Pressure sensing system

An apparatus for measuring fluid pressures which comprises a gas-type pressure sensor and a sensor control for supplying controlled purge gas to the sensor and for receiving vent gas from the sensor. The apparatus further includes a unique pressure-reducing device operable in response to a differential gas pressure in the sensor control for creating a zone of pressure less than any other in the apparatus, which zone is placed in fluid communication with gas in the apparatus to reduce the overall pressure thereof. This permits the accurate measuring of pressure at or below atmospheric. An integral design and a retrofit design are disclosed.

BACKGROUND AND SUMMARY OF THE INVENTION 
The present invention relates to pneumatic systems for accurately measuring 
and indicating fluid pressure, and to more particularly such systems for 
remotely measuring the weight, volume or depth of liquids in large 
processing or storage tanks. Cross-reference is made to the later-filed 
copending application, Ser. No. 359,581, filed Mar. 18, 1982, and assigned 
to the same assignee as the applicant's assignee herein. 
The type of system over which this invention is an improvement is 
illustrated in U.S. Pat. No. 3,161,051 issued Dec. 15, 1964 (the 
disclosure of which is incorporated herein by reference). It generally 
comprises a flush diaphragm-type sensing unit or transmitter mounted in 
the tank wall (near the bottom of the tank), one side of the diaphragm 
being in contact with the liquid being measured. The other side 
("indicator") side of the diaphragm is supplied a substantially constant 
volume of air (or other inert gas) which is vented through a nozzle to an 
extent proportionate to the position of the diaphragm. The result is that 
the pressure on the indicator side of the diaphragm is maintained 
substantially equal to the pressure on the liquid side of the diaphragm 
and the vent passage is constantly and dynamically proportional to the 
amount of such pressure (and hence the weight, volume or depth of the 
liquid in the tank). 
One of the problems encountered with systems of this type is that they will 
not operate on tanks which are under a vacuum (such as in many "septic" 
processes in the food and pharmaceutical fields), and that they are 
inherently not very accurate when the pressure in the tank is very close 
to the vent or ambient pressure (such as when the tank is completely or 
almost empty). This problem is aggravated in systems using stainless steel 
diaphragms because for some reason it has been discovered that stainless 
steel diaphragms introduce significantly greater errors into the pressure 
readings at close to vent pressure than do conventional elastomeric 
diaphragms. This means that accuracy has to be sacrificed in applications 
requiring tough, cleanable stainless steel diaphragms. 
It is therefore the primary object of the present invention to provide an 
improved system of the aforesaid type which has all the advantages of 
existing systems and yet overcomes the aforesaid problems. A related 
object resides in the provision of such a system which is very simple in 
construction, which does not materially increase operating costs, and 
which is readily adapted to be easily retrofit into existing systems with 
a minimum of difficulty. 
These and other objects, features and advantages of the present invention 
will become apparent from the subsequent description and the appended 
claims, taken in conjunction with the accompanying drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
Although the present invention is applicable to any pressure measuring 
system, it is disclosed herein embodied in a liquid level measuring 
system. The system generally comprises a sensor 10 sealingly disposed in 
the wall 11 of a liquid vessel or tank 12 and in pneumatic communication 
with a sensor control 14, which in turn is connected to a liquid level 
indicator or gage 16. The outside of the tank may, if desired, be covered 
with insulation 13. Sensor control 14 comprises several sections bolted 
together, including a main body 18, an optional over-pressure valve 20, 
optional filter 22 and a mounting bracket 24. Filter 22 may be provided 
with a conventional automatic drain 26 (FIG. 3). Except for the use, fluid 
flow location and structure of the vacuum producing means, sensor control 
14 is old. Also, none of the details of construction of sensor 10 form a 
part of this invention. Therefore, only so much of the construction as is 
necessary to understand the present invention is shown and described. 
