NBSIR 75-695

ULTRASONIC INTERFEROMETER MANOMETER

Peter L.M. Heydemann

National Bureau of Standards

Department of Commerce

Washington, D.C. 20234

April 1975

Interim Report

Prepared for

CCG-Army/Navy

US Army Missile Command

US Army Metrology and Calibration Center

Redstone Arsenal , AL 35809

Naval Plant Representative

Metrology Engineering Center

1675 W. Mission Blvd.

Pomona, CA 91766

ABSTRACT

This report describes the design of the 15 kPa - Ultrasonic Manometer and

the associated vacuum equipment. Lay-out drawings are provided. The possibility

of using diffusion pump oils as manometric fluid to achieve lower pressures is

discussed. Some preliminary thoughts on possible continuation of the project into

FY 76 are given.

Key words: Manometer; pressure measurement; ultrasonic interferometer.

Description of the Manometer and Vacuum Section

The assembly drawing (Fig. 1) gives a side view of the manometer now under

construction on a one to one scale. A large aluminum plate serves as the main

support. Three leveling screws are provided. The stainless steel base plate

rests on the aluminum plate. It is ground plane and parallel. The titanium

disks and the manometer tubes are tied to this steel base. A separate jig plate

helps to align the titanium disks. Three stainless steel tubes are welded into

the steel plate to connect the columns at the bottom. The connection is made

outside the steel plate to allow for the insertion of valves and dampers. The

vacuum ports at the top of the columns are connected through welded bellows

with the flanges mounted on the shield. The manometer is enclosed with an

aluminum shield to improve thermal stability. A thermostated enclosure should

surround the entire instrument.

The vacuum system is shown schematically in Fig. 2. Two sets of three

welded bellows are used to isolate the manometer from vibrations carried through

the plumbing. Reference vacuum is provided by a trapped, mercury diffusion

pump. The line to the fore pump contains a molecular sieve to allow roughing of

the system without fear of contaminating the system with oil vapors. The two

outer columns are connected to a system containing a trapped fore pump, a volume

control for adjustment of pressure and provisions to bleed helium into the

system from a compressed gas cylinder. Ports for the connection of instruments

to be calibrated are placed near the equalizing valve as close to the manometer as

practical

.

Errors caused by Misalignment and Tilt

Errors in the length measurement with a scale mounted on the manometer column

or with the ultrasonic interferometer can occur when the manometer is either

misaligned or tilted.

• Consider (Fig. 3) the case of a manometer that is misaligned but stable. It

is inclined by an angle 0 about an axis through the bottom closures of the outer

columns. The true difference in meniscus position is given by

where the dashed symbols refer to the measured values.

If the manometer was vertically aligned for the zero reading, but tilted

for the pressure measurement and assuming that the center column was offset

from the axis AA by i 2 (Fig. 4), we have

Ai-[^^^-%'d^Q) - ^

Finally assume that the manometer was vertically aligned for the zero

reading, but tilted about an axis normal to the connecting line AA by an

angle Q . Assume further that the outer columns are spaced ii and l 3 from the

center column (Fig. 5). We now have

These are, of course, only examples of the tilt and misalignment errors

that one might encounter in practice, and the possible errors have to be carefully

considered for each type of manometer.

In the cases considered here the cosine errors are proportional to the true

difference in meniscus position. For small angles Oj ciK (3 ^ I - ^ •

A misalignment by one degree (1.7 x 10" 2 rad) causes ai^error of about

1 in 7000. A misalignment by five minutes (1.45 x 10" 3 rad) causes an error of

about 1 ppm.

However, the terms containing the sine of a in the cases considered here

are independent of the difference in meniscus position but are proportional to

the offset l s and are therefore mostly felt at low pressures. In the case

considered in Fig. 4 the error is £ 2 sin .With sin(2 ~ 0 and i 2

= 0-5 mm the

error amounts to 8.7 x 10" 6 m for G = 1° (1.7 x 10" 2 rad) and to 7.3 x 10" 7 m for

0= 5

1 (1.45 x 10~ 3 rad). A manometer with a resolution of 1.3 mPa (10" 5 torr)

and an offset l 2 of .5 mm should not tilt more than 4 seconds of arc (2 x 10" 5 rad).

Such stability is difficult indeed to achieve and only the best possible

alignment (small fc 2 ) can ensure the expected resolution. We are therefore considering various ways of ensuring near perfect centering of the transducers on

the titanium plates and proper alignment of these plates themselves.

Distortion of the Manometer Base Plate due to Shifting Load

Since in the ultrasonic manometer all column height measurements are made

relative to the titanium closure disk, distortion of the disk or of the entire

mounting plate is a possible source of error. Measurements were made on an

existing mounting plate that is similar to the one to be used in the 10 kPa -

Manometer. Fig. 6 gives major dimensions of the plate with its three mercury

tubes. The distortion of the mounting plate was measured by placing the feeler

rod of an LVDT-transducer at point A. The body of the transformer was mounted

on a rigid support placed on the floor. Mercury was alternately moved from the

center column to the side columns and back to the center. The average vertical

motion of point A due to loading a one meter mercury column into the center tube

was 1.3 x 10" 7 m. However, the vertical motion of the titanium plates in the

outer tubes relative to that in the center tube is expected to be much smaller.

