Harwood
Engineering Company, Inc.
Harwood
Engineering Company, Inc.
Manganin Pressure
GagesManganin as a pressure sensor was brought into prominence by Bridgman1, following the suggestion of Lisell2. Bridgman calibrated manganin gage coils directly3 and indirectly against a free piston primary-gage4, and employed the calibrated coils for nearly all of his subsequent high-pressure experimentation. His example has been widely followed5.
Manganin is an alloy composed approximately of 84 parts copper, 12 manganese and 4 nickel. It has long been used in the better grades of electrical instruments because of its favorable mechanical properties and low thermal-EMF against copper. Within the usual range of room temperature, there are few materials whose resistivities are less affected by temperature change.
Its value as a pressure sensor lies in the fact that, under pressure, its resistivity is appreciably increased, and the increase is nearly proportional to the applied pressure. The pressure coefficient of the manganin currently used in pressure cells by the Harwood Engineering Company is approximately 1.65 x 10-7 ohm per ohm per PSI (2.39 x 10-6 ohm/ohm/kP).
The behavior of manganin varies with its composition and its history. It is supplied by several manufacturers in U.S.A. and abroad, but it is impossible to procure a supply having a guaranteed pressure-resistance relation. Bridgman was fortunate in having at hand some German manganin which the Jefferson Physical Laboratory had acquired, probably before 1900, and which appears to have been particularly well-seasoned. In later years he bought spools of wire from two American manufacturers; one spool he found unsatisfactory, the other he used for his experiments to 30,000 Kg/cm2.
The pressure coefficient is affected by temperature. Compared with the
value at 25°C, Birch and Roberson18 reported an increase
of 0.2% at 50°, 0.8% at 75°, 1.7% at 100° and 2.8% at 125°;
others5,19 are in rough agreement, but suggest wide variation
of the effect according to the particular material used. The use of manganin
as a pressure sensor has been a matter of interest at the National Bureau
of Standards, and an attempt is being made to establish optimum specifications
for material to be used for this purpose6.
It is important that the structure be free of strain. Otherwise, variations
of 2% or 3% in the pressure coefficient may result5,7. The winding
process can be expected to impose some strain on the coil. The preferred
way to overcome this is to subject the coil to extremes of temperature,
alternatively -140° or as much as the insulation will stand, for several
hours and then "dry-ice" temperature (or better, liquid air or nitrogen).
Repetition may continue as much as a week. The coil is then pressure-seasoned
by exposure well above the full use range (Harwood pressure-seasons at
30 kb). One application will probably be sufficient, but occasional coils
may require more — if after a pressure run the coil does not return to
its initial resistance, thus indicating an unstable zero.
Both types of cell are provided with a reference coil of manganin, having the same resistance as the active coil — 120 ohms ± 1/10 ohm — and set within a protective cap attached to the outer end of the closure. Its temperature matches that of the active coil as closely as possible. In contrast to this, since tubing for pressures above 200,000 PSI (1400 MP) is not available, it is necessary in the case of the 30-kilobar apparatus to place the gage coil in the same pressure vessel that contains the furnace, tensile jig or other experiment device.
The connection of the manganin wire to the coil leads is a matter of considerable delicacy. Harwood practice, following Bridgman, is to ground one end of the coil at the inner end of the closure and connect the other end to a lead passing through the closure. In view of possible future disconnections and re-connections it is convenient to attach short lengths of copper wire, about ½ inch long, permanently to the coil ends. Shifting of the coil, as from one closure to another, then can be made without changing the resistance of the coil.
There is some doubt as to the best type of connection of the copper to the manganin. Spot-welding may be the best, but to produce a secure joint with confidence requires artistry. Hard solder should give a good result, but special equipment and considerable care are necessary as there is danger of over-heating the wire. Probably the safest method is to use rather soft solder, but one must make the joint as small as possible — be meticulous in trimming the ends and apply only as much heat and solder as required.
Gages at the National Bureau of Standards are provided with four leads
in order that lead resistance may be eliminated. This imposes the use of
two more leads through the closure, with consequent greater chance for
leak, as well as more complex instrumentation for measuring resistance
change. The standard Harwood coils are 120 ohms and, since the lead resistance
is 1/100 ohm or less, the added refinement appears
unjustified for most work.
The Carey-Foster
bridge is not popularly well-known, but has long been used for measuring
small differences of resistance between two resistors.8 Almost
from the beginning of his experiments with high pressure, Bridgman used
such a bridge in connection with his manganin gages. Such a bridge, as
used by Bridgman, is shown in Figure 1. The gage
coil is represented by X and its resistance by x. C is a manganin coil
of nearly the same resistance, c, as X, mounted outside of the pressure
cell, at atmospheric pressure. It might be called the reference coil. R1
and R2 are ratio coils, each approximately 120 ohms. W is a
slidewire, 100 cm long, of bare manganin of about 20 mil diameter, stretched
over a good meter stick. S is a shunt selected to give the desired effective
resistivity to the slidewire. B is a voltage source consisting of 2 dry
cells. RB is a current-control rheostat and KB a
contactor. G is a sensitive galvanometer. RG is a protective
rheostat, and KG a contactor. A slider-contactor can be moved
along the slidewire to balance the bridge.
