A relatively simple and compact apparatus is described for producing and/or monitoring a plurality of reference pressures wherein all the pressures must be precise and, especially, precisely related to each other. The apparatus includes a plurality of manometers, each including a reservoir and an elongated vertical tube connected together at their lower ends and partially filled with a manometer liquid such as mercury. As an example, a first (lower) reference pressure such as 150 torr (150 mm Hg) is connected to the reservoir of a first manometer while a second reference pressure such as 300 torr is connected to the reservoir of a second manometer of the same construction. The liquid in the vertical tubes will be at the same level only when the second reference pressure is precisely twice as great as the first.

BACKGROUND OF THE INVENTION 
There are applications where a group of gas pressures are required that are 
accurately related to each other, as in the testing of transducers. The 
usual approach in monitoring such pressures is to use a group of 
manometers, each being used to measure one of the pressures, or to use a 
single monometer connected in turn to each of the pressures. Manometers 
containing mercury are often used. The height of a column of mercury in a 
transparent tube can be measured by comparing the top of the column to 
gradations etched in the tube. However, it is difficult to determine 
precisely where the top of the column lies with respect to a marking, 
because of the meniscus at the top of the column. A lighter manometer 
liquid such as water is also used, which enables greater precision in 
measurement. For most measurements of pressures, water columns are too 
high to enable their inclusion in compact instruments. Even a mercury 
column may be too tall for inclusion in a compact instrument where the 
highest pressure exceeds 3 psi. For example, to measure a pressure of 300 
torr (about 6 psi) requires a mercury column whose top is about 360 mm 
tall (about 14 inches). A manometer arrangement which substantially 
reduced the height required for manometers to be used for measuring two or 
more pressures, while also enhancing the accuracy of comparative 
measurements of the tops of manometer liquid columns with respect to 
markings and to each other, would be a considerable improvement. 
SUMMARY OF THE INVENTION 
In accordance with one embodiment of the present invention, a compact 
apparatus is provided for monitoring and producing a plurality of 
reference outlet pressures that are accurate, and accurately related to 
each other. The apparatus includes at least two manometers, each having a 
reservir and an elongated vertical tube whose lower ends are connected 
together and which contain a manometer liquid. The upper end of the first 
(lowest pressure) manometer tube is open to a predetermined constant base 
pressure such as atmospheric, while the upper end of the first reservoir 
is connected to the upper end of the second manometer tube. A first 
pressure regulator is coupled to the pressure at the upper end of the 
first reservoir, while a second pressure regulator is coupled to the upper 
end of the second reservoir. In one case, the liquid levels in the 
reservoirs are at the same levels at zero pressure. When pressure is 
applied and the liquid columns in the two vertical tubes are of an 
identical height, the pressure at the upper end of the second reservoir is 
precisely twice the pressure at the upper end of the first reservoir. The 
heights of the liquid columns in the two vertical tubes can be accurately 
compared to one another to produce a second pressure precisely twice that 
of the first. The total height of the manometer is reduced. 
Even though the tubes are separated, they can be compared as though they 
were very close together, by placing a prism, or for symmetry, a pair of 
parallel prisms in front of the tubes, so that when a person faces the 
V-shaped groove, formed by the prisms, each tube's refracted image may be 
made to appear to be very close to the other, even partially superimposed. 
The novel features of the invention are set forth with particularity in the 
appended claims. The invention will be best understood from the following 
description when read in conjunction with the accompanying drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 1 illustrates an apparatus which has a pair of reference pressure 
outlets 12, 14 which deliver a gas, such as air, at reference gage 
pressures P.sub.1 and P.sub.2, respectively. The reference pressures are 
required to be precisely related, with the second reference gage pressure 
P.sub.2 at 14 being required to be precisely twice the reference gage 
pressure P.sub.1 at 12 ("gage" means with respect to atmospheric 
pressure). The equipment includes a pressure gas source 16 which is 
connected to a pair of pressure regulators 18, 20 that supply pressure 
through valves 22, 24 to the reference outlets. Each pressure regulator is 
of the type which can be finely adjusted to provide a stable desired 
outlet pressure. The pressure regulators' outputs are connected to a pair 
of manometers 32, 34. The first manometer 32 includes a first reservoir 36 
and a first elongated vertical tube 38, whose lower ends 36b, 38b are 
connected together through a lower coupling 40. A manometer liquid such as 
mercury lies within the reservoir and tube, and in the coupling 40 that 
extends between them. The second manometer 34 is similarly constructed, 
with a second reservoir 42, second tube 44, and second coupling 46 
connecting their lower ends. The upper ends of the reservoirs and tubes 
contain a gas such as air. 
