Pressure responsive control unit employing snap action diaphragm

A control unit is disclosed comprising a pressure transducer assembly and an attached output switch assembly. The transducer includes rigid body members clamped together and defining a chamber between them, a snap acting diaphragm extending through the chamber, and high and low pressure event controlling mechanisms for adjusting the pressure levels at which the diaphragm snap moves. A motion transfer pin extends from the transducer to the switch assembly to operate a control switch in response to diaphragm motion.

BACKGROUND 
1. Field of the Invention 
The present invention relates to differential pressure responsive control 
units and more particularly to differential pressure responsive control 
units which can be set to respond to predetermined differential pressure 
levels. 
2. Prior Art 
Differential pressure responsive control units employing snap acting 
diaphragms for actuating a control switch, valve member, or the like have 
been proposed in the past. Generally speaking these kinds of units were 
constructed using housings which communicated a source of pressure being 
monitored to one side of a snap diaphragm while the other side of the 
diaphragm was exposed to some reference pressure, such as atmospheric air. 
The diaphragm motion was typically transmitted to the control switch or 
valve by an operating pin so that when the monitored pressure increased 
sufficiently above the reference pressure the diaphragm snap moved to 
alter the condition of the switch or valve member, etc. When the sensed 
differential pressure acting on the diaphragm reached a second level the 
diaphragm snapped to its alternate position resulting in the switch or 
valve member resuming its initial condition. 
Pressure responsive units employing snap acting diaphragms have been 
desirable because they are of relatively simple construction and can be 
manufactured inexpensively; but such units have usually had to be utilized 
in environments where sensed differential pressure levels effective to 
shift the diaphragm can vary relatively widely from nominal values. The 
relatively wide tolerance requirements for their usage were due to 
inability to calibrate some designs with sufficient accuracy and 
inadequately strong diaphragm mounting and support in the control units 
themselves. For these reasons such controls could not always be relied 
upon to respond to sensed pressure levels required for many end uses. 
Attempts have been made to construct snap diaphragm units which could be 
calibrated to respond more closely to predetermined sensed pressures, but 
these have not been uniformly successful. Generally, approaches to 
presetting the differential pressure levels at which the diaphragms 
snapped between alternate positions have involved establishing a desired 
operating differential pressure across the diaphragm and then permanently 
deforming the diaphragm itself, and/or shifting diaphragm supporting 
elements in the unit, until the diaphragm snapped to the alternate 
position. Theoretically at least, such diaphragms would thereafter snap to 
the alternate position at a consistent applied differential pressure; 
however, in practice the calibration procedures did not achieve sufficient 
accuracy on a consistent basis. 
In some of these proposals, for example, the diaphragm supporting housing 
members were mechanically deformed to deform the diaphragm and alter its 
response to pressure. This kind of calibration was often difficult to 
control and required the use of housing constructions which were 
relatively weak and subject to yielding during the useful life of the 
control. Moreover, critical weld joints were required in some of these 
kinds of units and the weld integrity often could not be assured until 
after calibration since the area of the weld was yielded during the 
calibration. Many such control units had to be completely assembled, 
including an associated switch assembly, before calibration and pressure 
testing. 
These controls were particularly subject to drifting from calibrated 
settings when exposed to automotive type environments where ambient 
operating temperatures for the controls vary from -40.degree. to 
121.degree. C. (-40.degree. to 250.degree. F.). The controls can also be 
subjected to sustained high pressure at temperature as well as mechanical 
shocks and vibration in the automotive environments. 
Other approaches to diaphragm calibration also resulted in the diaphragms 
being subjected to stresses which eventually caused fatigue failure of the 
diaphragm. For example, in some prior art proposals adjusting screws were 
advanced into engagement with the central sections of diaphragms to limit 
their travel. The screws were impacted by the diaphragms during their 
travel which tended to both overstress the diaphragm material in the 
vicinity of the screw and to loosen and change the position of the 
adjusting screw. 
Furthermore the adjustment screws were themselves only subject to loading 
when engaged by a diaphragm and tended toward being advanced or retracted 
from their adjusted positions particularly when subjected to vibrations of 
the sort encountered in automotive vehicles, houshold appliances and so 
forth. This caused the control units to drift from their calibration 
settings. 
