A pressure sensor comprising a main body having an inlet connectable to a source of liquid under pressure, an outlet, a passage extending through the main body from the inlet to the outlet and a surface outside of the passage. A flexible diaphragm confronts the surface and is coupled to the main body to at least partially define a sensing chamber. The main body has a port extending from the passage to the sensing chamber to provide communication between liquid under pressure in the passage and the sensing chamber. The port has a transverse cross section with at least one dimension which is no greater than about 0.060 inch.

BACKGROUND OF THE INVENTION 
Various parenteral liquids are administered to a patient under a positive 
pressure through an administration set. The positive pressure may be 
generated by gravity flow or by an appropriate pump. It is often necessary 
or desirable to monitor the pressure under which the fluids are delivered 
to the patient. This can be accomplished, for example, with a diaphragm or 
membrane type pressure sensor and a pressure transducer. 
One problem with the sensing of pressure in an administration set is that 
the pressure sensor should not be allowed to trap any gas or bubbles that 
may exist in the liquid. If bubbles were trapped and, subsequently 
released and infused into the cardiovascular system of the patient, the 
results could be fatal to the patient. 
One technique for pressure monitoring is to place a diaphragm over the top 
of an elongated channel through which the liquid flows and devices of this 
type are shown, for example, in German Pat. No. 888,933 and Cunningham et 
al U.S. Pat. No. 4,398,542. According to Cunningham et al, this approach 
does not trap air in the pressure sensor. 
Another approach, which is structurally simpler, is to simply tap into the 
conduit of the administration set using a Tee to couple the pressure 
sensor to the conduit. However, this approach makes it difficult to purge 
air from the system and creates a likelihood of air entrapment in the Tee 
connection or in the pressure sensor. 
SUMMARY OF THE INVENTION 
This invention employs a pressure sensor which is more akin to the Tee 
connection approach, but does so in a manner that facilitates purging of 
air from the system and which minimizes the likelihood of air or bubble 
entrapment in the pressure sensor. With this invention, a port leads from 
a flow passage in a main body to a pressure sensing chamber. The port is 
constructed in a way so as to substantially prevent the passage of bubbles 
into the sensing chamber. 
In a preferred construction, the pressure sensor includes a main body 
having an inlet connectable to a source of liquid under pressure, an 
outlet, and a flow passage extending through the main body from the inlet 
to the outlet so that liquid can flow through the passage. Means including 
a diaphragm coupled to the main body define a sensing chamber. The 
pressure sensor also includes means defining a port within the main body 
which extends from the passage to the sensing chamber to provide 
communication between the liquid under pressure in the passage and the 
sensing chamber. 
The invention employs a number of features substantially preventing the 
sensing chamber from serving as a bubble trap. This is accomplished 
primarily by limiting at least one dimension of the transverse 
cross-section of the port to a dimension which enables the surface tension 
to substantially exclude bubbles for the viscosity and pressure of 
interest. For parenteral liquids, this dimension is preferably no more 
than about 0.060 inch. When so limited, the surface tension substantially 
excludes bubbles from passing through the port for the liquid viscosity 
levels and pressures of interest in the parenteral administration of 
liquids to a patient. Parenteral fluids normally have a viscosity no less 
than the viscosity of water and are delivered at pressures no greater than 
about 22 psi. Preferably the port has a lesser cross-sectional area than 
the cross-sectional area of the passage. The port may be circular, 
rectangular, irregular or of virtually any configuration provided that the 
dimensional parameters of the invention are followed. 
The pressure sensing chamber senses only static pressure, and so 
theoretically, the controlled dimension of the port does not have a 
minimum. However, to facilitate manufacture, minimize the danger of 
plugging and to eliminate unfavorable time delays in sensing pressure 
changes, the controlled dimension of the port is preferably no less than 
0.020 inch. Furthermore, if the port is to be formed in a molding 
operation, it is preferred that the controlled dimension be no less than 
0.030 inch. 
To further minimize the likelihood of the transmission of bubbles from the 
flow passage through the port to the sensing chamber, the port preferably 
has sharp edges with essentially no radius at the location where the 
controlled dimension of the port exists. Preferably, the sharp edges are 
at the juncture of the port with the flow passage. 
