Pressure transducer

A pressure transducer of the manometer type comprises two interconnected columns of liquid with means for applying differential pressure across the columns. The liquid is a magnetic liquid and a magnetic field is impressed on the columns to attract the liquid by magnetic force to a reference position. The applied differential pressure acts counter to the magnetic force to shift the columns so that their difference in lengths reflects the pressure. By using capacitor plates to define the walls for the liquid columns and a magnetic liquid which is a dielectric, the capacitance between the plates is a function of the column lengths and therefore is a function of the applied differential pressure.

This invention relates to a pressure transducer and particularly to a 
pressure transducer having a magnetic liquid in a magnetic field 
responding to changes in differential pressure. 
It is often required to use pressure transducers to obtain electrical 
measurements proportional to or representing a pressure occurring in a 
fluid such as air. Such measurements are used not only for scientific 
measurements but also to obtain data for the control of industrial systems 
or control for machines. Many types of pressure transducers are known, 
however, they are mainly applicable to high pressure ranges. Manometer 
types of pressure sensors are useful for measurement of small pressure 
differentials, however, for making pressure measurements over a large 
range of pressures the traditional manometer becomes very large. It is 
desirable to obtain the sensitivity and the large pressure range of a 
manometer in an instrument of small dimension. 
It is a general object of the invention, therefore, to provide a transducer 
for measuring small pressure differentials with the design potential of a 
large pressure range and having small dimensions. It is a further object 
to provide such an instrument not requiring a vertical orientation or a 
stable orientation. 
The invention is carried out by providing in a magnetic field a slug of 
magnetic liquid in a passage such that the magnetic field biases the slug 
of liquid toward a reference position and pressure applied at ports in the 
passage displace the slug of liquid from a reference position against the 
force of the magnetic field, and an electrical sensor measuring the 
displacement of the slug for varying an electrical parameter representing 
the pressure differential across the slug. The invention further 
contemplates a passage shaped to provide two parallel columns of the 
magnetic liquid and a magnet positioned to equalize the column lengths in 
the absence of the pressure differential.