Other similar proposals involved providing a deformable member which 
extended peripherally along and engaged the diaphragm in one of its 
positions. The deformable member was engaged by a similarly shaped 
threaded member which was turned to deform the member and control its 
engagement with the diaphragm. This kind of adjustment scheme tended to 
fatigue the diaphragm and to unduly stress the diaphragm along the region 
of its engagement with the deformable member. Furthermore when the member 
was deformed to calibrate the pressure level at which the diaphragm 
snapped away from the deformable member, the pressure at which the 
diaphragm snapped toward engagement with the diaphragm tended to be 
altered as well. 
Some pressure responsive controls of the sort referred to were constructed 
to respond to applied differential pressures having first and second 
predetermined levels. In order to enable the controls to respond to a 
predetermined relatively high level pressure, the position of the 
diaphragm when dished into a sealed chamber section of the control had to 
be adjustable, at least for calibration purposes. 
Because the diaphragm was usually dished into the sealed chamber during 
assembly of the controls, the adjustment structure had to be constructed 
to enable the diaphragm to extend sufficiently into the sealed chamber 
section to enable its subsequent calibration. Accordingly various 
structures were proposed including calibration screws which exhibited the 
problems referred to above. 
In still other proposals a plunger was spring biased into engagement with 
the diaphragm. The spring force was adjustable to enable calibration of 
the controls by increasing the biasing force on the plunger. These 
proposals did not always produce highly accurate pressure responses 
because biased plungers did not form positive diaphragm stops. 
In addition to the problems attendant prior art diaphragm calibration and 
support structures, many prior art pressure controls did not adequately 
support the diaphragms around the periphery of the dished central 
portions. In some constructions the control assembly itself was not rigid 
enough to insure against the diaphragm shifting over a number of cycles 
and this "drifting" from the calibrated setting. 
SUMMARY OF THE INVENTION 
The present invention provides a new and improved pressure responsive 
control unit employing a snap acting differential pressure responsive 
diaphragm, the unit being so constructed and arranged that the diaphragm 
consistently responds to predetermined sensed differential pressure levels 
after a large number of cycles of operation, and notwithstanding the unit 
being subjected to over-pressure conditions, vibrations and extremes of 
ambient temperature. 
According to a preferred embodiment of the invention the new control unit 
includes a pressure transducer assembly having first and second rigid body 
members defining a chamber between them with a snap acting diaphragm 
extending across the chamber. The diaphragm is hermetically bonded to one 
body member so that it can move from one side of the chamber toward the 
other in response to differential pressure forces acting on it. A pressure 
event controlling mechanism is employed for adjustably calibrating the 
differential pressure level at which the diaphragm snaps in one direction. 
The controlling mechanism includes a diaphragm supporting member and an 
adjusting member. The diaphragm supporting member is anchored between the 
body members and defines a smooth diaphragm engaging face extending 
substantially across the chamber. The adjusting member reacts between the 
supporting member and its associated body member and resiliently deflects 
the diaphragm supporting member between adjusted positions to enable 
control unit calibration. 
The diaphragm supporting member is a strong, stiffly resilient plate-like 
structure having its outer periphery engaged with and supporting the 
diaphragm periphery so that the bond between the housing member and 
diaphragm is rigidly supported and isolated from stresses and straining 
which could otherwise be caused by the differential pressures applied to 
the diaphragm. The supporting member is resiliently deflected by the 
adjusting member in the vicinity of the center of the diaphragm and 
rigidly bears against the diaphragm at a location between the diaphragm 
center and the bond. The bearing engagement between the diaphragm 
supporting plate and the diaphragm assure consistently accurate snap 
movement of the diaphragm. The location of bearing engagement with the 
diaphragm is spaced from the diaphragm center so that it moves relatively 
little when the adjusting member shifts the central portion of the 
supporting plate. 
The pressure transducer formed by the body members, diaphragm and 
controlling mechanism is assemblable for calibration setting without 
requiring the presence of an output device, such as a switch assembly. The 
structural strength of the body members is such that the spring forces 
created by deflection of parts in the unit and applied fluid pressure 
forces are borne by the body members without requiring assembly of an 
output device to the transducer assembly for calibrating it. 