As best shown in FIG. 4, sensor 10 comprises a shell 28 welded at its inner 
end to the wall 11 of the tank and having disposed therein a transmitter 
assembly including a body 30 having affixed to the inner end thereof a 
ring 32 having a stainless steel diaphragm 34 sealingly affixed to the end 
surface thereof. There is a very small clearance space between the outside 
or indicator side of the diaphragm and the inner end of body 30 which is 
placed in fluid communication with the interior 36 of shell 28 by means of 
passages 38 and 40, and a clearance between the shell and body 30. The 
transmitter assembly is held in the position shown by means of a hollow 
post 42 which has shoulder-retained washers 44 and 46 bearing on body 30 
and a connector 48, respectively. The parts are urged to the right as 
shown in FIG. 4 by means of a nut 50 which threadably engages shell 28. 
The parts are configured to provide a substantially flush surface on the 
inside of the tank (i.e., the liquid side of the diaphragm) and an O-ring 
52 provides a seal between the transmitter assembly and shell 28. Body 30 
has a central bore in which is threadably disposed a nozzle 54 having an 
inner face in very close proximity to the indicator side of diaphragm 34. 
The passageway through the nozzle is in sealed fluid communication with 
the interior of post 42, which in turn communicates with a flexible tube 
56 via a central passage 60 in connector 48 and a fitting 58 sealingly 
disposed within connector 48. Fitting 58 also contains flexible tubes 62 
and 64 which communicate with a passage 66 in connector 48 (shown in part) 
which in turn communicates with interior 36 of shell 28 and ultimately the 
clearance space between diaphragm 34 and the transmitter assembly. 
The present invention is also fully applicable to sensors utilizing 
elastomeric diaphragms, such as the exemplary construction illustrated in 
FIG. 5, wherein there is shown an alternative transmitter construction. 
The remaining parts of the sensor are identical to those disclosed in the 
first embodiment and identical reference numerals are used to identify 
like parts. The transmitter of this embodiment comprises a body 126 over 
the outer surface of which is stretched a diaphragm 128 formed of suitable 
elastomeric material. Body 126 has a nozzle opening 130 corresponding to 
nozzle 54 of the first embodiment and passageways 132 and 134 
corresponding to passageways 38 and 40 in the first embodiment. Rigidly 
affixed to body 126 is a hollow post 136 which corresponds to post 42 in 
the first embodiment. The remaining structure is the same. 
The portion of sensor control 14 which is most important to an 
understanding of the present invention is the portion disposed within body 
18, the basic structure of which is best illustrated in FIGS. 2 and 3. A 
supply of air under pressure, designated by reference character S, is 
communicated to the control 14 via a flexible line 67 (shown schematically 
in FIG. 2) connected to a supply port 68, from which the air is 
communicated via a passage 70 through a conventional filter 22, as is also 
shown schematically in FIG. 2, which removes undesired liquid particles 
and particulate matter. The filtered air then passes from filter 22 
through passages 72 and 74 to a first pressure regulator means comprising 
a diaphragm 76, a compression spring 78 and a Schroder-type valve 80 which 
opens on downward movement of the diaphragm. This regulator means is fully 
equivalent to regulator 44 in the aforesaid '051 patent; it functions to 
maintain (via passages 82 and 83) a substantially constant pressure 
differential across a flow control orifice 84. The filtered supply air 
thus passes through valve 80, passage 82 and orifice 84, which causes it 
to be delivered into a passage 86 at substantially constant volume (cfm). 
From passage 86 it flows via a passage 88 to a port 90 to which the other 
end of flexible tube 62 is connected. This regulated volume (cfm) of air 
is the purge air supplied to the indicator side of the diaphragm in the 
sensor 10, as described above. 
This regulated air activates the sensor by opposing the liquid pressure on 
the liquid (or tank) side of the diaphragm. When these pressures reach 
equality excess air is vented from the sensor via nozzle 54 shown in FIG. 
