Harwood
Engineering Company, Inc.
A High–Pressure Wire Gage Using Gold–Chrome Wire
by H.E. Darling(1) and D.H. Newhall(2)
Precision experiments with manganin wire for the measurement of high pressures
are made difficult by virtue of the temperature–resistance response of
the manganin. The equilibrium time required for this material to steady
out after rapid pressure changes makes manganin generally unsuited for
uses in industrial–control applications. For this reason, a study was made
of the pressure and temperature response characteristics of several new
materials. An alloy of 2.1% chromium in gold was found to have much less
sensitivity to temperature, varying from +1 to -1 ppm per degree Fahrenheit
over the range of 40°F to 200°F, while manganin varies from +5
to -40 ppm in this same interval. This alloy also has a strong pressure
coefficient. Typical values are 0.67 to 0.72 X 10-7 ohm/ohm/psi
for gold chrome as compared to 1.67 to 1.72 X 10-7 ohm/ohm/psi
for the manganin tested. Although the pressure sensitivity is only 33%
that of manganin, the smaller temperature sensitivity of the gold chrome
results in good discrimination between temperature and pressure changes.
Gold chrome responds rapidly to pressure changes, quickly coming to equilibrium,
and does not show the annoying drift so characteristic of manganin. It
is generally pressure seasoned with a single application of pressure, as
compared to the several cycles usually required for manganin. Its long-term
stability compares favorably with manganin.
Introduction
In the laboratory investigation of high-pressure phenomena, the use of
a coil of manganin wire whose resistance changes linearly with exposure
to fluid pressure, has been adopted, universally as a pressure-sensing
device, as a result of the pioneering work of Dr. Bridgman(3).
Manganin is especially suitable for the measurements of pressure from
50,000 psi up to values that are limited only by the structure used. Its
resistance changes in the order of 1.7 ohms per 100 ohms of wire per 100,000
psi, a value which makes accurate resistance measurements quite simple.
It is well known that manganin changes its resistance slightly with
temperature, and this resistance change is small only over a fairly narrow
temperature range. If the temperature of the fluid departs very much from
room temperature, an appreciable change in coil resistance results, and
this cannot be distinguished from a pressure change. When precision measurements
are attempted on fluids whose pressure has undergone a rapid change, a
fairly long period of time is usually required for the manganin coil to
come to equilibrium. This is due in part, at least, to the temperature
change produced in the liquid by the pressure change. Although the equilibrium
time is a nuisance in laboratory measurements, it can be tolerated, but
this is not the case where high-pressure sensing devices are required for
control of continuous industrial processes.
A great deal of work has been done in Germany on the development of
alloys of gold, silver, and copper, for the purpose of developing resistance
wire that has better temperature-resistance properties than manganin(4).
Of these, alloys containing 2 to 4 per cent chromium in gold have shown
the most promise. The remarkable temperature properties of these alloys
have been investigated at length by the Bureau of Standards with the objective
of using gold chrome as a primary standard of resistance(5).
Material similar to that supplied to the Bureau of Standards was obtained
from Sigmund Cohn and tested at Foxboro. An excellent agreement between
our samples and those reported by the Bureau has been a gratifying check
on the accuracy of our measurement technique. Their probable sensitivity
to pressure also was predicted by the Germans. Experiment has shown that
gold chrome is suitable for measurement of high pressure to values of the
same order as manganin.
Gold–chromium wire has properties which make it particularly suitable
for a pressure–sensing element for industrial purposes. It responds rapidly
to pressure changes, it has a high degree of discrimination between temperature
and pressure effects, it has adequate pressure–resistance sensitivity,
and material with a high degree of uniformity is available.
Temperature Versus Resistance Properties — Experimental Procedure
Manganin
Samples to be tested were either loosely wound on a ceramic bobbin of small
size to facilitate handling, or were self-supporting coils as in the case
of pressure-sensing units. The coils thus wound contained locked-in mechanical
strains which of necessity had to be removed before the coils became stable.
For manganin coils, this stabilizing process was accomplished by the use
of the Bridgman cycle. This consisted of exposing the completed coils to
temperatures of -100°F for a period of at least 2 hours, and then placing
them in an oven at a temperature of +250°F for a period of at least
8 hours. This cycle was recommended by Dr. Bridgman to produce the greatest
stability in manganin wire, and frequently it was found that several such
cycles were necessary before complete stability was attained. Coils were
wound noninductively, since all measurements were made at 1000 cycles,
in order to avoid contact-potential difficulties.