The pressure sensor of FIG. 1 comprises a hollow cylindrical housing 10 of 
insulating material closed at one end which has a base flange 12 and open 
at the other end which has a flange 14. A metallic tubular sleeve 16 is 
disposed in the hollow housing 10 and is spaced from the inner wall 
thereof to form a first annular chamber 18 between the housing and the 
sleeve 16. One end of the sleeve 16 is slightly spaced from the closed end 
of the housing 10 while the other end of the sleeve has a flange 20 
engaging and seating against the housing flange 14 thereby providing a 
support of the sleeve 16 within the housing. A conductive rod 22 within 
sleeve 16 and concentric therewith is spaced from the sleeve to form a 
second annular chamber 24. One end of the rod 22 is seated against the 
closed end of the housing while the other end extends beyond the flange 20 
of the sleeve where it is centered by an annulus 26 of insulating material 
which in turn seats within a central bore 28 in the flange 20. An 
insulating cap 30 seats on the flange 20 and is secured by bolts 32 to the 
housing 10. The cap has a central inner recess 34 which receives an end of 
the rod 22 and a compression spring 36 which engages the inner surface of 
the recess and a shoulder on the rod 22 to hold the rod against the closed 
end of the housing 10. A port 38 extending through the side of the cap 30 
communicates with the recess 34 and a passage 40 in the annulus 26 
provides communication between the recess and the chamber 24 so that 
pressure applied to the port 38 is also applied to the chamber 24. Another 
port 42 in the side of the housing 10 near the flange 14 communicates 
directly with the chamber 18. External lead wires 44 and 46 connect to the 
sleeve 16 and the rod 22 respectively. 
The chambers 18 and 24 contain a slug of magnetic liquid 48 which each 
chamber forms into a column, the two columns being interconnected by the 
space between the end of the sleeve 16 and the inner end surface of the 
housing 10. A magnet 50 seats against the flange 12 of the housing 10 to 
establish a magnetic field in a direction to attract the slug of magnetic 
liquid 48 toward the magnet so that in the absence of any applied pressure 
differential the two columns of liquid in the chambers 18 and 24, 
respectively, will be of equal lengths. When pressure differential is 
applied to the ports 38 and 42 the magnetic liquid will flow from one 
column to another so that the difference in column lengths represents the 
measure of the applied pressure differential. Thus the sensor is analogous 
to a conventional manometer which relies upon gravity to bias the column 
lengths toward equality, however the conventional manometer must be held 
with the columns in an upright position as dictated by the direction of 
gravitational force. In the sensor according to the invention, however, 
the magnetic liquid and the magnet strength are so selected that the force 
on the liquid is several times stronger than that which is due to gravity 
so that the sensor need not be restricted to any particular orientation. 
It can be utilized with the columns disposed horizontally or even "upside 
down" with respect to a conventional manometer setting. 
The magnetic liquid, also called a ferrofluid, comprises an oil-like 
synthetic liquid called a diester or diester-based solvent used as a 
carrier in which particles of magnetite are suspended. The particles 
having a size of approximately 100 angstrom units are held in permanent 
suspension by the molecular motion of the diester medium. The magnetite 
particles provide the liquid with magnetic properties so that an 
externally imposed magnetic field will apply a force on the liquid. 
This magnetic liquid is also a dielectric and the dielectric properties are 
used for the electrical measurement of the column length. The rod 22 and 
the concentric sleeve 16 spaced therefrom comprise two plates of a 
capacitor and the column of magnetic liquid is the dielectric medium 
between the plates which varies in extent according to the applied 
pressure differential so that the capacitance of the capacitor thus formed 
is dependent upon the applied pressure. The leads 44 and 46 are 
conveniently attached to circuitry for measuring the capacitance or for 
otherwise utilizing the pressure information. 
By using a strong magnet and by optimizing the magnetic field by 
appropriate magnetic circuit design a given column length of the magnetic 
liquid can represent a pressure differential which is much larger than 
that ordinarily obtained by a conventional manometer. In other words a 
small instrument with a short column length can measure pressure ranges 
equivalent to that of much longer manometers. Moreover the source of the 
magnetic field is not limited to permanent magnets but electromagnets or 
solenoids are useful as well. In that case, the applied current can be 
adjusted to select a desired field strength to obtain a given pressure 
range for a particular instrument. In an instrument constructed according 
to the FIG. 1 configuration the gap between the rod 22 and the sleeve 16 
is on the order of 1/2 to 1 mm and the maximum column length was about 45 
mm so that throughout the range of the instrument the capacitance varied 
from 60 pF to 150 pF. This covered the pressure differential range of 0-90 
mm of water. The gap between the housing 10 and the sleeve 16 is made 
large relative to the gap for the column being measured so that the level 
of liquid in chamber 18 has relatively small variation over the range of 
the instrument. In that case the column of liquid in the chamber 18 serves 
primarily as a reservoir supplying the liquid required for the measured 
column in the chamber 24. 
Another embodiment of the invention as shown in FIG. 2 comprises a housing 
60 of insulating material which is hollow and has on its inner surface a 
cylindrical metallic liner 62 and contains at its center a metallic rod 64 
spaced from the liner 62. A cylindrical metallic sleeve 66 is spaced 
mid-way between the liner and the rod and spaced from an end of the 
housing thereby defining a pair of concentric chambers 68 and 70 of 
substantially equal volume. As in FIG. 1, a body of magnetic liquid 72 
forms a pair of columns interconnected at one end of the sleeve 66 and a 
magnet 74 in one end of the housing provides a field for attracting the 
magnetic liquid toward the magnet. The chambers 68 and 70 are connected to 
pressure input ports 76 and 78, respectively. In this configuration the 
liner 62 and the sleeve 66 form one capacitor while the sleeve 66 and the 
rod 64 form a second capacitor, the liner 66, of course, being a common 
plate for both capacitors as well as forming a common wall for both 
chambers 68 and 70. As pressure is applied to the columns to shift the 
liquid from one chamber to another one capacitance increases while the 
other decreases. Conveniently, these two capacitors are connected by 
external leads 80 to differential oscillator circuitry 82. The two 
capacitors thus form two arms of a bridge arrangement to improve thermal 
stability and reduce the effects of component and material aging as well 
as decreasing sensitivity to electromagnetic interference. 
While the preferred embodiments of the invention have been described 
herein, the principles of the invention apply to other configurations. The 
principle features are that a magnetic field is provided to maintain a 
body of magnetic liquid in a reference position when no pressure is 
applied and means for applying pressure differentials across the body of 
liquid to displace it by an amount depending on the magnitude of the 
pressure differential, and finally, means for electrically sensing the 
position of the body of liquid to provide an output signal representing 
the pressure differential. While a capacitive transducer has been 
described an inductive transducer may be used as well. That is, the 
inductance of a coil disposed around a column of magnetic liquid will vary 
according to the position of the magnetic liquid in the coil. Further, the 
configuration of the pressure transducer is not limited to the concentric 
annular columns as illustrated herein but may equally well be applied to a 
U-shaped manometer or indeed may apply to a straight tube disposed 
horizontally containing a slug of magnetic liquid, pressure input ports at 
each end, a solenoid disposed around the tube for applying a magnetic 
field which urges the slug of liquid toward the center of the solenoid, 
and an electrical sensor for measuring the displacement of the slug of 
liquid caused by applied pressure. 
It will be apparent that the pressure transducer according to the invention 
uses the principles of the traditional manometer but has the advantage of 
smaller dimensions for a given pressure range and it is not limited to a 
particular orientation.