The pressure at the outlet of the first pressure regulator 18 is connected, 
through an upper coupling 50, to the upper end 36u of the first reservoir. 
The upper end 38u of the first vertical tube is at a predetermined, or 
base, pressure such as atmospheric pressure by opening it to the 
atmosphere. As a result, the pressure on the reservoir 36 causes liquid in 
the tube 38 to rise to a particular level 52 which is proportional to the 
pressure on the first reservoir. For the particular example shown in FIG. 
1, wherein the manometer liquid 54 is mercury and the pressure P.sub.1 
applied to the first reservoir is 150 mm Hg, or 150 torr, the height 
H.sub.1 of the top 52 of the column above the top of the reservoir, 
represents the difference in pressure at the top of the column and at the 
top of the reservoir (with a minor correction for mercury column wall 
repulsion depending upon the vertical tube diameter and material). 
The upper coupling 50 also connects the upper end 36u of the first 
reservoir to the upper end 44u of the second tube. With the second 
pressure regulator 20 applying a pressure P.sub.2 of 300 torr to the upper 
end 42u of the second reservoir, the difference in pressures (P.sub.2 
-P.sub.1) between the upper ends of the second tube 44 and second 
reservoir 42 is 150 torr, and the height H.sub.2 of the second column 56 
will also be 150 mm. As a result, the top 58 of the second column will be 
precisely at the same level as the top 52 of the first column when the 
second pressure P.sub.2 is precisely twice as great as the first pressure 
P.sub.1. This 2:1 ratio will exist so long as the tops 52,58 of the two 
columns are at the same level. Of course, if the pressures P.sub.2 and 
P.sub.1 are to be 200 torr and 100 torr respectively, then the heights 52, 
58 of the columns will be lower, but they will still be at precisely the 
same level. 
It would be possible to measure the two pressures P.sub.2 and P.sub.1 using 
two separate and unconnected manometers. However, the connection of the 
manometers as shown in FIG. 1, has important advantages over the use of 
such separate manometers. A first advantage is that the tops of the two 
columns 52, 58 can be placed adjacent to one another so that they can be 
compared. It is generally difficult to determine when the top of a 
manometer column is precisely at a gradation marked on a tube. Such 
difficulty arises because the top of the column forms a meniscus, which is 
convex in the case of mercury lying in a glass tube. There is a difference 
in height between the top, middle, and bottom of the meniscus. However, 
when two identical tubes are placed side-by-side, it is easier to 
precisely determine when the meniscuses of the two tubes are at precisely 
the same level. 
Another advantage of the apparatus of FIG. 1, is that the manometer whose 
reservoir is at a higher pressure P.sub.2, does not have to be of great 
height in order to accurately measure that pressure. Where the second 
reference pressure P.sub.2 is 300 torr, a prior art manometer would 
require a mercury column of 300 mm height. In FIG. 1, the same pressure of 
300 torr is obtained using a column height H.sub.2 of only 150 mm. Where 
only the higher pressure P.sub.2 is needed, and the two-manometer 
arrangement is used merely to reduce the height of the second manometer, 
the first reference pressure outlet 12 is not needed. 
In order to enable operation of the apparatus as described above, the ratio 
of the diameter of the first tube 38 to the diameter of the first 
reservoir 36, must be the same as the ratio of diameters of the second 
tube 44 to the second reservoir 42. This can be accomplished by using 
similar manometers, having the same diameters of vertical tubes and 
reservoirs. 
FIG. 2 illustrates another apparatus 62 which produces four pressures 
P'.sub.1, P'.sub.2, P'.sub.3, and P'.sub.4, that are precisely related to 
each other in that each pressure is a certain level above the previous 
one, such as 100 torr above the previous pressure. Thus, the first 
pressure P'.sub.1 is 100 torr, while the fourth pressure P'.sub.4 is 400 
torr. The apparatus includes four manometers 64,66,68 and 70. The first 
manometers 64 has a tube 72 whose upper end is open to the atmosphere, and 
a reservoir 74 which is connected to a pressure regulator 76. With the 
pressure regulator 76 producing a pressure P'.sub.1 of 100 torr, the 
vertical height V of the column in the first tubes 72 is 100 mm above the 
top of mercury in the first reservoir. The top of the first reservoir 74 
is connected to the top of the second tube 78. The top of the second 
reservoir 80 is connected to a second pressure regulator 82 whose output 
P'.sub.2 must be precisely 200 torr in order that the height of the 
second column in tube 78 be equal to the height V of the first column. The 
reservoirs and tubes are simialrly connected, with the top of the 
reservoir of the last manometer 70 connected to a pressure P'.sub.4 of 400 
torr. It can be appreciated that the height V of mercury in the fourth 
column 84 is only one-fourth the height that would be required if a single 
manometer were to be used to measure a pressure of 400 torr. The four 
tubes 72, 78, 84, and 86 should be arranged so that they are closely 
adjacent so that their heights can be compared to each other, or at least 
to etch marks on their respective tubes. 
