Transmitter with moisture draining housing and improved method of mounting RFI filters

A transmitter transmitting a sensed process variable over a conductor includes a cylindrical housing having a terminal compartment and an electronics compartment separated by a bulkhead. The terminal compartment used for conductor connection is subject to moisture accumulation. An access channel fully intersects an internal surface of terminal compartment such that the access channel drains moisture from the terminal compartment across a range of mounting orientations. The transmitter includes a circuit in the electronics compartment for compensating a process variable and providing the compensated process variable via a feedthrough circuit assembly in the bulkhead, which in one embodiment, includes an encapsulated radio frequency interference filter, to terminals in the terminal compartment for coupling to a conductor that is connected to an external controller.

FIELD OF THE INVENTION 
The present invention relates to transmitters used in industrial process 
control systems. In particular, the present invention relates to a process 
variable transmitter suitable for high humidity or wet environmental 
operational conditions. 
BACKGROUND OF THE INVENTION 
Transmitters sense process variables in a variety of applications such as 
oil and gas refineries, chemical storage tank farms, or chemical 
processing plants. A process variable (PV) is a sensed parameter of a 
process or sensed property of a product, including absolute pressure, 
differential pressure, temperature, flow, material level, etc. One common 
transmitter application uses a transmitter to sense a PV representative of 
a process and transmits the sensed PV to a controller over cabling. For a 
two wire transmitter, the cabling is a twisted two wire cable set. The 
transmitter and controller are electrically cabled in series forming a 
current loop. The transmitter receives no external power and derives all 
its operating power from the current loop. Typically, the transmitter 
regulates the magnitude of current in the current loop, as a function of 
the sensed PV. In one standard protocol, the current ranges between 4 and 
20 mA. Three and four wire transmitters use other cabling as appropriate. 
Transmitters commonly have a cylindrically shaped housing with a bulkhead 
separating the housing into two compartments, with each compartment capped 
by a threaded cover. A cylinder as used in this specification is defined a 
solid bounded by a given curved surface and two parallel planes. An 
electronics compartment houses electronics for sensing and compensating 
the PV, and a terminal compartment houses terminals to connect the 
compensated PV to the cabling. The bulkhead has an electronics feedthrough 
between the compartments. The terminal compartment includes an externally 
threaded access channel through which the cabling enters the transmitter 
housing to connect to the transmitter. Many transmitter housings have two 
threaded access channels in the terminal compartment for connecting to 
external hollow electrical conduit. The hollow conduit forms a passageway 
between the controller and the transmitter which protects the cabling 
inside. The cabling typically contains two conductors for a two wire 
transmitter. The location of the access channels varies on transmitter 
housings, ranging from the top to the bottom of the housing. For 
temperature transmitters, the top is that side of the transmitter housing 
opposite the mounting boss. For pressure and other types of transmitters, 
the top is that side of the transmitter housing opposite the process 
sensor location. 
Although transmitters are commonly used in various rugged industrial 
applications, problems have arisen when a transmitter is installed in a 
humid or high moisture operating environment. With the exception of 
hermetically sealed transmitters, moisture accumulation in the terminal 
compartment is a common problem encountered by transmitter designs. 
Hermetically sealed transmitters are costly and difficult to configure or 
repair as the hermetic seal is typically welded shut. In non-hermetically 
sealed transmitter, moisture condenses within the housing, sometimes 
filling the housing if not drained periodically. This moisture 
accumulation causes electrical shorting between terminals in the terminal 
compartment, cross talk or growth of organic or dendritic metallic matter 
which degrade the transmitter's performance. A dendritic growth is caused 
by a metallic filament formed from metal ions transported by a liquid on 
an insulating surface, the filament growing under the influence of a DC 
voltage bias. If the filament bridges across conductors, it can create low 
impedance leakage paths. A related problem to the condensate accumulation 
is intrusion of moisture into the transmitter. An unsealed or even an 
improperly hermetically sealed transmitter accumulates moisture inside if 
subjected to directed moisture such as pressure washing or driving rain. 
The detrimental effects of accumulated moisture are the same as condensate 
accumulation. In PRIOR ART FIG. 1, a transmitter shown generally at 50, 
has a terminal compartment 52 from which it is difficult to drain 
accumulated moisture. A pair of access channel openings 54 are located at 
a top 56 of transmitter 50 such that any moisture entering through the 
access channels 54 falls to the bottom of terminal compartment 52 and is 
trapped. Even if transmitter 50 is rotated and mounted 90.degree. in 
orientation, terminal compartment 52 remains partially filled to a 
waterline 58 as access channels 54 do not fully drain trapped moisture. 
