Seal for process pressure to current transmitter

A pressure to current transmitter has an internal fill fluid chamber separated from a process fluid by a diaphragm and backup plate. A fluid-tight seal is achieved between the backup plate and the wall of the fluid chamber by a continuous weld bead while the wall of the chamber is arranged on a flexible cantilever to accommodate differential thermal expansion between the backup plate and the chamber wall.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to a fluid pressure to electrical signal 
transmitter, and more particularly to an improved welded fluid tight seal 
between the diaphragm backup plate and fill fluid chamber. 
2. Description of the Prior Art 
Conventional prior art, pressure to current transmitters have four parts: 
namely, a process head arranged to receive into an internal cavity, a 
process fluid pressure under measurement, a flexible diaphragm and 
diaphragm backup plate used to form a flexible, fluid-tight side of the 
cavity within the process head, a center body having a fluid chamber 
located adjacent to the diaphragm and backup plate and containing a 
captive fluid through which pressure changes in the process fluid under 
measurement are transmitted from the diaphragm to a strain gauge in the 
fluid chamber, and a base portion arranged to protect electrical 
connections to the strain gauge for carrying a transmitter output signal 
to a remote measuring or controlling device. The four pieces of the 
process head are held in a fluid-tight stack by conventional nut and bolt 
connectors. 
A resilient O-ring seal is employed in the aforementioned prior art devices 
between an outer face of the diaphragm backup plate and an outer mating 
face of the fluid chamber to prevent leakage of the captive fluid from the 
fluid chamber while allowing a pressure transmitting motion of the 
diaphragm. 
Such an O-ring seal often suffers from several inherent problems, including 
non-uniform thickness defects, improper assembly producing defective 
seating and corrosion due to the chemical action which takes place between 
the captive fluid and the O-ring. These problems, either singly or in 
combination, can cause the O-ring to loose its proper sealing 
characteristic resulting in an undesired loss of fluid from the fluid 
chamber. This leakage, in turn, reduces the pressure that is applied by 
the captive liquid against its side of the diaphragm whereby the diaphragm 
will then have a tendency to be moved by the pressure of the process fluid 
under measurement into solid engagement with its backup plate. Since the 
diaphragm is then no longer in a flexible state it is not capable of 
accurately transmitting changes in pressure of the process fluid under 
measurement to the strain gauge. 
Additionally, in prior art transmitters the diaphragm and diaphragm backup 
plate are often exposed to the process fluid which necessitates making 
them from a non-corrosive and expensive metal rather than made out of a 
mild steel since some process fluid under measurement can corrode mild 
steel. The use of such corrosion-resistant materials in combination with a 
mild-steel body for the transmitter is a source of further leakage at the 
interface using conventional seals due to the differential thermal 
expansion of such dissimilar materials. 
SUMMARY OF THE INVENTION 
It is an object of the invention to provide an improved pressure to current 
transmitter. 
It is another object of the invention to provide an improved pressure to 
current transmitter having a welded seal for the fluid chamber to insure 
retention of the captive fluid. 
It is a further object of the invention to provide an improved pressure to 
current transmitter having unitary welded fluid chamber and diaphragm 
backup plate construction that allows a proper operation of a diaphragm 
over a wide temperature range. 
In accomplishing these and other objects, there has been provided, in 
accordance with the present invention, a pressure to current transmitter 
having an integral cantilever ring formed on an internal surface thereof 
surrounding the fluid chamber and the diaphragm backup plate. A continuous 
fluid tight welded seam joint is located between the ring and a peripheral 
edge of the diaphragm backup plate. The diaphragm is attached at its 
peripheral edge to the backup plate to form a fluid-tight seal inwardly of 
the welded seam to complete the sealing of the captive fluid in the fluid 
chamber.

DESCRIPTION OF A PREFERRED EMBODIMENT 
Referring now to FIG. 1 there is shown a process pressure to current 
transducer 10 consisting of three major portions, or subassemblies, 
namely, a process head portion 12, a central body portion 14 and a base, 
or end, portion 16 which are held together in a stacked relationship by a 
suitable number of connectors, e.g., bolts 18 and 22 and corresponding 
nuts 20 and 24. 
The head portion 12 has an inner chamber 26 therein that extends through 
the head portion 12 to communicate with both ends of the head portion 12. 
A process fluid supply conduit 28 is connected by a fluid-tight 
connection, e.g., a threaded connection, to a first end of the head 
portion 12. The other end of the head portion is arranged as a cylindrical 
abutment surrounding the chamber 26 and in contact with the body portion 
14. The open end of the chamber 26 is sealed by a convoluted disc-shaped 
diaphragm 30 which is attached along its circumference by a suitable 
method, e.g., electron-beam welding, to a disc-shaped backup plate 32. The 
backup plate 32 and diaphragm 30 are generally made of a relatively 
expensive corrosive resistant metal, such as Monel, stainless steel, etc. 
while the body portion 14, is usually made of a cheaper material, e.g., 
mild steel. These corrosion resistant metals tend to expand, when they 
experience an increase in ambient temperature, at a faster rate than the 
body portion 14 and tend to contract at a faster rate than the body 
portion 14 when they experience a decrease in ambient temperature. On the 
other hand, in some applications, the corrosive action of the particular 
process fluid may be such that it is necessary to use other types of 
corrosive resistant material, such as Hastalloy for the diaphragm 30 and 
backup plate 32. Such materials expand and contract at a slower rate than 
the body portion 14 in response to increases and decreases in ambient 
temperature. 