4 and flexible tubing or vent line 56 to a vent port 92 in the sensor 
contro1 14, as shown schematically in FIG. 3. From this vent port 92 the 
vent air passes through a passage 94 to a second pressure regulator means 
comprising a diaphragm 96, a compression spring 98 and a valve 100 which 
is urged open by spring 98. The second regulator means is fully equivalent 
to the "back pressure" regulator 72 in the aforesaid '051 patent. It 
maintains a substantially constant pressure differential between the purge 
air and the vent air, and thereby improves system accuracy by eliminating 
the effects of distortion of the diaphragm at different pressures. 
In prior systems air from this second regulator means was vented directly 
to atmosphere. In accordance with the present invention, however, this 
does not occur. Instead, the vent air is communicated via passages 102, 
104 and 106 to an eductor 108 disposed within a bore 110 in the flow 
control. Eductor 108, which is conventional per se, comprises a converging 
inlet nozzle 112 in communication with the relatively high pressure (with 
respect to atmospheric) air in the cavity of diaphragm 76 immediately 
upstream of orifice 84, a chamber 114 and a converging/diverging outlet 
nozzle 116 communicating with an outlet vent port 118 via a reduced 
diameter passage 120 which provides sound attenuation. Chamber 114 is in 
fluid communication with passage 106 via a plurality of openings 122. Air 
flowing through eductor 108 from the first regulator to atmospheric vent 
port 118 creates (in accordance with known principles) a vacuum in chamber 
114. This causes the system vent pressure to drop substantially below 
atmospheric pressure, which thereby makes it possible to measure negative 
gage pressures, or liquids in vacuum tanks. 
In the present system pressures are indicated by a gage or indicator, such 
as the manometer 16 in FIG. 1, connected by a flexible tube 124 to the 
over-pressure valve 20 which in turn is connected by flexible tube 64 to 
sensor 10, and thus also to tube 62 through fitting 58 as discussed above 
and shown in FIG. 4. The pressure of the purge air in sensor 10 is 
proportional to the weight, volume and/or depth of the liquid fluid or in 
the tank and it is air at this pressure which is communicated to gage 16. 
Valve 20 (details not shown) is normally open to permit the free flow of 
air from indicator tube 64 to tube 124, and closes only in the presence of 
excessive pressures which would damage the indicator 16. 
The manner in which the system functions may be easily understood by 
referring to FIG. 6, which illustrates the relationship between tank 
pressure and gage pressure. If it is assumed that the system is designed 
to provide a one-to-one ratio between these pressures the relationship 
would be theoretically represented by a straight line a extending at 
45.degree. and passing through zero gage pressure (normally atmospheric 
pressure, which is normal vent pressure). In actual practice, however, it 
has been discovered that the relationship between these pressures more 
closely follows the dash line b (in the absence of a back pressure 
regulator). In a standard atmospherically vented system, when the tank 
gage pressure is zero (i.e., the tank is empty) there is a small indicator 
pressure reading, indicated at c on the graph. This is often referred to 
as the "air on" pressure and this is undesirable to the extent that it 
indicates that there is liquid in the tank, whereas in fact there may not 
be. As the pressure builds up in the tank (e.g., on filling) curve b stays 
somewhat below theoretical curve a until a point d when it crosses a and 
then curves upwardly (shown exaggerated). Ideally, the second pressure 
regulator (i.e., the back pressure regulator) is set to make substantially 
constant the differential between these pressures at point d on the curve. 
This causes the indicator pressure to thereafter accurately track tank 
pressure as it increases. The air-on pressure c, which distorts indicated 
readings at pressures close to ambient or vent pressure, is particularly 
aggravated when stainless steel diaphragms are used in the sensor. It has 
been discovered that the air-on may be more than two or three times 
greater with stainless steel diaphragms than it is with elastomeric 
diaphragms. 
The present invention overcomes this problem by reducing overall system 
pressure below atmospheric or normal vent pressure, such as to the point 
represented by the negatively displaced axes shown in phantom lines. Under 
these conditions the system performs exactly as before and therefore the 
same references to the graph are used except that they are primed. In the 
system of the present invention there is still an air-on signal when 
trying to sense pressures near venting pressure, but it is fully corrected 
by the back pressure regulator prior to reaching atmospheric pressure. 