In a preferred embodiment of the invention a second pressure event 
controlling mechanism is provided for assuring that the diaphragm snap 
moves to the supporting plate when a predetermined relatively high 
differential pressure is applied to it. The second pressure event 
controlling mechanism includes a shiftable diaphragm support member and an 
adjusting member reacting between the housing and the diaphragm support 
member. The diaphragm support member is initially fixed in the housing so 
that it is spaced from the diaphragm to facilitate bonding the diaphragm 
to the housing. The adjusting member deflects the diaphragm support member 
to an adjusted position where the diaphragm, engaged by the support 
member, responds to a predetermined differential pressure level by snap 
moving away from engagement. 
The adjusting member is preferably moved by operation of a screw and the 
diaphragm support member is constructed and arranged to resiliently engage 
the adjusting member at all times after calibration so that the adjusting 
member position is maintained by the support member engagement. This 
assures that the diaphragm support member and its adjusting member are 
maintained in their adjusted positions even when subjected to relatively 
severe vibrations. 
The body members are fixed to each other with the diaphragm and controlling 
mechanisms assembled in place. The assembly is calibrated by subjecting 
the diaphragm to differential pressures and adjusting the differential 
pressure level control mechanisms so that the diaphragm snaps from one 
position to another when predetermined pressure differentials are 
established. 
One of the body members defines a locating surface against which output 
device assembly is attached. A motion transmitting pin extends between the 
diaphragm and the output device so that operation of the diaphragm causes 
some output indication. The output assembly is, in the preferred 
embodiment, a switch whose conductive state is altered when the diaphragm 
operates. The switch housing abuts the body member locating surface so 
that an accurately fixed relationship exists between the assemblies. 
Other features and advantages of the invention will become apparent from 
the following detailed description of a preferred embodiment made with 
reference to the drawings which form part of the specification.

DESCRIPTION OF A PREFERRED EMBODIMENT 
An automotive air conditioning system 10 employing a pressure responsive 
control unit constructed according to the present invention is illustrated 
by FIG. 1. The system 10 is a conventional compressor-condenser-evaporator 
system including a refrigerant compressor 12, a condenser 14, an 
evaporator 16 and an expansion valve or throttling orifice 18 between the 
condenser and evaporator. The refrigerant compressor 12 compresses gaseous 
refrigerant and discharges it to the refrigerant condenser 14 which 
functions to transfer heat from and liquefy the refrigerant. As the 
liquefied refrigerant flows through the expansion valve 18 it is vaporized 
and as it passes through the evaporator absorbs heat. This heat absorption 
results in the evaporator cooling its surroundings. The relatively low 
pressure gaseous refrigerant exiting the evaporator 16 returns to the 
inlet of the compressor 12 and in the illustrated system a refrigerant 
accumulator 20 is disposed between the evaporator and compressor to 
accommodate changes in volume of the refrigerant in the system created by 
environmental temperature changes, etc. 
The compressor 12 is driven from the vehicle engine through an electrically 
operated clutch shown schematically and indicated by the reference 
character 22. Operation of the clutch 22 is governed by various control 
switches to assure that the compressor 12 is not driven from the engine 
when undesirable. The clutch 22 is typically connected in a circuit with 
the vehicle power supply such as a battery (not shown) and a manually 
operated switch (not shown) associated with the ignition switch which 
prevents the compressor 12 from being driven during cranking of the 
vehicle engine and a pressure responsive control unit 30, constructed 
according to the invention. 
The control unit 30 is illustrated as connected in pressure communication 
with the accumulator 20 and is effective to interrupt and close the clutch 
energizing circuit in response to detection of predetermined refrigerant 
pressure levels relative to atmospheric pressure. For example, the 
pressure control unit 30 can be set to cut in, or energize, the clutch 22 
when the refrigerant pressure rises to 46 pounds per square inch greater 
than atmospheric pressure and maintain the clutch energized until the 
refrigerant pressure falls below 26 pounds per square inch in excess of 
atmospheric pressure. 
This operation enables cycling of the compressor because the refrigerant 
pressure detected by the control unit 30 is reflective of the 
refrigeration load on the system 10. When the system refrigeration load is 
small the air flowing past the evaporator is relatively cool and the 
refrigerant passing through the evaporator absorbs less heat than it 
otherwise would. Hence the pressure detected by the control unit 30 is 
reduced relative to atmospheric air pressure, causing the clutch to be 
deactivated. When the air passing the evaporator is relatively warm, 
signifying a greater system refrigeration load, the refrigerant in the 
evaporator is heated and the pressure detected by the control unit 30 
rises until the clutch is reactivated and the compressor operates again. 