To facilitate priming, the length of the port should be minimized. The 
length of the port preferably does not exceed 2.5 times the controlled 
dimension of the port or 0.150 inch maximum. By limiting the length of the 
port, the height of the column of air that must be initially purged from 
the pressure sensor is reduced. 
The pressure sensing chamber is also preferably of minimum volume and may 
have a volume of 0 when there is no pressure differential across the 
diaphragm. However, to facilitate purging, the sensing chamber preferably 
has a conical surface into which the diaphragm can be deflected with thumb 
pressure to eject air from the pressure sensor during purging by a 
reciprocating manual pumping action. Thus, the conical surface facilitates 
air elimination from the system during purging. 
Prior to purging, there is air in the administration set and in the sensing 
chamber. To facilitate removal of the air from the sensing chamber, the 
port at the location where it meets the conical surface or other 
corresponding surface preferably has a radiused edge. 
The main body is preferably integrally molded from a suitable plastic 
material and includes a surface outside of the passage which the flexible 
diaphragm confronts to define the sensing chamber. The main body can 
advantageously be mounted on external supporting structure, such as a 
cassette frame. To accomplish this, the main body preferably has a lug 
integral therewith partly defining a slot for use in coupling the main 
body to such external supporting structure. 
The invention, together with additional features and advantages thereof may 
best be understood by reference to the following description taken in 
connection with the accompanying illustrative drawing.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
FIGS. 1-3 show a pressure sensor 11 which comprises a main body 13 and a 
flexible diaphragm or membrane 15. The pressure sensor 11 is adapted to be 
used with a pressure transducer 17, which may be of conventional 
construction and which is shown in broken lines in FIG. 2. 
Although the main body 13 can be constructed of metal, it is preferably 
integrally molded from a suitable plastic material, such as PVC. The main 
body 13 includes an inlet 19, an outlet 21 and a generally cylindrical 
flow passage 23 extending linearly between the inlet and the outlet. The 
inlet 19 is adapted for connection to a source of parenteral liquid under 
pressure. The main body 13 has a conical surface 25 and a port 27 leading 
from the passage 23 to the apex of the conical surface 25. The main body 
13 has a lug 29 which forms one side of a slot 31 for use in mounting the 
pressure sensor 11 as described hereinbelow with reference to FIGS. 4 and 
5. 
The main body 13 has an annular flat surface 33 surrounding a shoulder 35 
on its exterior. The diaphragm 15, which is circular in the embodiment 
illustrated, has an annular peripheral zone ultrasonically bonded to the 
annular surface 33. The conical surface 25 cooperates with the diaphragm 
15 to form a sensing chamber 37 of very small volume which communicates 
with the passage 23 via the port 27. Accordingly, the static pressure 
within the sensing chamber 37 is approximately equal to the static 
pressure within the passage 23. The static pressure of the liquid in the 
chamber 37 can deflect the diaphragm 15, and these deflections are sensed 
by the pressure transducer 17 in a conventional manner to provide an 
indication of the static pressure of the liquid under pressure within the 
passage 23. 
Although the port 27 could pass through a separate member in addition to, 
or in lieu of, the main body 13, manufacture is facilitated if the port 
extends through the main body. Although the port 27 could be of various 
different configurations, in this embodiment, it is cylindrical. 
To exclude bubbles or air from passing from the passage 23 through the port 
27 into the sensing chamber 37, the port 27 has a diameter of no more than 
about 0.060 inch. The preferred minimum diameter of the port 27 may be 
0.020 inch or 0.030 inch as discussed above. Although this diameter could 
exist at various locations along the length of the port, in this 
embodiment it exists at the juncture of the port 27 with the passage 23 
and for substantially the full length of the port. To further tend to 
exclude air, the port 27 has a sharp circular edge 39 (FIG. 2a) with 
essentially no radius where it meets the passage 23. 
