Differential pressure indicator

A differential pressure indicator has a piston across which inlet and outlet pressures are applied with a spring preventing movement of the piston until a predetermined differential pressure is exceeded. At this point, a port is opened to allow the inlet pressure to be applied to an indicator button to move the indicator button from a retracted position to an operative extended position to indicate that the pressure differential has been exceeded. The use of fluid pressure to operate the indicator button applies a large force to the indicator button and allows it to be securely held in the inoperative position to guard against accidental movement by high G-forces.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The invention relates to differential pressure indicators. 
2. Brief Review of the Prior Art 
Differential pressure indicators are used to provide an indication, such as 
a visual indication or an electrical signal, when the difference between 
two fluid pressures exceeds a predetermined value. This is often achieved 
by applying the pressures to opposite sides of a piston which moves from 
an inoperative position to an operative position when the predetermined 
value is reached. 
In one known indicator of this kind, the piston carries a rod which moves 
from a non-visible position to a visible position as the piston moves from 
the inoperative position to the operative position. The rod thus provides 
a visual indication that the predetermined pressure differential has been 
exceeded. 
Such indicators are used, for example, in filters to indicate the onset of 
clogging of the filter element which increases the pressure differential 
upstream and downstream of the filter in the fluid being filtered. 
SUMMARY OF THE INVENTION 
According to the invention, there is provided a differential pressure 
indicator comprising a housing containing a piston having opposed first 
and second surfaces subject to respective higher and lower fluid pressures 
so that the piston moves from an inoperative position to an operative 
position when the difference between the higher and lower pressures 
exceeds a predetermined value, and an indicator being mounted in the 
housing and being movable between an inoperative position and an operative 
position to produce an indication. The indicator element includes a region 
acted on by one of the fluid pressures under the control of the piston so 
that said fluid pressure is applied to the region of the indicator element 
when the piston moves from said inoperative position. 
In this way, the piston and the indicator element are not mechanically 
interlinked and this relieves various design constraints on the indicator. 
In addition, the fluid under pressure can apply a significant force to the 
indicator so providing a very positive indication and allowing the 
indicator element to be held securely in the inoperative position against 
accidental movement by G-forces.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
The differential pressure indicator 9 comprises a housing 10 having an 
inlet passage 11 extending through the housing 10. A first bore 12 is 
formed in the housing 10 to one side of the inlet passage 11, extending in 
a direction normal to the length of the inlet passage 11, and is separated 
from the inlet passage 11 by a wall 13 provided with a port 14. 
On the opposite side of the inlet passage 11, the housing 10 is provided 
with a second bore 15 which extends normal to the length of the passage 11 
and intersects the passage 11. An outlet passage has a first portion 16a 
leading into the bore 15 on one side of the bore and a second portion 16b 
leading from the bore 15 on a side of the bore diametrically opposed to, 
but axially staggered from, the outlet passage portion 16a. 
The bore 15 receives an annular sleeve 17 having an end adjacent the inlet 
passage 11 which sits in an annular rebate 18 provided around the end of 
the bore 15. An 0-ring seal 19 carried on the outer surface of the sleeve 
17 seals against the rebate. 
The end of the sleeve 17 adjacent the inlet passage 11 is provided with an 
inwardly directed flange 20 having a radially extending surface 21 facing 
away from the inlet passage 11 and provided with an annular seal 22 for a 
purpose to be described below. 
The sleeve 17 also has a pair of diametrically opposed apertures 23 which 
provide fluid communication between the interior and the exterior of the 
sleeve 17. 
The end of the sleeve 17 remote from the inlet passage 11 is provided with 
an outwardly directed flange 24 that locates on the interior surface of 
the bore 15. There is thus formed between the outer surface of the sleeve 
17 and the inner surface of the bore 15 an annular chamber 25 which is in 
communication with the outlet passage portions 16a and 16b. 
A piston 26 is received in the sleeve 17 and is slidable relative to the 
sleeve 17. The piston 26 has a first working surface 27 which, in the 
inoperative position of the piston 26 shown in the upper part of the 
drawing, engages the seal 22 on the flange surface 21 of the sleeve 17. 
