma control limits. All but six of the B. points fall inside the two

sigma band indicating that the differences in the B. are mostly due to

random error. Consequently, the systematic shifts observed between towers

(fluctuations in A.) are for all practical purposes constant for all pressures. It follows that calibration of any tower at a single pressure,

say 28 psi, would result in a considerable improvement in obtaining uniform

readings from different air towers.
The previous analysis is concerned only with precision, i.e., with

agreement between towers. From the viewpoint of aQouraoy t a comparison

must be made between the x., i.e., the average pressures obtained by all

49 towers, and the corresponding nominal values of pressure. The results

are shown as follows:

Study of Accuracy

Nominal

Pressure 20 24 28 32 36

Difference

19.87 23.86 27.89 31.72 35.69

0.13 0.14 0.11 0.28 0.31

It is seen that the average pressure for all towers is consistently smaller

than the nominal pressure. The differences, however, are relatively small

when compared to the random error. The latter is characterized by a standard deviation of 0.54 psi. Thus, if systematic errors are eliminated

through calibration, a random error will remain which, with 95 percent

probability, will fall within approximately ±1 psi. The additional inaccuracy measured by the differences shown above is then practically

negligible.

In order to obtain a visual appraisal of the large systematic differences between the pressures for the various air towers, frequency diagrams

are shown for the deviations from nominal pressure settings in figure 4 for

the three settings, 24, 28, and 32 psi, which are the usual pressure limits

for passenger car tires. The calculated mean and standard deviations for

the three distributions were and 4 psi, respectively. For the purpose

of the following discussion, the gage reading differences were approximated

to be normally distributed.

Analysis of the data in figure 4 showed the following results:*

(1) A motorist who uses an air tower, which has the characteristics

of the sample discussed here, to inflate his tires has only a 20 percent

chance of obtaining a pressure within ±1 psi of the setting he chooses on the air tower.

(2) The chance that a motorist will inflate his tires to a pressure

which differs from the nominal air tower pressure by ±x psi is given as

fol 1 ows

:

Pressure Deviation (psi) Probability (percent)

±2 psi or more 62

±3 psi or more 45

±4 psi or more 32

*The calculations are based on a normal distribution with zero mean and a standard deviation of 4 psi.
lOr

8

6

4

2 QJl I i I i I i I I i I

TOWER SETTING = 32 psi

h.n j i i i i i i rn.rn

-8 -6 -2 2 6 8 10 12 14 16

I0r

CO

uj 8

6-

o

a: 4

2-

adid

TOWER SETTING^ 28 psi

TTTThn -6-4-2 2 4 6

I L_l I I I rn.rn

10 12 14 16

°r n ""I TOWER SETTING = 24psi

8-

6- —

« m i m 1 1 1 1 1 i i i

i . i i rrrn i n i II i -6-4-20246

DEVIATIONS (psi)

10 12 14 16

Figure 4. Inflation pressure deviations between air tower

and test gage.
5. Recommendation

Results of the survey showed that a motorist, using an air tower in

the areas described above, has only a 20 percent chance of inflating his

tires within ±1 psi of the pressure he chooses on the tower's gage. The

test team was informed by a number of service station managers that to the

best of their knowledge the air towers were never calibrated. As indicated

by the analysis, the air towers which were incorrect at one pressure setting were off on the four other settings by about the same amount. This

indicates that a simple periodic calibration at a setting of, for example,

28 psi would be sufficient to reduce the standard deviation of the obtained

pressure to 0.5 psi.

6. Acknowledgment

The statistical analysis was done by Dr. John Mandel , Statistical

Consultant, Institute for Materials Research, whose assistance is gratefully acknowledged.

7. References

[1] Stevens Institute of Technology Report on Characteristics of Tire

Usage in the Eastern United States, A Survey, I.R. Ehrlich and

M.P. Jurkat, October 1968.

[2] "Automobile Tire Gauges," Consumers Reports 33, No. 2, 94-96 (1968).

[3] "The Interlaboratory Evaluation of Testing Methods," J. Mandel and

T. W. Lashof, ASTM Bull. 239, July 1959, p. 53-61 (TP133).

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