The control unit 30 also protects the compressor from damage as a result of 
being operated when the quantity of refrigerant filling the system 10 is 
inadequate. Inadequate refrigerant supply causes the control unit 30 to 
detect low refrigerant pressure and terminate operation of the compressor. 
In addition the control unit 30 discontinues operation of the compressor 
when ambient atmospheric temperatures are sufficiently low that 
refrigerant pressure falls below the predetermined low level. Operation of 
the compressor is generally unnecessary at low atmospheric temperatures. 
The illustrated and preferred control unit 30 comprises a differential 
pressure responsive transducer assembly 32 and an output assembly 34 the 
latter being, in the preferred embodiment, formed by an electrical switch 
assembly for completing and interrupting the clutch energizing circuit. 
Referring to FIG. 2 of the drawings the pressure transducer assembly 32 
includes first and second body members 40, 42, respectively, a pressure 
responsive snap acting diaphragm 44 which coacts with a diaphragm motion 
transmitting pin 46 to operate the output assembly 34, and high and low 
pressure event controlling mechanisms 48, 50, respectively, for 
controlling the differential pressure levels at which the diaphragm snap 
moves between alternate positions. 
The body member 40 is a rigid, structurally strong member preferably formed 
from steel and includes a generally cylindrical base 60 having a flange 62 
at one end and a through opening 64 extending along the axis of the base. 
The base opening 64 is internally threaded so that it can be screwed into 
place on a tube or valve stem of the refrigeration system 10 and the base 
may be provided with a sheet metal valve stem depressor 66 in the event 
the control unit 30 is used with a Schrader or similar valve of the same 
general type used to govern inflation of automotive tires. 
The flange 62 includes a bearing structure 68 which, in the preferred 
embodiment, is defined by an annular planar face disposed in a plane 
perpendicular to the axis of the base, a recessed section 70 radially 
inwardly from the bearing structure 68 and a cylindrical clamping wall 72 
disposed radially outwardly from the bearing structure and projecting 
axially beyond the plane of the bearing face 68. 
The body 42 is constructed from structurally strong, rigid material like 
the body 40 and includes a base 80, a flange 82 extending outwardly from 
the base and a central through opening 84. The illustrated flange 82 
includes a bearing structure 86 forming an annular face confronting the 
bearing structure 68 and a recessed section 88 radially inwardly from the 
bearing structure 86. The opposite axial side of the flange 82 includes a 
peripheral clamping shoulder 90 and a rear locating surface 92 upon which 
the output assembly 34 is mounted in a fixed relation to the pressure 
transducer assembly. 
The body members 40, 42 are firmly clamped together with the recessed 
sections 70, 88 aligned to define a chamber between the body members. As 
illustrated by FIG. 2, the body members are assembled with the bearing 
structures 68, 86 aligned and the clamping wall 72 is peened over about 
the clamping shoulder 90 so that the body members are tightly clamped in 
the assembled condition. 
The diaphragm 44 is preferably formed from a thin sheet of spring metal and 
extends across the chamber to form, with the recessed sections 70, 88, 
separate chamber sections on its opposite sides. The chamber section 
associated with the body member 40 communicates with referigerant in the 
system 10 via the opening 64 while the other chamber section is vented to 
ambient atmospheric pressure via the opening 84. The diaphragm 44 snap 
moves back and forth between the chamber sections depending upon the level 
of differential fluid pressure across the diaphragm. 
In the preferred embodiment the diaphragm 44 comprises an outer marginal 
section 110 formed by a planar annular rim, a circumferential corrugation 
112 which projects into the chamber toward the body member 40, a central 
dished section 114 engaging the motion transmitting pin 46 at its center, 
and a narrow annular transition section 116 between the dished section 114 
and the corrugation 112. 
The outer marginal diaphragm section 110 is connected to the bearing 
structure 68 by a hermetic bond, indicated by the reference character 118, 
which extends continuously about the diaphragm. In the preferred and 
illustrated embodiment the bond 118 is a narrow weld joint created by 
scanning a laser beam around the diaphragm so that the diaphragm material 
is melted and fused to the body 40 essentially along a narrow line of 
contact. The corrugation 112 provides for absorbing and reducing stresses 
applied to the disc during welding and when the diaphragm is clamped 
between the body members. 
