Report CCG 71-57, NBS 2210471

NBSIR 74-450

ULTRASONIC INTERFEROMETER MANOMETER

P. HEYDEMANN

PRESSURE AND VACUUM SECTION

HEAT DIVISION, NBS

WASHINGTON, D.C. 20234

JANUARY 1974

INTERIM REPORT FOR PERIOD OCTOBER 1 - DECEMBER 31, 1973.

PREPARED FOR CCG-ARMY/NAVY

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Itork on ths Ultrasonic Manoaeter during the last quarter of calendar year

1973 was devoted to the p rob lens of solving the difficulties with the closures

previously discussed sad developing a acre reliable technique for determining the

length of the mercury colvmj.

CLOSURES

The closure at the bottom of the mercury columns must not only couple the

ultrasonic wave from the single crystal quartz transducer to the mercury, but

also provide a mechanically strong and stable and chemically inert seal for the

bottom of the manometer tube. For ultrasonic coupling fused quartz is a perfectly

acceptable material, its reflection coefficient with mercury being about 2.3%.

t Unfortunately extensive experience has shown that it is very difficult to mechanically

secure these disks in a manner that will assure that they are accurately aligned

with the rest of the apparatus without cracking them. In addition the low tensil

strength of quartz severely limits the differential pressure that can be placed across

the closure. Most other materials either amalgamate, are not available in the needed

form, or are a poor accoustic match to the mercury. For instance, we had hoped to

use stainless steel but its reflection coefficient of 19% causes unacceptably long

rise and fall times and amplitude modulation of the ultrasonic pulse. Further searching

revealed two available materials with promise, titanium and beryllium, with reflection

coefficients of 2.4% and 0.6% respectively. Both materials are strong and ductile

and Be has an exceptionally high modulus of elasticity and thermal conductivity. The

, chemical compatibility of these materials with mercury is not as well established as

in the case of quartz or stainless steel. Modern data indicate a solubility of

titanium in mercury of 5 x 10-6 % by weight at room temperature and a solubility of

beryllium in mercury of 10~5 % by weight at 100°C (solubility will be much lower

at room temperature)

Samples of commercially pure titanium and beryllium were obtained. The

titanium was fabricated into 1/4" thick disks with the surfaces ground plane and

parallel to within ±0.0002". Because of its toxicity the beryllium was used as

received, a 1/4" disk with a lathe finish. Neither sample worked at all well at first.

After many trials it was found that excellent results can be obtained with the

titanium by carefully degreasing and cleaning the entire apparatus with a final

cleaning of the titanium disk using an etch of 25-30% HNO3, 2% HF, balance water,

followed by a thorough rinsing with distilled water. Carefully and recently cleaned

mercury must then be used. Using similar techniques with the beryllium we were

able to obtain only slightly better results than with the as received material.

Although beryllium should give better results than titanium we do not intend to further

investigate it at this time as titanium works adequately well for present purposes.

LENGTH MEASUREMENT

One of the major drawbacks of the fringe counting ultrasonic interferometer

manometer is that the length measurement is a cumulative fringe count and susceptible

to error from counts lost or gained if the measurement is interrupted for any reason.

This is especially true when mercury is used since a mercury surface is so easily

disturbed. This problem is alleviated somewhat by the use of relatively long

ultrasonic wavelength and different smoothing techniques in the signal processing.

This problem is much more severe for the Infrared Laser Interferometer Manometer also

being developed in the Pressure and Vacuum Section because of the 10 micrometer wavelength used in that instrument. For this reason we investigated alternate techniques

to the fringe counting method to obtain the length information. It now appears that a

solution may be the classical excess fractions, exact fractions, or multiple color

technique, and that it may be easily applied to the ultrasonic manometer.

In the excess fractions technique one starts from the basic equation

2L = (Nj+F^) X^ where L is length being measured and N^ and F^ are the integral and

fractional fringes when a wavelength Xj_ is used. The X^ are of course known and the

F£ are measured but the are not known. However, if F.j_ is measured for a number

of different X^ there will, within limits, be a unique set of N^ and hence a unique L

that will simultaneously satisfy the basic equation for all of the measured wavelength

We have worked on two techniques to obtain L from the measured F^. One approach is to

simply search for the appropriate length using a computer program that generates

successive values of L and tests them against the F^ until it finds one that fits.

The second technique algebraically solves for increasingly better approximations to

the N^ until the error is less than 1. These equations are not complex and probably

could be programmed on a microcomputer. Although we have not developed complete

criteria for picking the optimum and we are not yet certain what the distance is

over which we obtain a unique solution in the case of the computer search routine both

approaches appear promising. With three wavelengths and a ±2.5% error in measuring

Sin8 and cos 6 (F = Tan" 1 S "^n

^ ) we can uniquely determine L to within 3000 fringes

cos6 J

(about 23 cm) using the algebraic technique and to within at least 6000 fringes (about

46 cm) with the computer search. Since any number of relatively crude devices

could be used to determine the length with an uncertainty less than this repeat

distance (the distance within which we have a unique solution) this technique can

be used over almost arbitrarily long distances. Furthermore, the ultrasonic interferometer is ideally suited for use with this technique since it can be set up to

make a fringe measurement at a number of wavelengths almost simultaneously. We can

therefore use it to make an almost instantaneous measurement of the length of

the mercury column that is independent of its previous history of the mercury column.

FUTURE

We will try its further quantify the choice of optimum wavelengths for

use in the exact fractions technique and the limitations of the technique. We

will then modify the interferometer to make measurements at the appropriate

wavelengths and use the new interferometer with the 1 meter range manometer that

is presently being fabricated.