Sierra Monitor Gas Sensor User Manual

SENSORS: SIMPLE TO SMART TO SYSTEMS

PC-based software plus an
intelligent remote controller
provide the flexibility to
configure sensor system
inputs, outputs, and data
acquisition to a plant’s
specific needs. The result
is greater visibility of plant
operations, allowing better
decisions to be made.

ensors are in our home,

S

transportation, and workplace
affect our safety , environment, and
productivity. You awoke this
morning in a warm room because a
sensor monitored the room
temperature. The car you drove to
work has sensors for temperature,
oxygen emissions, and more. Its
control panels are covered with
lights, gauges, and switches. Yet, conflicting data,
possible burned-out lights, and trying to read too many
gauges, lights and indicators repidly can make your job
difficult and prone to problems.

Sensors are the primary building blocks that provide the
vital function of acquiring data and transmitting it to
human operators. Just as sensors have permitted a
simplified way of flying commercial airlines, sensors
provide the necessary information for efficient and
effective operation of every process and facility .

Sensors must operate under diverse conditions, 24 hours
a day , 365 days a year . Some sensors are designed to
operate in extreme temperatures, pressures, flow, and
corrosive or hazardous conditions. Sensors development
incorporates specific industry needs, including hardened
sensors that are resistant to specific environmental
conditions, miniaturized sensors, disposable sensors,
and sensor systems. Purchasers of sensors must
consider the variables of their specific application. As
an example, purchasers of gas detection sensors must
consider the following when selecting a sensor for their
application: speed of response, measurement range,
accuracy , resolution, interfering gases, operating range
(temperature, humidity, pressure), temperature and
humidity effects, sensor life, ease of calibration,
calibration frequency , and replacement cost.

Figure 1. Typical field-mounted intelligent controller connected to
PC-based DCS.

Technical Reprint TR-004

Simple Sensors

Historically , sensors have been simple devices capable
of monitoring a specific variable. The human operator
reviews the data generated by these simple sensors and
determines what action needs to be taken. Limitations
include:

• All intelligence resides in the sensor controller;

• Communication from sensor to controller is

analog;

• Controllers typically are dedicated to specific
sensor types;

• Multiple sensors require multiple controllers;

• Calibration requires intrusive manual

adjustments of transmitters and a corresponding
adjustment of controllers.

Often, because of these limitations, operating decisions
are based on the input from only a few devices,
concerning a few parameters. Simple sensors generate
data but not information. The operator might be
concerned about data reliability if the sensor was overdue
for calibration, or if the data from a sensor was outside
the norm. The process can end in disaster due to
inaccurate data because of a burned-out bulb or a stuck
needle on a gauge.

Sierra Monitor Corporation 1991 T arob Court Milpitas, CA 95035 USA [phone] [phone] Fax: [phone]

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Smart Sensors

Smart or intelligent sensors improve the
communication from the sensor to
controller and improve the reliability of
sensor data by better signal processing.
Smart sensors use digital signals rather
than analog signals to transmit to the
controller. Digital signals provide the
opportunity for greater diagnostics and
data checking.

Smart sensor diagnostics involve
continuous checks to identify out-of­bounds system operating criteria. The
sensor accesses operating
characteristics, checks the operating
voltage, and continuously verifies that the
dynamic range of the sensor provides
adequate resolution. Warning messages
from the sensor identify items such as
low sensor sensitivity and low battery .

Figure 2. Typical sensor system at a wellhead accepts input from a variety of
sensors concerning safety, environment, process and security; performs
automatic control and alarm functions; and communciations to a central control
facility.

In addition, some smart sensors can deal
with detection of intangible properties
such as softness, freshness, sweetness, or aroma. New
smart sensors in the biological, medical and
pharmaceutical world can detect cholesterol and lipids.
In the industrial sector, smart sensors have enabled the
plant operator to obtain more reliable data.

Sensor Systems

But, more reliable data is still just waiting to be converted
into information. Sensor systems have significantly
improved operator efficiency by effectively and quickly
converting large volumes of data into useful information.
The operator needs input from a variety of sensors and
locations to make knowledgeable decisions. Today’s
more powerful sensor systems have the following
characteristics:

• Accepts input form various sensors (and various
manufactures);

• Provides local display of sensor readings;

• Allows for non-intrusive sensor calibration;

• Provides relays for local alarm action;

• Follows user-defined alarm strategy;

• Accepts feedback signals from final control

elements;

• Has independent battery backup;

• Is a low-cost link to the control room;

• Provides centralized monitoring.

Distributed intelligence is central to proper sensor system
design philosophy. Since the field controllers are
intelligent with the capability to implement alarm logic
and operate independently of the host computer,

interrupted communication with host computer does not
hamper field operation. In addition, sensor systems
provide the user with clear, understandable data that
enables the operator to analyze the situation quickly
and take the required action.

A sensor system not only provides the best features of
local control and centralized monitoring, but also
systemizes the entire monitoring process. The system
provides plant personnel with sufficient, reliable
information to ensure that decisions are made with
maximum knowledge of the current situation. Sensor
systems accept a wide range of sensor inputs, including
those with non-linear signals. The remote controller
conditions to signals, compares with data from various
inputs, and transmits the data to the control room.
Within the I/O controller, the input values are used for
equation processing, alarming, control, event sequencing,
and more. Complex control and logic algorithms and
parameters for each channel can be constructed and
modified by the operator.

Generally, all data from the system is accessible from
the control room via simple twisted pair cabling, telephone
lines, or radio signals. In the control room, the operator
can access data from a variety of sensors. Using PC­based software package combined with an intelligent
remote controller provides the user with the flexibility to
configure sensor system inputs, outputs, and data
acquisition to meet the plant’s specific needs. The result
is greater visibility of plant operations, allowing better
decision to be made.

© 1998 by Sierra Monitor Corporation

Sierra Monitor Corporation 1991 Tarob Court Milpitas, CA 95035 USA [phone] [phone] Fax: [phone]

Visit our Web site: www.sierramonitor.com E-Mail: [e-mail]