Wall structure 60 within the transmitter housing juts out from the inner 
surfaces of compartment 52 so that even when transmitter 50 is mounted 
sideways, moisture must accumulate to the level of waterline 58 before 
draining. 
Moisture can also degrade the effectiveness of a radio frequency 
interference (RFI) filter or feedthroughs in a transmitter. To minimize 
the effects of an electrically noisy process environment, PV electronics 
are commonly shielded in a Faraday cage formed in the transmitter by the 
electronics compartment, an access cover and RFI filters on electrical 
signal connections. If any one element of Faraday cage is compromised, the 
desired isolation is rendered ineffective and degrades transmitter 
performance. RFI filters typically include a mechanical case having a 
threaded exterior for screwing the RFI filter into the bulkhead. Moisture 
becomes a problem for the RFI filter when moisture accumulates across the 
conductor and the RFI filter case. A low impedance leakage path may be 
created between the case of the RFI filter and the conductor, thereby 
compromising the electrical isolation of the conductor. Another problem 
for the RFI filter arises in keeping the bulkhead watertight. The RFI 
filter screws into the bulkhead with conductors exposed on each side of 
the bulkhead, creating a seal. However, the sealing insertion force or 
torque required to screw in the threaded RFI filters undesirably stresses 
the RFI filters. A stressed RFI filter may sometimes degrade the 
transmitter performance by not providing the desired electrical isolation, 
and if detected during assembly can require substantial rework in the 
manufacturing process. 
Therefore, a transmitter is desired which promotes the draining of 
accumulated moisture from within the transmitter. Another characteristic 
desired of the transmitter is a reliable feedthrough circuit that is 
assembled in a manner that does not stress the components during assembly. 
SUMMARY OF THE INVENTION 
A transmitter transmitting a sensed process variable over a conductor 
includes a cylindrical housing having a terminal compartment and an 
electronics compartment separated by a bulkhead. The terminal compartment 
used for conductor connection is subject to moisture accumulation. An 
access channel fully intersects an internal surface of the terminal 
compartment such that the access channel drains moisture from the terminal 
compartment across a range of mounting orientations. The transmitter 
includes a circuit in the electronics compartment for compensating a 
terminal variable and providing the compensated process variable via a 
feedthrough terminal in the bulkhead, which in one embodiment, includes an 
encapsulated radio frequency interference filter, to process terminals in 
terminal compartment for coupling to a conductor that is connected to an 
external controller.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
In FIGS. 2 and 3, a transmitter 100 includes a housing 102. Housing 102 has 
two compartments separated by a bulkhead 106; an electronics compartment 
108 and a terminal compartment 110. Housing 102 has a top 112, a base 114, 
a threaded terminal end 116 and a threaded electronics end 118. Base 114 
of housing 102 has a suitable mounting boss area 126 machined into housing 
102. Housing 102 is cylindrical in shape with the central axis of the 
cylinder shape running from the center points of round electronics end 118 
and round terminal end 116. The terminal compartment 110 is formed by 
capping terminal end 116 with a cover 128. The electronics compartment 108 
is formed by capping electronics end 118 with a cover 130. Terminal 
compartment 110 and electronics compartment 108 share bulkhead 106 that 
forms the back of each compartment 108,110. The surface of bulkhead 106 in 
terminal compartment has a retaining wall 120 formed on it; retaining wall 
120 integrally attaches to the internal surface of the terminal 
compartment 110. The surface of bulkhead 106 in electronics compartment 
108 has a feedthrough mounting boss 124 disposed on it. A feedthrough 
circuit opening 141 is formed through bulkhead 106 to facilitate 
communication of electrical signals between compartments 108,110. Housing 
102 is preferably formed by a die or impression casting method and then 
machined to bring mounting bosses 126,124 and component areas 108, 110 
into a desired tolerance and smooth finish. Covers 128,130 preferably 
includes a male thread set 129A,131A and a matching female thread set 
129B, 131B contained in an inner rim of compartments 108, 110. The 
particular thread interface of covers 128,130 provides a stronger barrier 
to flame pathways than another thread interface. In the unlikely event of 
an explosion with in transmitter, the overpressure forces the threads 
129A-B,131A-B into each other rather than being forced apart. In FIGS. 
4A-B, terminal compartment 110 includes a terminal block assembly 132 for 
connecting a cable 134 containing a pair of conductors 134A-B to a 
controller (not shown). Integral retaining wall 120 divides terminal 
compartment 110 into a component mounting area 138 and a watershed area 
140. A pair of access channels 136,137 enter housing 102 through the wall 
of terminal compartment 110 at base 114 of transmitter 100. At the time of 
installation, conduit 158 is threaded into one of access channels 136,137 
through an NPT connector 160 or the like and conductors 134A-B are 
individually connected to terminals 132A-B on terminal block assembly 132. 