The diaphragm central section abruptly reverses its curvature in response 
to the existence of predetermined pressure force levels acting on it and 
in so doing snap moves between its stable positions. As the diaphragm 
central section moves between its oppositely dished conditions the central 
section outer periphery expands and the expansion is accommodated by 
resilient deepening of the corrugation 112 until the central section 
passes through center, i.e., becomes flat, after which the central section 
periphery is again reduced. 
The diaphragm forms a spring which, in the illustrated unit, is biased 
toward its dished position illustrated by FIG. 2. When the refrigerant 
pressure force acting on the diaphragm exceeds the sum of the atmospheric 
pressure force, the biasing force of the diaphragm itself and the force 
transmitted to the diaphragm by the pin 46, the central section snap moves 
so that it is dished away from the body member 40. In this position the 
diaphragm central section has reversed its curvature, or snapped 
over-center, and the biasing force of the diaphragm spring is 
substantially reduced. 
The diaphragm returns to its illustrated position when the refrigerant 
pressure force level is less than the algebraic sum of the atmospheric 
pressure force, the diminished diaphragm biasing force and the force 
exerted by the pin 46. The difference in magnitude of the diaphragm 
biasing forces determines the differential between the refrigerant 
pressures at which the diaphragm moves. In a typical system 10 employing a 
refrigerant such as that known as R12 the respective refrigerant pressures 
at which the diaphragm changes position are, for example, 26 pounds per 
square inch above atmospheric pressure and 46 pounds per square inch above 
atmospheric pressure. 
It should be noted that the biasing force with the diaphragm resists 
shifting between its positions is reduced as the dished central diaphragm 
section moves toward center, i.e., toward the plane of the outer marginal 
portion 110. The high and low pressure event adjusting mechanisms 48, 50 
function to limit the motion and displacement of the diaphragm central 
section away from "center" and thus control the level of the diaphragm 
spring biasing force. This in turn governs the refrigerant pressure levels 
at which the diaphgram snap movement occurs. The diaphragm 44 is formed so 
that, if completely unrestrained by the mechanisms 48, 50, the diaphragm 
will snap move from its position illustrated by FIG. 2 when refrigerant 
pressure is around 55 pounds per square inch above atmospheric pressure 
and will snap move to the FIG. 2 position when refrigerant pressure is 
about 18 pounds per square inch above atmospheric pressure. 
The high pressure event adjustment mechanism 48 supports the diaphragm 
control section at a position where its biasing force is reduced to 
require the 46 psig refrigerant pressure to shift the diaphragm. The 
mechanism 48 includes a diaphragm support member 130 and an adjusting 
member 132 which reacts between the body 40 and the member 130 to control 
positioning of the diaphragm central section. 
The member 130 engages and supports the diaphragm with minimal stress 
concentrations being induced in the diaphragm and is constructed and 
arranged particularly to facilitate assembly of the diaphragm in body 
member 40. Referring to FIGS. 2 and 3 the member 130 includes a peripheral 
base 134 anchored to the body member 40, a medial portion 136 forming a 
face engageable with the central diaphragm section, and a deformable 
resilient portion 138 between the base and medial portions for enabling 
the medial portion 136 to be shifted by the adjusting member 132. 
The diaphragm support member 130 must be assembled to the body 40 before 
the diaphragm 44 is bonded in place. This creates a manufacturing problem 
because the diaphragm central section is, as noted, initially "overformed" 
and dished more deeply than it need be in order to respond to desired 
refrigerant pressure levels. The central section must not be engaged with 
the support member 130 during bonding in order to insure against stressing 
the weld and/or destroying the seal created by the weld. 
In order to facilitate assembly, the base 134 is seated against a body 
member locating shoulder 140 and firmly staked in place by upsetting body 
material, indicated by the reference character 142, along the base 
periphery. The resilient portion 138 is formed by an annular corrugation 
having spaced openings defining wide struts 139 between them. The 
corrugation is sufficiently deep to assure the medial portion 136 is 
spaced from the diaphragm central section as the diaphragm is bonded to 
the member 40. 