Measurements of the resistance of the coils were made by an automatic
two-function recorder developed at Foxboro for this purpose. The recording
chart was positioned according to the temperature of an oil bath in which
the sample was placed, and the recording pen moved across the chart in
response to resistance changes. The temperature sensitivity of this instrument
had a reproducibility of ½°F, while the resistance measurement
was accurately repeatable to 10 ppm change. Samples were placed in a well-stirred
oil bath, and the temperature of the bath slowly raised through the temperature
interval desired. When the temperature cycle was completed, a continuous
record of resistance changes was obtained. From such records, the accompanying
curves were plotted. Thus several spools of manganin were found which had
exceptionally good temperature characteristics for the construction of
pressure coils.
Gold Chromium
Samples were prepared by either winding the wire on ceramic bobbins or
were formed into self-supporting coils, as in the case of the manganin
samples. It was found that the gold-chrome wire was quite sensitive to
mechanical handling or cold-working. These locked-in strains disappear
with extensive baking at 300°F. Subjecting the wire to temperatures
of -100°F was found to have little effect on its temperature properties,
so the Bridgman cycle, as used in the preparation of manganin coils, was
abandoned in favor of extensive baking at 300°F. Care must be taken
not to expose the wire to temperatures greater than 350°F, as the material
will be many times more sensitive to temperature change after such a treatment.
The gold-chrome wire amalgamates readily with soldier, so care must
be exercised when the soldering technique is used. Spot-welding is recommended.
Experimental Results
The temperature-resistance properties of a typical sample of gold chrome
are shown in Figure 1. This illustrates the effect
upon the temperature properties by extensive baking at 300°F. A temperature
coefficient of from -1 to +1 ppm per degree Fahrenheit can be obtained
readily over a temperature range of 80°F. The sample shown contained
2.1% chromium in gold and was in a dead-soft annealed condition. This appears
to be the combination that has the least sensitivity to temperature.
A comparison of the temperature-resistance properties of gold-chrome,
manganin, and Advance wire shown in Figure 2, all
being plotted to the same scale. The advantages of use of gold chrome over
manganin are at once evident. Advance, which also has an excellent temperature
property, unfortunately has a very small pressure coefficient.
The properties of alloys containing different percentages of chrome
in gold are shown in Figure 3, from which it is
evident that almost any coefficient, positive or negative or nearly zero,
can be obtained by suitable choice of alloy composition and heat-treatment.
Hard-drawn wires of all alloy proportions do not respond to baking procedures,
but always have a large temperature coefficient. For this reason, half-hard
or dead-soft wire is recommended for precision resistors and pressure coils.
Half-hard wire will have a 2 to 4 per cent elongation, while soft wire
will have 15 to 16 per cent elongation on a rupture test.
Pressure Measurements
Experimental Procedure
The material used in the construction of manganin coils was selected on
the basis of uniformity of resistance, and minimum temperature-resistance
properties. The manganin wire was of No. 38 gage, double-silk and double-silk
plus nylon-braid insulation. Coils were wound noninductively, using a small
piece of spaghetti as a starting form, and were interleaved for several
layers. then the coil was lashed securely with strong thread, forming a
firm, self-supporting unit, which could be handled readily for mounting
in pressure cells. although the finished coil appears bulky, its cross
section is mostly porous insulating material, allowing pressure fluids
readily to penetrate the structure. Where a smaller physical size was required,
the nylon braid was omitted, and the coils were wound as before with thin
paper between layers to minimize the possibility of shorts. Coils were
usually of 60 or 120 ohm resistance.
Manganin coils were first stabilized by the use of the Bridgman cycle,
and then exposed to several applications of pressure to a value greater
than that for which they were to be used.
The gold-chrome wire was of No. 36 gage, and was specially covered with
a quadruple layer of nylon. Coils also were wound noninductively, starting
with a spaghetti core, and using a double layer of onionskin paper between
layers to minimize the possibility of shorts and to make a firmer coil.
Lacing of the coil with thread, as before, makes a firmer structure.
The gold-chrome coils are stabilized by baking at 300°F for at least
36 hours. Pressure-seasoning was accomplished by a single application of
pressure.
Finished manganin and gold-chrome coils are shown in Figure
4.