The essential characteristic of a Carey-Foster bridge is that if the bridge is balanced with the contactor at some point r (identified as centimeters on the meter scale), and if coils C and X are then interchanged by a suitable switch, with a new balance point (r'), the difference between c and x is simply
In practice, Bridgman balanced the bridge at atmospheric pressure and
at the fixed point (the freezing pressure, 7640 kg/cm2**, of
mercury at 0°C) and computed the slider displacement for a pressure
difference of 1000 kg/cm2, or about 4½ cm. A scale then
was constructed with divisions corresponding to 1000 kg/cm2,
and 500 kg/cm2 subintervals. This was laid beside the meter
scale and was a convenient index by which approximate pressure could be
read at a glance for any bridge balance. A pressure setting was measured
as:
K[(r1-r0)
+ (r0'-r1')]
p = ----------------------
2
where p was the pressure in kg/cm2 and K was the calibrated
constant (kg/cm2 per centimeter displacement of the slider)
determined by the mercury fixed point experiment.
The type "S" Direct Reading Bridge (Figure 2) is intended for routine measurement of pressure. The active and reference coils are Harwood cells (described above) connected as shown. The four arms of the bridge are then nearly equal. The slidewire is a 1000-ohm, 1000-division potentiometer of high resolution, and the matching coil is trimmed to match it precisely. By the use of a 1-ohm calibrating coil and range-adjusting shunts, full scale of the slidewire can be made to coincide with full scale of the pressure range, normally either 100,000 or 200,000 PSI (700 or 1400 MP). In operation, with the cell at atmospheric pressure and the slidewire contactor set at zero, the bridge is balanced by means of the ZERO ADJUST. When pressure is applied, the bridge is again balanced by means of the slidewire, which gives a direct reading of the pressure in thousandths of the selected pressure range. The bridge operates with 3 No. 6 dry cells. Appropriate current controls are provided. For establishing bridge balance, a sensitive galvanometer (about ½ microvolt per millimeter deflection at 1 meter) can be used; preferably, a guarded DC null detector having a maximum sensitivity of 1/10 microvolt per scale division is recommended.
The Harwood type "G" bridge has practically the same components as the
type "S" but, in addition, provision is made for short-circuiting the slidewire
matching coil and for interchanging the reference and active for true Carey-Foster
operation. Additional calibrating coils are provided, enabling the operator
to check the resistivity of the slidewire or the resistance of a new gage
coil without resort to external means. Pressure can be measured in the
manner of the type "S" bridge or by the Carey-Foster method. The latter
should be twice as accurate because it has the advantage of two slidewire
displacements rather than one. Used in this way, the bridge should be good
to better than 0.1%.
For use as a pressure gage, the pressure coefficient of resistance,
or pressure factor, of the particular coil must be precisely known. As
previously stated, one has no assurance that the coefficient will be the
same for any two spools. One can expect uniformity within a particular
melt, but there may be slight difference in tension as the wire is wound
onto different spools, and it has recently been pointed out9
that strain may vary between the inner and outer layers of a spool. Differences
due to strain may presumably be ironed out in the seasoning process, but
this would not be true regarding variation among various melts. It is evident
that, for precise work, coils should be individually calibrated after seasoning.
Experiments at Harwood using a number of coils from a single spool indicate that, while there is some slight variation of pressure resistivity relation among the coils, the deviation from linearity is nearly uniform among them. A typical characteristic is shown in Figure 3. The upper curve, for the cell factor, was obtained by subjecting the coil to a series of pressures established by a controlled clearance free piston gage, and plotting the resistance response.
The lower curve shows the corrections to be applied if the cell factor found at 100,000 PSI is used over the whole range (0-200,000). The cell factor determined at any particular pressure may vary as much as perhaps 1% among coils from the same spool but, with this single determination, the curve of corrections should permit the accurate specification of pressure by resistance readings taken throughout the range. This is particularly fortunate because, lacking a free piston gage, many laboratories depend on a single fixed point, probably the freezing point of mercury at 0°C, for the calibration of their coils. Data is lacking for manganin from other sources, though one experimenter reports the use of gage coils having 3 times the slope shown in Figure 3.14
With present equipment at Harwood Engineering Co., correction curves (Figure 3) cannot be made for pressures greater than 200,000 PSI. Two fixed points are now well established — the mercury point mentioned and the transition pressure of BiI-II, which is has been investigated by several4, 11, 12, 13, most recently (and probably most authoritatively) by Heydemann.15 These two points should define the curvature of the resistivity curve. If further refinement is necessary, one might also use the freezing pressure of mercury at elevated temperatures,16, 17 and other satisfactory references will doubtless be established. If available, a free piston gage will remain the most trustworthy means for calibration.
With respect to this deviation from linearity, it is to be noted that
by special circuitry, and by special arrangement of the pen linkage, the
response of the Foxboro Dynalog to pressure can be made more nearly linear.
Unfortunately, this treatment cannot anticipate the employment of manganin
from a different melt.