Watershed area 140, which is the area below retaining wall 120, includes 
the internal openings of access channels 136,137 and a cable guide 156. 
In FIG. 3, electronics compartment 108 includes a process variable (PV) 
electronics 104 of a suitable design. Electronics 104 receive a sensed 
temperature signal from a temperature sensor (not shown). Electronics 104 
compensate the sensed temperature signal for known repeatable errors and 
output a current signal representative of the sensed temperature to 
terminal block 132. Electronics 104 and conductors 134A-B, attached to 
terminal block 132 are electrically connected through bulkhead 106 by a 
feedthrough circuit assembly 142 which includes eight RFI filters 144 and 
a mounting plate 166. All electrical signals connected to electronics 104 
pass through signal pins on RFI filters 144. Electronics compartment 108 
is protected from the environment by access cover 130. The feedthrough 
circuit assembly 142 and the feedthrough circuit opening 141 are filled or 
potted with an encapsulant 154 that seals the bulkhead 106 and electronics 
compartment 110. Transmitter 100 can be configured to provide an output 
representative of other sensed process variables such as absolute 
temperature, differential temperature, differential pressure, absolute or 
gauge pressure, flow, pH or others, when used with appropriate electronics 
104. 
In FIGS. 4A-B, a pair of standard mounting arrangements are shown with 
transmitter 100 being mounted to a bracket 162 which attaches to mounting 
boss 126. Bracket 162 is attached to a post 164 or the like. In either 
mounting orientation, one of the access channels 136,137 is connected to 
conduit 158 and acts as a drain. In an upright mounting orientation, as 
shown in FIG. 4A, either access channel 136 or 137 functions equally well 
as the drain. As transmitter 100 is mounted in a clockwise (or 
counterclockwise) direction from upright, the lower one of access channels 
136,137 is the drain. FIGS. 4A-B both show a lowest point 149, which is 
the point to which moisture drains within transmitter 100 before exiting 
compartment 110. Lowest point 149 changes location as transmitter 100 is 
mounted in various mounting orientations. 
The draining structure of compartment 110 includes retaining wail 120, 
watershed area 140 and access channels 136,137. All internal surfaces of 
the compartment 110 are smoothed to provide a continuous cast surface to 
concentrate moisture into watershed area 140. All joining surfaces within 
compartment 110 are filleted for the same purpose. Smooth surfaces and 
filleted joints limit the available area for droplets to attach, and with 
the force of gravity, urge formed droplets towards watershed area 140. 
The intersections of access channels 136,137 with the inner surface of 
terminal compartment 110 are flush with the inner surfaces of compartment 
110. No other structure inside compartment 110, such as that shown at 60 
in PRIOR ART FIG. 1, obstructs moisture from draining out channels 
136,137. Preferably, those sections of channels 136,137 which extend 
outside housing 102 are declined downward to facilitate enhanced drainage. 
In FIG. 4A, transmitter 100 fully drains moisture within a 120 degree 
mounting range (i.e. 60 degrees offset in either direction from upright), 
as indicated by dashed line 147. In the same drawing, transmitter 100 
allows a small amount of moisture to accumulate in compartment 110, but 
not enough to contact electronics or terminals, when mounted over a full 
180 degrees. 
A preferred embodiment of the present invention is shown in FIG. 4C, which 
enlarges the area in FIG. 4B around the intersection of access channels 
136,137 with the inner surface of compartment 110. Specifically, the 
corners in the space indicated at 145 in FIG. 4C have been leveled and 
flared out, and the resulting surface smoothed to allow drainage over a 
full 180 degrees of mounting orientations (i.e. 90 degrees offset in 
either direction from upright). The upper section of the internal 
intersection of access channels 136,137 with the internal wall of 
compartment 110 has been flared to lower the level of waterline 143 (shown 
in FIG. 4B) to the level of waterline 151. Terminal block 132 and 
conductors 132A-G are permanently above water line 151 and cannot be 
wetted by accumulated moisture. All electrical connections between 
terminal block 132 and conductors 134A-B are made above the access 
channels 136,137, keeping the electrical connections dry. Flared areas 
136A and 137A (not shown) are formed during casting or machined 
thereafter, but are formed in the existing wall structure without adding 
material to housing 102. Flared areas 136A,137A may be narrow channels 
formed in the internal wall of compartment 110 and extending from access 
channels 136,137 to lowest point 149, or widened drainage flowages as 
shown in FIG. 4C. In all cases, flared areas 136A,137A are blended into 
the internal surface of watershed area 140 to reduce sharp edges and 
facilitate moisture drainage. 
