Rice Lake Weigh Modules/Mount Assemblies - Manual (Page 12)

SYSTEM DESIGN
1-10
Design Elements
Choosing the Correct Load Cell
Misuse of any product can cause major cost and safety problems;
load cells are no exception. Unfortunately, the load cell protection
rating systems used in the industry today are inadequate in some
ways. That’s why Rice Lake Weighing Systems, with years of load
cell experience, has developed its own rating system for load cells.
Our system categorizes load cells in two major groups: hermeti-
cally-sealed (HS), and environmentally-protected (EP). Hermeti-
cally-sealed cells are then further characterized by IP (Interna-
tional Protection) numbers. We feel this system effectively matches
load cell to application for optimal results.
To choose the proper load cell protection qualities, a fundamental
understanding of the differences between “environmentally-pro-
tected” and “hermetically-sealed” load cells is necessary. The
inappropriate use of environmentally-protected load cells in harsh
conditions is a prescription for load cell failure. Because of the
extra manufacturing steps, hermetically-sealed load cells cost
more than environmentally-protected versions. Despite the higher
initial cost, hermetically-sealed load cells may be the best long-
term choice for harsh chemical, washdown, and unprotected
outdoor applications.
Environmentally Protected
Environmentally-protected load cells are designed for “normal”
environmental factors encountered in indoor or protected outdoor
weighing applications. By far the most popular type, these load
cells may employ strategies like potting, rubber booting, or redun-
dant sealing to afford some protection from moisture infiltration.
Potted load cells utilize one of several types of industrial potting
materials. The liquid potting material fills the strain gauge cavity
then gels, completely covering the strain gauge and wiring sur-
faces. While this may significantly diminish the chance of mois-
ture contamination, it does not guarantee extended waterproof
performance, nor does it withstand corrosive attack.
A second method of protection uses an adhesive foam-backed plate.
This protection affords some moisture and foreign object protec-
tion, but less than potted cells. In many cases, manufacturers will
use a caulking material to seal the plate to decrease the potential
for cavity contamination. A common approach among manufac-
turers to further decrease the entry of moisture to the strain gauge
combines both a potted cavity and a foam-backed plate, in a process
called redundant sealing.
Yet another strain gauge cavity protection strategy is the rubber
boot. Commonly employed with cantilever and bending beam
models, the boot covers the cavity and is secured by clamps. While
this provides easy access for repairs, the boot may crack if not
lubricated regularly, allowing contaminants into the load cell
cavity. Lubricating the rubber boot during routine inspections will
contribute to the long-term durability of the load cell.
Protecting the strain gauge cavity is just one consideration in
protecting a load cell from contamination. Another susceptible
area is the cable entry into the body of the load cell. Most environ-
mentally-protected load cells incorporate an “O” ring and cable
compression fitting to seal the entry area. This design provides
protection only in applications with minimal moisture. In high-
moisture areas, it is safest to install all cabling in conduit, provid-
ing both a moisture barrier and mechanical protection.
Although environmentally-protected load cells keep out unwanted
contaminants, they are not suited for high moisture, steam, or
direct washdown applications. The only long-term strategy for
these applications is to use true hermetically-sealed load cells.
Hermetically Sealed
Hermetically-sealed load cells offer the best protection available
for the weighing market. Using advanced welding techniques and
ultra-thin metal seals, these load cells handle the extremes of
harsh chemical and washdown applications. What makes the seal
unique is the process of laser-welding metal covers to protect the
strain gauge and compensation chambers. The cavities are then
injected with potting or, in the case of glass-to-metal seals, filled
with a pressurized inert gas, providing a redundant seal. As a final
assurance of the integrity of the seal, a leak test is conducted to
reveal any microscopic flaws in the sealing weld.
True hermetic protection addresses both the strain gauge cavity
and cable entry area. The most advanced cable entry design
employs a unique glass-to-metal bonding seal which makes the
cable termination area impervious to moisture. Cable wires termi-
nate at the point of connection to the load cell, where they are
soldered to hermetically-sealed pins that carry signals to the sealed
strain gauge area through a glass-to-metal seal. Water or other
contaminants cannot “wick up” into the load cell, since the cable
ends at the entry point. This design allows for field-replaceable
cable, since the connection is outside the load cell.
A word of caution: stainless steel load cells are not synonymous
with hermetically-sealed load cells. While environmentally-
protected stainless steel load cells may be suitable for dry
chemical corrosive environments, hermetically-sealed stainless
steel models are the appropriate choice for high moisture or
washdown applications.