The adjusting member 132 is preferably a hollow screw threaded into the 
body member opening 64. The projecting screw end 148 engages the 
supporting member medial portion 136 so that as the screw advances it 
reacts between the body member and the diaphragm supporting member to 
shift the medial portion 136 toward the diaphragm. The opening through the 
screw 132 communicates system refrigerant pressure to the diaphragm and is 
shaped to receive a tool for driving the screw. In the illustrated 
embodiment the screw end 148 engages standoffs embossed in the medial 
portion 136. The standoffs avoid the possibility of the engagement between 
the screw end 148 and the medial portion 136 blocking pressure 
communication through the screw opening. 
The struts 139 flex to enable substantial corrugation "rolling" when the 
screw 132 is advanced to move the supporting member medial portion 136 
into engagement with the diaphragm. The struts 139 resiliently resist 
advancement of the screw so that when the diaphragm support member 130 
reaches its adjusted position, illustrated by FIG. 2, the struts 139 
continue to be resiliently deflected. The diaphragm support member thus 
both resiliently engages the screw end 148 to frictionally lock the screw 
in its adjusted position and and rigidly supports the diaphragm central 
section in position. The force exerted by the diaphragm support member on 
the screw maintains the screw locked in its adjusted position when the 
diaphragm central section is disengaged from the support member 130 
notwithstanding the vibrations, temperature induced differential expansion 
and conraction; etc., to which the control unit is subjected in use. 
The low pressure event controlling mechanism 50 supports the diaphragm 
after refrigerant pressure in the system has increased to a level where 
the diaphragm is snapped away from the support member 130. The mechanism 
50 supports the diaphragm in a position where, when the refrigerant system 
pressure reaches a predetermined level below the high pressure event 
level, the diaphragm snaps back into position against the support member 
130. The mechanism 50 also rigidly supports and retains the diaphragm when 
the refrigerant system pressure increases substantially above the high 
pressure event level such as when the system is exposed to relatively high 
ambient atmospheric temperatures at a time when it is not operating. The 
mechanism 50 includes a diaphragm supporting plate 50 and an adjusting 
member 152 for reacting between the supporting plate 150 and the body 
member 42. 
The supporting plate 150 is formed by a relatively heavy spring metal disk 
having an outer annular peripheral portion 154 anchored between the body 
members and a stiffly resilient central portion 156 extending across the 
chamber defining a smoothly concave face with a small central guide 
opening 160 for the pin 46. The supporting plate outer peripheral portion 
154 engages and supports the diaphragm 44 radially outwardly from the 
diaphragm corrugation 112 while the supporting plate central portion 
defines a narrow zone 162 of bearing contact with the diaphragm transition 
section 116, just inside the diaphragm corrugation. 
The contact zone 162 engages the diaphragm throughout the operational 
pressure range of the control unit so that the diaphragm flexure is 
limited to the diaphragm central section radially inwardly from the zone 
162. The diaphragm section 116 rolls on the bearing zone 162 at the inside 
diameter of the corrugation when the central section changes its 
curvature. Because of this motion stresses at the juncture of the 
diaphragm central section and the transition section 116 are reduced. It 
should be noted that the refrigerant pressure is always greater than 
atmospheric pressure so that the diaphragm engages the bearing contact 
zone at all times. 
In the preferred embodiment of the invention the outer peripheral portion 
154 is very slightly frustoconical and merges with the concavely curved 
central portion 156 at a reversely curved juncture 158 (See FIG. 5). The 
juncture 158 is aligned with the diaphragm corrugation so that the 
diaphragm does not engage the juncture 158. This avoids the diaphragm 
material being reversely curved by being forced into conformity with the 
juncture 158. Stress concentrations which otherwise would quickly fatigue 
the diaphragm and cause fracturing are thus avoided. 
The body members 40, 42 are clamped together with sufficient force that the 
frustoconical peripheral plate portion 154 is flattened and urged against 
the diaphragm peripheral section radially outwardly from the corrugation 
to the diaphragm outer periphery, including that portion of the diaphragm 
secured to the body member 40 by the bond 118. This eliminates any 
tendency of the diaphragm peripheral section to toggle as a result of 
pressure applied to it and otherwise isolates the bond 118 from stress. 