Therefore the indicator accurately tracks the pressure in the tank in 
accordance with theoretical line a from zero gage pressure on upward. To 
get accurate readings from zero gage and upward, the eductor merely has to 
reduce system pressure by an amount equal to the value of e. Furthermore, 
if the eductor is designed to reduce system pressure to close to zero 
absolute, then the system will operate accurately to track tank pressure 
for a substantial range of vacuums below atmospheric. Thus a system 
incorporating the present invention is capable of not only handling vacuum 
tank installations, but also is one which is not susceptible to air-on 
signal errors at low liquid levels, even in those installations where a 
stainless steel diaphragm is used. 
In FIGS. 7 and 8 there is illustrated an alternative embodiment of the 
invention in which the eductor is retrofit to existing pressure sensing 
systems. Where parts are the same, the same reference numerals are used. 
In this embodiment the sensor control, indicated at 140, has no internal 
eductor; instead, there is provided a vent adapter or a second body 142 
for providing a similar function. Vent adapter 142 connects between the 
compressed air supply and the sensor control, and between the sensor vent 
and the sensor control. To accomplish this it has an internal through 
passage 144 communicating at one end with a fitting 146 connected to 
supply port 68 in the sensor control and at the other end with a port (not 
shown) to which flexible tube 67 is connected for the supply of compressed 
air. The vent adapter contains a bore 148 in which there is disposed an 
eductor 150 including an inlet nozzle 152, a vacuum chamber 154 
communicating with bore 148 via opening 155, and an outlet nozzle 156 
communicating with a port 158. The latter port is connected via a flexible 
tube 159 to vent port 92 in the sensor control. The vent adapter has a 
vent inlet port 160 communicating via a passage 162 with openings 155. A 
set screw 164 may be utilized to retain the eductor in position. 
Supply compressed air, in addition to passing directly into port 68 of the 
sensor control, passes from passage 144 through a pressure regulator 
comprising a conventional Shroeder-type valve 166 having an inlet 168, a 
regulator chamber 170 having a piston 172 therein and a spring 174 urging 
the piston against the valve actuator in a direction to open the valve. 
The spring side of the piston is vented at 176. Regulated compressed air 
which is passed through valve 166 into chamber 170 thereafter passes into 
passageway 180 and the inlet to the eductor via an intermediate passage 
178. This pressure regulator is optional in that it is not essential to 
the operation of the apparatus, but is provided for the purpose of 
limiting the amount of air supplied to the eductor, for conservation 
reasons. In this embodiment the eductor functions to reduce the pressure 
in the vent line 56 from the sensor to the back pressure regulator, which 
serves to reduce overall system pressure sufficiently to provide the 
aforesaid advantages. Even though this retrofit arrangement of the second 
embodiment may not be as good as the first embodiment for measuring the 
pressure in vacuum tanks, this is not a problem because all retrofit 
systems are for liquid tanks which never go below atmospheric pressure. 
This is because prior systems were not capable of operation below 
atmospheric pressure. 
Although the above system has been described using compressed air, it 
should be appreciated that any suitable inert gas may be used. It should 
also be understood that the system is not limited to the measuring and 
indicating of the weight, volume or depth of liquids in tanks, but is 
fully applicable to the measuring and indicating of any pressure which can 
be sensed by the sensor. Although the apparatus is disclosed utilizing an 
eductor of conventional construction for the purpose of creating the 
vacuum desired, it will be appreciated that other devices for generating a 
vacuum using compressed air may be used. Furthermore, in lieu of the 
diaphragm-type sensor other gas-type sensors may be used, such as those 
utilizing pistons or other pressure balancing mechanisms. 
Thus there is disclosed in the above description and in the drawings an 
improved pressure sensing system which fully and effectively accomplishes 
the objectives thereof. However, it will be apparent that variations and 
modifications of the disclosed embodiments may be made without departing 
from the principles of the invention or the scope of the appended claims.