Pressure Equipment
Initial pressure measurements were made with the co-operation of the high-pressure
laboratory of the Watertown Arsenal, where pressures up to 150,000 psi
were available. Measurements were referred to the manganin-coil standards
as calibrated in Dr. Bridgman's laboratory.
Later measurements were made in the Foxboro high-pressure laboratory
with equipment designed and built by D.H. Newhall. A typical pressure-cell
assembly is shown in Figure 5.
Our present high-pressure laboratory was designed and built by the
Harwood Engineering Company. A view of the laboratory is shown in Figure
6. The low-pressure pump, intensifier, controls, and pressure cell
are plainly visible in this view.
In addition to the high-pressure equipment, a dead-weight assembly was
constructed for measurement of pressures up to 30,000 psi (Figure
7). Pressure coils were calibrated with reference to this dead-weight
assembly, and periodic checks were made with the Watertown standard and
Dr. Bridgman's laboratory.
All measurements were made with specially adapted Foxboro instruments
or with specially designed alternating-current bridges. Accuracies of measurement
were in most cases better than 0.1%.
Experimental Results
All measurements on gold chrome were compared to manganin standards. Figure
8 shows the pressure-resistance characteristics of two typical gold-chrome
samples. The two samples had received a different degree of annealing after
the drawing process, and had been baked after bing formed into a coil as
described previously.
The pressure coefficients of 0.716 and 0.673 X 10-7 ohm/ohm/psi
are typical of several samples tested. A remarkable degree of linearity
of resistance change versus pressure was noted and, even more important,
the complete absence of drift, so annoying with manganin. Linearity was
good to less than 0.25%.
The gold-chrome alloy was found to follow rapid changes of pressure,
coming to equilibrium within a second or so. No hysteresis during pressure-cycling
was observed. When first exposed to pressure, the gold-chrome coils showed
an initial zero shift, but in all cases this was well under 1% of the initial
value. Furthermore, all measurable zero shift occured in the first pressure
cycle, as compared with manganin, which often required several cycles before
repeatable results would be obtained.
Figure 9 shows the temperature-resistance properties
of a 2.1% gold-chrome coil which had been pressurized to greater than 100,000
psi several times. It is interesting to note that oneof the effects of
exposure to pressure-cycling is to perform the same type of stabilization
to temperature as accomplished by extended baking of the new coil.
For purposes of comparison, Figure 10 shows
the pressure sensitivity of a Foxboro manganin coil as compared to a typical
gold-chrome coil. The manganin coils had pressure coefficients ranging
from 1.69 to 1.72 X 10-7 ohm/ohm/psi. Therefore it appears that
gold chrome has about 33% of the sensitivity of good-quality manganin,
but its ability to discriminate between pressure changes and temperature
changes more than overweighted this loss in sensitivity.
Conclusion
Recent observations on gold chrome have confirmed these earlier findings
on the linearity and stability of the pressure characteristics. Since 2.1%
gold chrome so nearly fulfills all of the requirements of pressure-sensing
device for industrial-control applications, it is recommended that this
material be considered seriously as a standard for pressure measurements,
replacing the existing manganin standard for application to industrial-control
problems.
Footnotes
(1) The Foxboro Company, Foxboro, Massachusetts.
(2) Harwood Engineering Company, Inc., Walpole, Massachusetts;
formerly with the Foxboro Company. June. ASME.
(3) "The Physics of High Pressure," by P.W. Bridgman, The
MacMillan Company, New York, NY, 1931. Contributed by the Industrial Instruments
and Regulators Division of The American Society of Mechanical Engineers
and presented at The Seventh National Instrument Conference, Cleveland,
Ohio, 9-10 September, 1952. Note: Statements and opinions advanced in papers
are to be understood as individual expressions of their authors and not
those of the Society. Manuscript received at ASME Headquarters, 30 June,
1952. Paper No. 52-HRD-1.
(4) "Werkstoffe für Widerstandsmanometer und Widerstandsthermometer,"
by Alfred Schulze, Chemiker-Zeitung, vol. 19, July, 1943, p. 228.
(5) "Gold Chromium Resistance Alloys," by J.L. Thomas, U.S.
Bureau of Standards, Journal of Research (Research Paper No. 737),
vol. 13, 1934, pp. 681-688.
Harwood Engineering Company, Incorporated – © May, 1999
455 South Street, Walpole, Massachusetts 02081
phone: 508-668-3600
fax: 508-660-2276
http://www.ultranet.com/~harwood/
email: harwood@ma.ultranet.com