The piston 26 has a skirt 28 which, in this position, covers the apertures 
23. 
The centre of the working surface 27 is formed with a tube 29 which 
projects axially along the bore 15 into the inlet passage 11. A rod 30 is 
slidably mounted in this tube and a seal 31 carried by the tube 29 at an 
end of the tube 29 remote from the inlet passage 11 seals between the tube 
29 and the rod 30. The rod 30 is provided with a pointed tip 32 at an end 
of the rod 30 projecting from the tube 29, and with an annular rib 33 at 
the opposite end within the piston 26. The interior of the piston 26 is 
provided with an annular flange 34 co-axial with the axis of the piston 26 
and the rod 30. 
The interior of the piston 26 also accommodates a cup 35 whose open end 
receives the flange 34 and the end of the rod 30 and the rib 33. A helical 
spring 36 acts between the closed end of the cup 35 and the rib 33 and, in 
the inoperative position of the piston 26 shown in the upper half of the 
drawing, loads the rod 30 so that the tip 32 is urged into the port 14 so 
sealing the port 14. 
The end of the bore 15 remote from the inlet passage 11 is closed by a 
cup-shaped end closure 37 which is in threaded engagement with the bore 
15. A rim at the open end of the end closure 37 engages an end of the 
sleeve 17 to urge the sleeve 17 against the rebate 18. In addition, a 
spring 38 acts between the closed end of the end closure 37 and the piston 
26 to urge the piston 26 into the inoperative position shown in the upper 
half of the drawing where the working surface 27 engages the seal 22 on 
the flange surface 21. In this position, the tip 32 of the rod 30 is urged 
into the port 14 to prevent fluid communication between the inlet passage 
11 and the first bore 12. 
The first bore 12 is generally cylindrical with its axis coaxial with the 
axis of the second bore 15. The bore 12 contains an indicator device which 
comprises a mounting indicated generally at 39 and having a cylindrical 
portion 40 having an exterior screw thread in threaded engagement with a 
co-operating thread on the bore 12. The mounting 39 also has a head 41 
provided with diametrically opposed bores 42 extending radially relative 
to the common axis of the first bore 12 and the second bore 15. The head 
41 is provided with a central aperture 43 through which passes an 
indicator element comprising an indicator button 44 which is generally 
cylindrical in shape with a closed end wall 45. The upper half of the 
drawing shows the button 44 in an inoperative position where the end wall 
45 is generally level with an end surface of the head 41. The button 44 is 
provided adjacent the end wall 45 with an exterior annular groove 46. Each 
bore 42 contains a ball 47 which, in the position of the button 44 shown 
in the upper half of the drawing, is urged into the groove 46 by an 
associated spring 48 carried in the associated bore 42 and acting between 
the ball 47 and a stop 49. The stops 49 are held in position by a 
retaining ring 50 engaging over the outer edge of the head 41. 
The diameter of the aperture 43 is less than the internal diameter of the 
cylindrical portion 40 so that there is an annular step 52 between the 
aperture 43 and the inner surface of the cylindrical portion 40. Within 
the cylindrical portion 40, the button 44 is provided with an annular 
outer portion 51 which has an exterior surface in sliding engagement with 
the interior surface of the cylindrical portion 40. This portion 51 
carries two seals: a first annular seal 53 carried by a radially extending 
annular face 54 facing the step 52 and an annular wiper seal 55 sealing 
between the portion 51 and the inner surface of the cylindrical portion 
40. An orifice 56 extends through the portion 51 between the interior of 
the button 44 and the exterior of the button 44. It can be seen from FIG. 
1 that the indicator button 44 includes a piston portion on which fluid 
pressure can act to move the indicator button 44 and an indicating portion 
which is associated with the piston portion and which produces an 
indication. 
A further seal 57 seals between the exterior surface of the cylindrical 
portion 40 and the bore 12. 