Its measurement is being considered for the near future.

Use of Manometric Fluids other than Mercury

There are several reasons to use manometric fluids other than mercury.

Mercury's high density makes it an ideal fluid for measurements at relatively

high pressures, say 10 torr and up. At lower pressures a fluid with lower

density might be advantageous, if column length could be determined with the

same absolute uncertainty. For example, if visual observation with sighting

ring, photo cell etc. or ultrasonic measurements allow for a resolution of the

column height measurement of 0.05 mm, the resolved pressure with a mercury

column is 6.7 Pa (5 x 10"2 torr) and with a diffusion pump oil column of density

0.85 g/cm 3 it is 0.4 Pa (3 x 10" 3 torr). A further advantage for some

applications is the low vapor pressure of diffusion pump oils (10~ 6 Pa, 10" 8 torr)

as compared to mercury (10-1 Pa, 10~ 3 torr).

In the ultrasonic manometer the height of the column is deduced from the

ultrasonic transit time, speed of sound and ultrasonic absorption are therefore

important parameters. The lower the speed of sound is» the higher is the

resolution. A low ultrasonic absorption allows use of higher echoes for added

resolution.

Some qualitative measurements of speed of sound and attenuation were made.

Optimum acoustic match of the ultrasonic transducer to the sample fluids required

a redesign of the bottom closure of an interferometer. The new closure is

shown in Fig.7 . A steel disk with plane and parallel end faces is drilled out

to 25 mm in the center. A single crystal quartz transducer of 19 mm diameter is

placed flush with the top surface of the disk, backed with bakelite and cast in

epoxy. A metal layer is evaporated across the surface for electrical contact. The

other electrode is brought out through an opening at the lower side. Fig.#

shows the mounting of this sound projector in the fixed-path interferometer.

Measurements were first made with water as a test substance. The speed of

sound in water measured with this simple device differed only by 0.05% from the

expected value. A variety of diffusion and fore pump oils were then tested.

Mineral oils and some silicone oils showed excessive attenuation. Results are

shown in Table I.

TABLE I Properties of Pump Oils

Type Speed of sound

(m/s)

Speedivac

DC 703

DC 704 1430

Octoil

Octoil S 1403

di -butyl pht. 1390

Convoil 20

Ki nney

Attenuation Density Vapor pressure

(g/cm 3

) (torr)

excessive

very high 1.089

high 1.066 10" 8

very high 0.983 lO" 7

medium 0.912 10~ 8

medi urn 1.044 10" 5

very high 0.86 10" 7

very high

The attenuation in Octoil S is sufficient ' low to permit operation on the

sixth echo with a column length of about 10 mm. In the ultrasonic interferometer

this renders a resolution per count at 10 MHz of

Due to the high viscosity of the oil the reflecting surface of the short oil

column will be rather quiet compared to mercury and one might expect to interpolate between counts to 0.1 or 0.05 of a fringe. The ultimate resolution then

is 10" 5 torr s well into a range where it might compete with McLeod gages. The

ultrasonic manometer thus would be a welcome alternative to the McLeod gage.

Both instruments being primary standards it might be attractive to use these

instruments together to establish a pressure scale in the medium vacuum range.

The electronic instrumentation for ultrasonic manometers using mercury or

oil as manometric fluids is essentially the same.

With the density of 912 kg/m3 the equivalent pressure is

Possible Future Tasks

We expect to complete the 10 kPa - ultrasonic manometer by the end of

FY 75. The next task then will be to operate the manometer, evaluate its

performance, modify where necessary, and calibrate the instrument. To achieve

the target accuracy of 10 ymHg at 100 mmHg pressure requires that the temperature

of the manometer is stable and known to better than 0.1 °C. It may be advisable

at a later date to design a thermal enclosure for the manometer to achieve

greater accuracy. This design could be based on our experience with a similar,

but much larger enclosure that has recently been constructed.

Barring unforeseen difficulties the 10 kPa - manometer would be completely

calibrated, evaluated and documented late in 1975. At that time duplication of

the manometer for use in other laboratories should be possible without difficulty.

Since some parts for a second ultrasonic manometer are available we might

cc consider to develop and construct ^manometer for use with a diffusion pump oil to

operate at pressures, where it can be compared with a McLeod gage.

Cooperation with the Army Metrology Laboratory at Redstone would be most welcome

for the purpose of comparison.

Alternatively the second set of parts could be used to assemble a manometer

for higher pressures, for example up to 1000 mmHg.

The present electronic system for the ultrasonic interferometer is designed

to print data on demand. The actual pressure measured by the manometer must then

be computed. Since in our present system a microprocessor is used for the control

of the electronics, it is feasible to add a computing unit, which, under the

control of the microprocessor, will compute the pressure in real time and indicate

it in engineering units.

.wj)

4©<p \htu>

<5L

-Mc -^we) -

(•«,'«*>© -«„>) + ( »* S - K )

a.

,\ujl>-^V . C<^*Wr "WW c^vd: £^<x>^ AA

Alt " ^4

CM- tl - -Hje

J

cL

"PiL^W (A) , CKxJut^ ^VcA-C_ v~, AJUUS*4jL H^-^-4- ,>>\«Uv. - JUT

V

* 0

z

cS