A process fluid 34 is supplied under pressure through conduit 28 to the 
chamber 26 where it is applied to one side of the diaphragm 30. 
An annular fluid-tight seal 36 is retained in an annular recess 38 in the 
face cylindrical abutment of the head portion 12. The seal 36 is arranged 
to contact the backup plate 32 outwardly of the edge of the diaphragm 30. 
The diaphragm backup plate 32 has an radially extending annular lip 40 to 
form the outer peripheral surface thereof. The lip 40 mates with a step, 
or recess, 42 of a cantilever annular ring 44 extending outwardly from a 
face of the body portion 14. Thus, the ring 44 is arranged to surround the 
outer edge of the backup plate 32. 
Referring now to FIG. 2 for more detail, the lip 40 of backup plate 32 is 
connected to the recess 42 of the ring 44 by means of a suitable seam weld 
bead 46. The periphery 47 of the disc shaped backup plate 32 below the lip 
40 is a tapered surface extending away from an inner wall 48 below the 
recess 42 in the ring 44. The taper of the periphery 47 is arranged to 
progressively separate the periphery 47 from the wall 48 with the point of 
closest approach being located at the bottom of the recess 42 in the ring 
44. This point can be either an actual contact to provide press fit 
between the plate 32 and the ring 44 or a small separation to provide a 
slip fit between the plate 32 and the ring 44. The step 42 is provided 
beneath the weld bead 46 to prevent any extraneous weld material from 
entering the tapered space between backup plate 32 and the wall 48 which 
would prevent proper operation of the present invention, as described 
hereinafter. The center of the body 14 is arranged to form a hollow 
chamber below the backup plate 32 with an inwardly extending step 51 
located at the other end of the wall 48 from the recess 42. The step 51 is 
arranged to extend a short distance beneath an inner face of the backup 
plate 32 to provide a support for the backup plate 32. The remainder of 
the inner face of the plate 32 is exposed hollow chamber formed in the 
interior of the body 14. The cantilever ring 44 may be formed by machining 
out an annular groove 50 behind the ring 44 in the upper surface of the 
body portion 14. 
The diaphragm backup plate 32 has a fluid passageway 52 extending 
transversely between the inner face of the backup plate 32 and an outer 
face located beneath the diaphragm 30. The outer face of backup plate 32 
has a convoluted surface 54 similar to the convolutions of the diaphragm 
30 in order to provide an approximately uniform separation from the 
diaphragm 30 to achieve a desired maximum movement of the diaphragm 30 
without physical contact with the backup plate 32. 
A pressure sensor 56, which may be silicon material into which has been 
diffused a resistive Wheatstone bridge pattern 58, is mounted on an 
refractory chassis 60 positioned beneath the backup plate 30 within the 
body 14. The chassis 60 is retained in a fixed position by any suitable 
means on a wall 62 that surrounds the chassis 60 and which forms a 
fluid-tight end for the chamber within the body 14. An electrically 
nonconductive liquid 64 of a preselected volume is used to fill the space 
between the diaphragm 30 and the convoluted surface 54, the space in the 
passageway 52 and the space between the inner face of the diaphragm backup 
plate 32 and the hollow chamber within the body 14. The chassis 60 may 
have its underside vented to the atmosphere by passageway 68 to allow the 
elements of the bridge pattern to respond to changes in pressure of the 
liquid 64. Electrical connections to the bridge pattern 58 are made by a 
terminal 72 and fine electrical conductors 74. The conductors 74 are 
connected to pin terminals 76 of feedthrough header 78 forming a 
fluid-tight seal with the wall 62. 
External electrical leads 80 which may be in the form of a flexible cable 
82 are connected to the lower multipin connections 84 of the feedthrough 
header 78 through an insulator 86 and the hollow base portion 16. 
MODE OF OPERATION 
PRESSURE TRANSMITTER OPERATION UNDER A NORMAL AMBIENT TEMPERATURE CONDITION 
A process fluid 34 whose pressure is to be transduced into an equivalent 
electrical output signal for transmission is applied by way of conduit 28 
and the chamber 26 in the head portion 12 to the outer side of the 
diaphragm 30. An increase in the pressure of the process fluid 34 causes 
the diaphragm 30 to move toward the convoluted surface 54 of the diaphragm 
backup plate 32 and thereby affect a compression of the nonconductive fill 
liquid 64 against the pressure sensor 56 and vice versa. The bridge 58 of 
the pressure sensor 56 generates an electrical output signal whose 
magnitude is proportional to the magnitude of the pressure of the process 
fluid under measurement. The electrical signal is then transmitted by 
conductors 70, the conductor leads 74, the pins 76, header 78, pins 84 and 
by the electrical conductors in the flexible cable 82 to a utilization 
device which will indicate the pressure of the process fluid 34. 