The differential pressure indicator operates as follows. The operation will 
be described in relation to its use in connection with a filter although 
it will be appreciated that it may operate similarly in other 
applications. 
As shown in FIG. 2, a source 60 of fluid to be filtered is connected via a 
first conduit 61 to one end of the inlet passage 11. The other end of the 
inlet passage 11 is connected via a second conduit 62 to an inlet 63 of a 
housing 64 containing a filter element 65. A third conduit 66 is provided 
for conveying filtered fluid from an outlet 67 of the housing 64 to the 
outlet passage portion 16a and a fourth conduit 68 is connected to the 
outlet passage portion 16b for conveying filtered fluid away from the 
indicator device. In use, while the degree of clogging of the filter 
element 65 remains acceptable, fluid is passed from the source 60 via the 
inner passage 11 to the housing 64 where it is filtered and then passed 
through the indicator via the outlet passage portions 16a, 16b. 
The inlet pressure acts on the area of the working surface 27 of the piston 
26 indicated at Al in the drawing. The outlet fluid passing through the 
outlet passage portion 16a, 16b and the chamber 25 also passes through the 
apertures 23 in the sleeve 17 and then passes in both directions between 
the sleeve 17 and the skirt 28. This means that the outlet pressure acts 
both on the interior surface of the piston 26, applying the outlet 
pressure to an area indicated at A3 in the drawing, and also acts in the 
reverse direction on the portion of the working surface 27 between the 
annular seal 22 on the flange surface 21, and the outer surface of the 
skirt 28. Thus the outlet pressure acts on a net area which is equal to 
the area acted on by the inlet pressure. 
The spring 38 acting between the end closure 37 and the piston 26 is 
designed to apply a force to the piston 26 holding the piston in an 
inoperative position until the difference between the inlet pressure and 
the outlet pressure exceeds a predetermined value. In the case of use with 
a filter element, this pressure difference is the difference at which the 
filter element reaches an unacceptable level of clogging. 
At this point, the inlet pressure exceeds the outlet pressure by an amount 
such as to move the piston 26 relative to the sleeve 17 and against the 
action of the spring 38. After a short length of travel, the flange 34 
engages the rib 33 on the rod 30 and the rod 30 begins to move with the 
piston 26. As a result of this, the tip 32 disengages from the port 14 to 
open the port 14. Thus, inlet fluid passes through the port 14. 
The effect of this is to apply the inlet pressure to the piston portion of 
the button 44, with the pressure acting on an area indicated in the 
drawing as A2. The force generated is sufficient to overcome the detent 
provided by the balls 47 and to force the balls 47 out of the grooves 46 
so allowing the button 44 to slide relative to the mounting 39 and the end 
of the button 44 to project from the mounting 39, so giving a visual 
indication that the predetermined pressure differential has been exceeded. 
At the end of its travel, the face seal 53 engages the step 52 to prevent 
leakage of the fluid through the head aperture 43. 
The orifice 56 ensures that any leakage through the port 14 prior to 
movement of the piston 26 from the inoperative position simply passes 
through the orifice 56 and escapes through the head aperture 43. 
The continued movement of the piston 26 away from the inoperative position 
moves the piston 26 to a position in which the skirt 28 at least partially 
uncovers the apertures 23. When this happens, inlet fluid from the inlet 
passage 11 flows past the piston 26, through the ports 23, into the 
chamber 25 and out through the passage 16b. 
When the filter element has been replaced, the button can 44 can be 
manually pressed to re-engage with the balls 47 and the spring 38 will 
return the piston 26 to the inoperative position. 
It will be appreciated that there are a number of variations that can be 
made to the arrangement described above. Although in the indicator shown 
in the drawing, movement of the piston 26 from the inoperative to the 
operative positions diverts the outlet flow back to the inlet, this is not 
essential. In addition, although the fluid pressure is applied to the 
button 44 by a rod opening a port, the rod/port combination is also not 
essential. The piston 26 could control the application of the inlet 
pressure to the button 44 in any suitable way. Indeed, it need not be the 
inlet pressure that is applied to the button 44; it could be the outlet 
pressure.