The adjustment member 152 is formed by a hollow screw threaded into the 
body member opening 84. The motion transfer pin 46 extends through the 
screw opening between the diaphragm 44 and the output assembly 34. The end 
of the screw 152 projecting into the chamber between the body members 
engages the diaphragm supporting plate central portion. As the screw is 
advanced the diaphragm supporting plate 150 is resiliently deflected 
toward the diaphragm to increase the refrigerant pressure level at which 
the diaphragm snaps toward engagement with the diaphragm support member 
130. Retraction of the screw is accompanied by resilient return of the 
diaphragm support plate toward its undeflected position which in turn 
provides for greater diaphragm flexure and reduces the refrigerant 
pressure level required for the diaphragm to move away from the supporting 
plate. 
The stiffness and structural strength of the diaphragm supporting plate 
150, together with the support offered by the screw 152, firmly support 
the diaphragm against overstressing even under conditions where the 
refrigeration system pressure is extremely great. Overpressure conditions 
of this sort can yield an unsupported diaphragm as well as a diaphragm 
supported (or partially supported) by a member of lesser strength and 
rigidity. In cases where partially supported diaphragms are subjected to 
overpressure conditions the diaphragms tend to be reversely curved and 
fracturing of the diaphragms tends to ensue. 
The supporting plate 150 resiliently engages the screw 152 constantly so 
that the screw is frictionally maintained in position in the body member 
42 without requiring a separate thread locking device and notwithstanding 
vibrations to which the unit is subjected. The end of the screw 152 remote 
from the diaphragm projects from the body member 42 and is formed with 
external tool engaging flats which permit adjusting the screw's position 
by a suitable driving tool. 
The pressure transducer assembly 32 is assembled by pressing the body 
members 40, 42 together and clamping them in place by deforming the 
clamping wall 72 to turn its projecting end portion radially inwardly to 
grip the clamping shoulder 90. As noted, the clamping force is 
sufficiently great to assure that the diaphragm supporting plate 
peripheral poriton 154 is flattened against the diaphragm peripheral 
section. The mechanisms 48, 50 are fully retracted. In this initially 
assembled condition the transducer assembly is stress relieved by placing 
it in an atmosphere at around 450.degree. F. and maintained at temperature 
while the pressure in the chamber section 70 is cycled several times 
between atmospheric pressure and about 250 psi above atmospheric pressure. 
Calibration of the pressure transducer assembly 32 involves a number of 
operational steps and adjustments requiring continued adjustability of the 
diaphragm supporting plate and the member 30. An important advantage of 
the control unit construction resides in the ability to accurately 
calibrate its pressure response characteristics before the output assembly 
34 is attached to it. 
In order to calibrate, the stress relieved transducer assembly 32 (with the 
fitting 66 removed, or not yet in place) is connected to a controllable 
source of pressurized calibrating gas via the body opening 64. The 
calibration gas source (not illustrated) is provided with a rotatable 
screw driving tool which extends into the high pressure event adjusting 
screw 132 for turning it while the source gas pressure is applied to the 
diaphragm 44. The calibration source pressure is elevated until the 
unbalanced force applied to the diaphragm is sufficiently great to move 
the diaphragm into engagement with the supporting plate 150 (the 
calibration source pressure is raised to about 55 psi above atmospheric 
pressure). 
The low pressure event controlling mechanism 50 is then precalibrated by 
reducing the calibration source pressure to a few pounds per square inch 
less than the desired low pressure event level (about 23 psi) and the 
screw 152 is advanced to resiliently deflect the plate 150 and shift the 
diaphragm toward the high pressure event support member 130 until the 
diaphragm snaps into engagement with the member 130. 
The high pressure event controlling mechanism 48 is next precalibrated by 
adjusting the calibration source pressure to a few pounds per square inch 
above the desired high pressure event level (for example 49 psi) and 
advancing the screw 132 until the diaphragm 44 snaps into engagement with 
the plate 150 again. 
At this juncture the calibration source pressure is elevated to about 450 
psi (substantially greater than the highest predictable pressure 
encountered during use of the control) so that the plate 150 is seated 
firmly against the screw 152. 
The low pressure event controlling mechanism is next calibrated by reducing 
the calibration source pressure to the desired low pressure event level 
(26 psi) and advancing the screw 152 until the diaphragm 44 snaps into 
engagement with the member 130. 