When the process fluid is an acid, a caustic (alkaline) or some other 
corrosive fluid of strong concentration it has a tendency to destroy 
ordinary metals by corrosion, and it is then necessary to make the metal 
parts of the transmitter, i.e., especially the diaphragm 30 and backup 
plate, that contacts such active fluids of corrosive resistant materials, 
e.g., Monel, stainless steel, etc. Such materials have a different rate of 
thermal expansion and contraction than the material, e.g., mild steel, 
from which the body portion 14 and its associated integral cantilever ring 
44 is constructed. The diaphragm 30 and the backup plate 32 are usually 
made of the same material to avoid temperature induced motions 
therebetween. In order to avoid a leakage of the fill fluid 64 in a 
structure combining such different materials, the transmitter of the 
present invention is arranged to accommodate a differential temperature 
expansion between the backup plate 32 and diaphragm 30 unit and the body 
14 of the transmitter 10. Specifically, the backup plate 32 is welded to 
an integral flexible ring 44 in the body 14 whereby a fluid-tight seal is 
formed by the weld bead 46 while allowing a flexible connection to be 
maintained by the flexing of the ring 44. 
Under normal ambient temperature operating conditions, as illustrated in 
FIG. 2, no stresses will be introduced into the welded bead 46 that is 
formed between the ring 44 and the backup plate 32. In other words, under 
these normal ambient temperature operating conditions only a very slight 
difference in expansion will be exhibited between the different materials 
used for the backup plate 32 and the body portion 14. The cantilever ring 
44 will, therefore, be maintained in a substantially straight 
configuration. Since the cantilever ring 44 normally extends straight up 
from the body portion 14 with which it forms an integral part, no stress 
will be introduced into the weld bead 44. 
PRESSURE TRANSMITTER OPERATION UNDER A HIGH AMBIENT TEMPERATURE 
During an increase in the ambient temperature of the transmitter 10 from 
the previously described normal ambient temperature condition to a higher 
ambient temperature condition the metal from which the highly corrosive 
resistent backup plate 32 is made may expand at a faster rate in a radial 
direction than the mild steel material from which the body part 14 and 
ring 44 is constructed. The tapered edge of the plate 32 will accommodate 
the radial movement of the plate 32 and aid in forcing a bending of the 
ring 44 by concentrating the stress at the weld bead 46. Under such 
conditions, the flexibility of the cantilever ring 44 allows the backup 
plate 32, the weld 46 and the ring 44 to move radially outward as a single 
unit from the position as shown in FIG. 2 to the position shown in FIG. 3 
when these parts experience an increase in ambient temperature. The welded 
end of the ring 44 will, thus, be forced outwardly in a cantilever fashion 
to its bent position as shown in exaggerated form in FIG. 3 due to the 
resulting difference in rate of expansion between the backup plate 32 and 
the body 14. 
If the corrosive resistant material that is selected for the backup plate 
32 is made of a material having a lower coefficient of thermal expansion, 
an increase in ambient temperature will cause the plate 32 to expand 
outward at a slower rate than the body portion 14 to which the ring 44 is 
attached. This will cause the welded end of the ring 44 to be canted 
inwardly to the position shown in exaggerated form in FIG. 4 since the 
weld bead 46 follows the motion of the backup plate 32. 
PRESSURE TRANSMITTER OPERATION UNDER A LOW AMBIENT TEMPERATURE CONDITION 
During a decrease in the ambient temperature of the transmitter from its 
previous described normal ambient temperature condition to a lower ambient 
temperature condition and assuming the metal from which the backup plate 
32 is made shrinks at a faster rate in a radial direction than the 
material from which the body portion 14 is constructed, the flexibility of 
the cantilever ring 44 allows the backup plate 32, the weld bead 46 and 
the ring 44 to move radially inward as a single unit from the position 
shown in FIG. 2 to the position shown in FIG. 4. The nonattached end of 
the ring 44 will thus be pulled inwardly in a cantilever fashion to its 
bent position as shown in FIG. 4 due to the resulting difference in 
coefficient of thermal expansion which occurs during the decrease in 
ambient temperature of the ring 44 and the backup plate 32. 
If the material selected for the backup plate 32 has a smaller coefficient 
of thermal expansion than the body 14, then a decrease in ambient 
temperature will cause the backup plate 32, weld bead 46 and plate 32 to 
move radially outward as a single unit in a cantilever fashion to the 
position shown in FIG. 3. This is due to the fact that a greater rate of 
contraction of the body portion 14 takes place than the rate at which the 
backup plate 32 is being contracted. 
In summary, it can be seen that there has been provided by the present 
invention an improved seal for retaining a fill liquid in the fill 
chamber, or body portion, of a pressure to current transmitter while 
accommodating differential expansion between differing materials used in 
the fluid pressure retaining elements of the transmitter.