Cable guide 156 deflects inserted twisted pair conductors 134A-B towards 
terminal block assembly 132. The redirection of cabling 134 facilitates 
the distribution of conductors 134A-B and prevents the conductors from 
entangling. Access channels 136,137 are preferably located opposing each 
other, on the base 114 of terminal compartment 110 to allow one cable 
guide 156 to direct cabling for both channels 136,137. Cable guide 156 is 
cast or mounted on the inner surface of compartment 110 between access 
channels 136,137 and is beveled to allow moisture flow off cable guide 156 
into access channels 136,137. Conductors 134A-B are splayed out to 
separate connections on terminal block 132. Conductors 134A-B are 
typically the same length, and each conductor will have excess length in 
all but the longest connection path to terminal block 132. A terminal 
block bezel 131 is made of a non-conductive plastic and has a horseshoe 
curved shape to allow conductors to be centrally distributed. The excess 
lengths of conductors 134A-B are stored within compartment 110. Removing 
non-essential structure in watershed area 140 and the shape of terminal 
block assembly 132 allows more room in compartment 110 to store excess 
lengths of conductors, thereby obviating shorting caused by pinched 
conductors when the cover is installed. 
In FIGS. 3 and 6, an interface circuit card 152 mounts into component 
mounting area 138 in compartment 108, and then plastic bezel 131 is placed 
over card 152 with screw terminals 132 protruding through bezel 131. Bezel 
131 insulates and spaces terminals 132 and also protects electrical 
components on interface circuit card 152. The case of each of the eight 
RFI filters 144 is soldered into a conductive mounting plate 166, so as to 
make a completed feedthrough circuit assembly 142. Assembly 142 is 
inserted through opening 141 in bulkhead 106 and mounted with metal screws 
to an integral mounting boss 124. Mounting plate 166 also positions RFI 
filters 144 for encapsulation. The screws which mount assembly 142 to 
bulkhead 106 electrically ground the case of filters 144 to housing 102. 
Interface circuit card 152 provides mechanical mounts for screw terminals 
132 and electrically connects signal pins on filters 144, some of which 
represent the compensated process variable, to the screw terminals. 
An encapsulant 154 is introduced around circuit assembly 142, on the 
terminal compartment 110 side of bulkhead 106, so as to completely seal 
one compartment from the other. Encapsulant 154 is preferably an epoxy 
potting compound, but may also be made of any curable potting compound. 
Component mounting area 138 is filled with enough encapsulant 154 to rise 
to the level of the height of retaining wall 120. Once filled, there are 
no recesses or hollows within component mounting area 138 where moisture 
can collect and which contribute to electrical leakage. Any moisture which 
may form within area 138 is urged, with the force of gravity, toward 
watershed area 140 and out access channels 136,137. Aside from the 
enhanced draining feature encapsulant provides, encapsulant 154 provides a 
substantially infinite impedance between the case and the signal pins of 
the RFI filters, thereby substantially limiting leakage current between 
signals and electrical ground. The present invention permanently installs 
RFI filters 144 without a potentially damaging torque action, while 
augmenting the environmental isolation between compartments and the 
electrical isolation between signals and ground. Once encapsulant 154 
cures, signal pins of filters 144 on the terminal compartment side are 
connected to signal connection points on electronics 104. 
RFI filters 144 are typically composed electrically of the .pi., L or C 
filter types for suppression of high frequency noise. A commercially 
available RFI filter 144 configuration typically consists of a ceramic 
capacitor shaped as a hollow cylinder. A conductive material on both the 
inside and the outside surfaces of the cylinder forms the capacitor. The 
external surface of the cylinder is grounded to the case. The inside 
surface of the cylinder is electrically connected to a conductor (i.e. the 
signal pin), which vans the length of the filter. Other forms of RFI 
filters 144 may include an inductor/capacitor combinations, a current 
shunt, a series inductive barrier or other types of electrical noise 
filtering electronics. 
The present invention provides a transmitter design that resists electrical 
faults due to accumulated moisture by draining away moisture as it 
accumulates over a broad range of mounting orientations. The transmitter 
of the present invention drains moisture over a wide range of mounting 
configurations without additional external hardware or special conduit 
drains. The placement of the oppositely located access channels 136,137 at 
the base 114 also orients cabling 134 entering the transmitter 100, 
facilitating connection of conductors to terminals. Encapsulation 154 
provides improved electrical and environmental isolation, while 
permanently positioning RFI filters 144 and obviating torsional stress 
during installation. Furthermore, filling component mounting area 138 with 
encapsulant to a level coincident with the height of retaining wall 120 
ensures that moisture is channeled into watershed area 140 and ultimately 
exits the access channels. 
The manner and content of the present invention disclosed herein is 
described with reference to a preferred embodiment. Workers skilled in the 
art will recognize that changes may be made in form and detail without 
departing from the spirit and scope of the invention.