Final calibration of the high pressure event controlling mechanism 48 is 
accomplished by increasing the calibration source pressure to the desired 
high pressure event level (46 psi) and advancing the screw 132 until the 
diaphragm snaps back into engagement with the plate 150. 
It should be noted that the resilient movement of the supporting plate 
central portion 156 to its calibrated position causes a slight movement of 
the zone of bearing contact 162 towards the member 130. This slight 
movement of the bearing contact zone 162 results in a change in the high 
pressure level at which the diaphragm is snapped away from the member 130. 
Accordingly the low pressure event controlling mechanism 50 must always be 
calibrated before the final calibration of the high pressure event 
controlling mechanism 48. 
After the pressure event levels have been set the calibration source 
pressure is again increased to 450 psi briefly to assure seating the plate 
150 on the screw 152. The calibration source pressure is then reduced to 
correspond to the low and high pressure event settings to determine 
whether the diaphragm snaps back and forth at the desired pressure event 
levels. 
If the low pressure event level is not responded to within an acceptable 
tolerance range the calibration proceedure just outlined is repeated to 
recalibrate the transducer. The resilient flexure of the plate central 
portion 156 by the screw 152 enables recalibration since the plate 150 can 
readily be repositioned for the precalibration step. 
If the high pressure event level is not responded to accurately enough the 
high pressure event controlling mechanism 48 can merely be readjusted to 
the desired level without affecting the calibrated low pressure event 
level. This is due to the fixed location of the bearing engagement between 
the supporting plate 150 and the diaphragm along the bearing region 162 
which is maintained whether the diaphragm is supported by the plate 150 or 
the member 130. 
After the transducer assembly calibration the output assembly 34 is fixed 
in place to the transducer assembly 32 by a deformable clamping collar 
170. The output assembly 34 comprises a switch 172, output terminals 174, 
176 and a support housing 178. The housing 178 is a rigid dielectric 
plastic molded part having a barrel 180 surrounding the switch, an end 
flange 182 abutting the transducer assembly and engaged by the clamping 
ring 170 and a terminal supporting end 184 through which the terminals 
extend. 
The switch 172 is formed by a nonmoving contact arm 190 adjustably 
supported by the housing end 184 and a moving contact arm 192. The moving 
contact arm 192 is formed by an electrically conductive leaf spring which 
carries a contact at its free end and is connected to the terminal 174 at 
its opposite end. The leaf spring is engaged by the pin 46 and when the 
diaphragm is snapped between its positions the leaf spring resiliently 
deflects to open or close the switch contacts. 
The nonmoving contact arm 190 is formed by a leaf spring supporting a 
contact at its projecting end and fixed to the terminal 176 at its 
opposite end. An adjusting screw 194 is threaded in the housing end wall 
184 and engages the nonmoving contact arm 190. That contact arm can be 
shifted by advancing or retracting the screw to accomodate for tolerance 
variations in the output and transducer assemblies. 
Both the moving and nonmoving contacts are supported cantilever fashion by 
their respective leaf springs so that the snap closure of the contacts is 
cushioned and the contacts are able to roll slightly with respect to each 
other upon opening and closure of the contacts. Overtravel of the pin 46 
causes resilient deflection of the springs improving the electrical 
continuity between the contacts and aiding abrupt separation of the 
contacts when they are disengaged. 
The head of the screw 194 is recessed in the housing end 184 to isolate the 
screw from contact after adjustment of the nonmoving contact position. It 
should be noted that the motion transmitting pin 46 carries an end cup 
engaging the switch arm 192 which is formed of a dielectric material. This 
electrically insulates the transducer assembly from the switch contact arm 
192. 
The switch housing end flange 182 is formed with an annular locating land 
196 which engages the locating face 92 on the transducer assembly when the 
transducer and output assembly are assembled together. This coaction 
assures accurate location of the assemblies relative to each other. 
The assemblies are maintained forcibly urged together while the collar 170 
is placed about the assembly and crimped to provide opposed collar lips 
170a, 170b which engage the body member flange 62 and the switch housing 
flange 182, respectively. 
While a single preferred embodiment of the invention has been illustrated 
and described in detail the invention is not to be considered limited to 
the precise construction shown. Various modifications, adaptations and 
uses of the invention may become apparent to those skilled in the art to 
which the invention relates and the intention is to cover all such 
modifications adaptations and uses which come within the spirit or scope 
of the appended claims.