The reservoirs and elongated vertical tubes of two or more manometers can 
be rigidly held together by forming them as bores in a single housing. 
FIG. 3 shows the pair of tubes 54, 56 of FIG. 1 as bores in a transparent 
plastic housing 58. While it would be desirable to place the vertical 
bores 54, 56 immediately adjacent to each other, it is necessary to 
separate them in constructing the device. Even though the tubes are 
separated, the heights of liquids in them can be compared as though the 
tubes were immediately adjacent to one another, by forming the outer 
surface 90 of the housing into dual parallel prism by a V-shaped groove 
92. The prismatic regions or prisms 91, 93 formed by the groove, results 
in a pair of walls 94, 96 that converge. The walls 94, 96 are positioned 
so that when a person views the tubes 54', 56' through the walls 94, 96, 
the two tubes appear as indicated at 54', 56', as though they were much 
closer together, even partially superimposed, than in reality. This is due 
to the fact that the material of the housing 58, of a material such as 
acrylic plastic, has a higher index of refraction (above 1.0) than air, so 
that light is refracted at the plastic-air interface. The result of the 
two tubes 54, 56 appearing to be closer together than they actually are, 
facilitates comparision of the levels of manometer fluids in the tubes. 
While a pair of identical manometer such as 32,34 in FIG. 1, can be used to 
generate and measure two pressures that are in a 2:1 ratio, they also can 
be used to measure two pressures that differ by a predetermined amount. 
FIG. 4 illustrates two manometers 100, 102 similar to those of FIG. 1, 
except that the top 104 of the second reservoir is at a level which is a 
distance H.sub.5 of 50 mm higher than the top 106 of the first reservoir. 
This can be accomplished by using a support 107. When the tops 108, 110 of 
the two liquid columns 112, 114 are at equal heights, the second reservoir 
pressure P.sub.6 will be 50 torr less than twice the first reservoir 
pressure P.sub.5. By contrast, in FIG. 1 the manometer liquid in the two 
tubes and reservoirs are all at the same level 99 when the base pressure 
(atmospheric) is applied to the upper ends of all tubes and reservoirs, so 
P.sub.2 is precisely twice P.sub.1. 
In the above examples, the pressures such as P.sub.1 and P.sub.2 have been 
positive gage pressures, that is, pressures above atmospheric. However, 
the same apparatus can be used to measure vacuum, or negative gage 
pressures. FIG. 5 illustrates an apparatus similar to that of FIG. 1, 
except that the pressures P.sub.11 and P.sub.12 are vacuum gage pressures, 
with the absolute value of P.sub.12 being precisely twice P.sub.11. The 
apparatus 120 includes two identical manometers 122, 124, with the first 
reservoir 126 open to the atmosphere and the first vertical tube 128 
connected to the second reservoir 130. The first tube 128 is connected to 
a first pressure regulator 132 while the second tube 134 is connected to a 
second pressure regulator 136. When the tops 138, 140 of the two columns 
are equal, the negative gage pressures are in a 2:1 ratio. For example, 
P.sub.11 may be at a gage pressure of 100 torr (below atmospheric) while 
P.sub.12 would be at a gage pressure of -200 torr. 
Thus, the invention provides apparatus for monitoring at least two 
reference pressures, wherin the second pressure is precisely related to 
the first, such as where it is twice as great as the first, in a 
relatively compact and accurate instrument. This can be accomplished by 
connecting the reservoir of a first manometer to the top of the vertical 
tube of the second manometer. For positive pressures, first and second 
pressure regulators are connected to the reservoirs of the first and 
second manometers. For negative gage pressures, the pressure regulators 
are connected to the tops of the first and second vertical tubes of the 
manometers. 
Although particular embodiments of the invention have been described and 
illustrated herein, it is recognized that modifications and variations may 
readily occur to those skilled in the art and consequently, it is intended 
that the claims be interpreted to cover such modifications and equivalents 
.