The port 27 at the location where it meets the conical surface 25 has an 
annular radiused edge 41. Preferably, the radius is relatively long and 
may extend for all or a substantial portion of the length of the port 27, 
if so desired. The port 27 has an axial length or column height of no more 
than about 2.5 times its diameter, and in this embodiment, the length of 
the port is about 0.150 inch. The port 27 also has a lesser 
cross-sectional area than the cross-sectional area of the passage 23. 
The inlet 19 is formed in a nipple 43 so that it can be inserted within one 
end of a flexible tube 45 (FIG. 5) of an administration set 46. The outlet 
21 has an enlarged diameter so that an end portion of a flexible tube 47 
may be inserted into it as shown in FIG. 5. Although the pressure sensor 
11 can be caused to communicate with tubes carrying a fluid under pressure 
in various different ways, the pressure sensor is particularly adapted to 
be mounted on a frame 49 as shown in FIGS. 4 and 5. When so mounted, the 
slot 31 frictionally receives a tongue 51 of the frame. 
Although the frame 49 can be of various different constructions, in this 
embodiment it has a central opening 53, and it carries at the end opposite 
the pressure sensor 11 a tubing coupling 55. The tubing coupling 55 joins 
the other end of the tube 45 to a tube 57 which extends to a source 9 of 
parenteral liquid. 
The tube 45 is compressible and suitable for use in a peristaltic pump. The 
opening 53 provides a space in which the tubing 45 may be sequentially 
compressed in peristaltic pumping fashion by a peristaltic pump (not 
shown). This generates a pressure head for transmitting liquid from the 
source 59 of parenteral liquid through the pressure sensor 11, the tube 47 
and a suitable device, such as an IV needle 61 to a patient. Accordingly, 
conduit means is provided for administering the parenteral liquid. 
Prior to use, all of the components of the administration set including all 
of the tubes, the port 27, and the sensing chamber 37 contain air, and 
this air must be purged from the system. To do that, the attendant 
manually depresses the diaphragm 15 tightly against the conical surface 25 
to expel the air from the sensing chamber 37. Then liquid from the source 
59 is allowed to flow by gravity through the IV needle 61. By repeatedly 
depressing the diaphragm 25 with thumb pressure and then releasing it, 
liquid can be drawn through the port 27 into the sensing chamber 37 to 
thereby completely expel air from this portion of the system. The 
expulsion of air is facilitated by the radiused edge 41. Thus the port 27 
makes it somewhat easier for a bubble to leave the sensing chamber than to 
enter it. 
The sensing chamber 37 is not a flow-through chamber in that the port 27 
effectively isolates the chamber from the liquid flowing in the passage 
23. During use, there is essentially no flow through the port 27 to or 
from the sensing chamber 37. During operation the static pressure in the 
sensing chamber 37 is essentially the same as in the passage 23, and such 
pressure is monitored by the transducer 17. If air or bubbles are present 
in the liquid being pumped, it is highly unlikely that any bubble will 
pass through the port 27 into the sensing chamber 37 because of the 
surface tension being too great to allow the bubble to pass through the 
small diameter in the port 27. The sharp edge 39 also makes it more 
difficult for a bubble to travel from the passage 23 through the port 27 
to the sensing chamber 37. 
FIG. 6 shows a pressure sensor 11a that is identical to the pressure sensor 
11, except that the port 27a is rectangular rather than cylindrical. 
Portions of the pressure sensor 11a corresponding to portions of the 
pressure sensor 11 are designated by corresponding reference numerals 
followed by the letter "a." 
The port 27a has a long dimension 63 and a controlled or short dimension 
65. The short dimension 65 is no more than 0.060 inch and preferably has a 
minimum in accordance with the parameters discussed above for the 
controlled dimension of the port. The long dimension 63 may, if desired, 
exceed 0.060 inch because, the short dimension will be controlling in 
preventing the passage of bubbles through the passage port 27a. The length 
of the port 27a preferably is no greater than about 2.5 times the short 
dimension 65. 
Although exemplary embodiments of the invention have been shown and 
described, many changes, modifications and substitutions may be made by 
one having ordinary skill in the art without necessarily departing from 
the spirit and scope of this invention.