Rice Lake Z6 Single-Ended Beam, SS Welded-seal, IP67, OIML C3 - Manual (Page 8)

4 Load Cell and Weigh Module Handbook
3.0 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: hermetically sealed (HS), and
environmentally protected (EP). Hermetically sealed cells are
then further characterized by IP (Ingress 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 protected” 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.
3.1 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 redundant 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 surfaces. While this may significantly diminish the
chance of moisture contamination, it does not guarantee
extended waterproof performance, nor does it withstand
corrosive attack.
A s e c o n d m e t h o d o f p r o t e c t i o n u s e s a n a d h e s i v e
foam-backed plate. This protection affords some moisture
and foreign object protection. In many cases, manufacturers
will use a caulking material to seal the plate to decrease the
potential for cavity contamination. A common approach
among manufacturers 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
environmentally 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, providing 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.
3.2 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 terminate 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.