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Control of “Noise” in Instrumentation Circuits
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- Precaution should be taken during
design, engineering and installation to reduce
the effects of noise. The type of signal transmitted
by the sensor is related to its sensitivity to
noise. The lower the voltage level and the higher
the impedance of a circuit, the greater the circuits
sensitivity to noise of all types. The following
discussion describes the major types of “noise” and
commonly accepted solutions for each.
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Common
Mode |
- A result of different ground potentials
at each location in a process plant. Noise created
by current flow between grounds. Occurs even
with high common mode rejection when shields
are improperly grounded. Is particularly critical
with thermocouple extension wire circuits.
1. To protect against common mode noise pickup
within the wire and cable, a shield circuit
should be grounded at the point which the instrument
circuit is grounded and isolated from
all other grounds; i.e., with a grounded couple,
ground the shield on the extension wire at
the couple. As the shield circuit is carried back
to the control room through a junction box
and a multipair cable, connect the pair shield
in the cable to the single pair which leads to
the couple without grounding the shield in the
junction box or connecting it to any other
shield (on other pairs). The shield should not
be grounded in the control room.
2. Ground all shields. An ungrounded shield will
not provide noise protection.
3. Ground a shield at one point only.
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Cross
Talk |
- Occurs with ac instrument signals,
especially pulse-type signals where more than
one circuit is carried in the same cable. It
is the tendency for a signal to be coupled from
one pair to another within the cable, resulting
in noise being superimposed on a circuit. Cross
talk noise may be eliminated by the use of cables
with individually shielded, isolated pair shields.
The pair shield protects against noise picked
up from adjacent pairs, as well as reducing noise
radiated by the pair it surrounds
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| Static |
- Caused by the electric field radiated
by a voltage source being coupled capacitively
into the instrument circuit. The best way of
fighting static noise is to place the circuit
inside a total coverage shield which isolates
the pair of wires from outside influence. The
grounded shield intercepts static interference
and carries it off to ground. The shield must
be grounded in order to reduce static noise;
an ungrounded shield will not reduce noise.
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| Magnetic |
- Produced by currents flowing through
conductors and pieces of electrical equipment
such as motors, generators, etc. As the current
flows through equipment, a magnetic field is
radiated around the conductor. As this field
passes through the space between the conductors
in a circuit, a current is set up in the circuit
to oppose the magnetic field (transformer action).
This current causes a noise to be superimposed
on the signal in the instrument circuit. The
best way of compensating for this type of noise
is to twist the wires in the instrument circuit.
Twisting causes the noise to be cancelled in
adjacent sections of the wire. This is the least
expensive, most effective way of combatting magnetic
noise.
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Environment vs. Relative Noise Levels
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| Environment |
Relative
Noise Level |
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Wiring
located far from power lines, motors; motors less
than 5 hp;
no induction heating, arcs, control or power relays
nearby:
tank farms, material storage areas, light process plants,
blending
operations, fall into this classification.
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Low |
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Instrument
wire run near medium sized motors, control relays:
the average process plant falls into this classification. |
Medium |
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Electrolytic
processes, large motors, generators, transformers,
induction heating, relay controls, power lines or control
wire nearby:
heavy industry, metals, utilities, fall into this classification. |
High |
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| Class of Wire |
Single Pair Or Triad |
Multipair/Multi Triad Cable |
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| I |
Twisted pair or triad, non-shielded |
Overall shield, individual pairs
or triads twisted but not shielded |
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| II |
Twisted pair or triad, shielded |
Overall shield, individual pairs
or triads twisted but not shielded |
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| III |
Twisted pair or triad, shielded |
Overall shield, individual pairs
or triads twisted and shielded |
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Process
Instrumentation Wire and Cable Selection Guide |
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| Signal
Type |
Sensor
Type |
Noise
Sensitive To |
Noise
Environment
Level |
Wire
Class |
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DC,
low level
<100 mV |
Thermocouples |
Static
Magnetic
Common Mode |
Low
Medium
High |
Class
III
Class III
Class III |
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DC,
low level
<100 mV |
Bridge
circuits,
thermistors, RTD’s,
chromatographic ph,
magnetic flow meter |
Static
Magnetic
Common Mode |
Low
Medium
High |
Class
III
Class III
Class III |
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DC,
medium
level 100 mV-5V |
Analog
computer
outputs |
Static
Magnetic |
Low
Medium
High |
Class
II
Class III
Class III |
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DC
high level
75 V |
Retransmission
potentiometers,
annunciators
alarms |
Static
Magnetic |
Low
Medium
High |
Class
II
Class II
Class II |
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AC
low level
<1OO mV |
Bridge
circuits,
“carrier”
transducers |
Static
Magnetic
Common Mode
Cross Talk |
Low
Medium
High |
Class
III
Class III
Class III |
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AC
medium level
1000 mV-5V |
Turbine
flow meters,
tachometers |
Static
Magnetic
Cross Talk |
Low
Medium
High |
Class
III
Class III
Class III |
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AC
high level
75 V |
Annunciator
pick-up circuits |
Static
Magnetic
Cross Talk |
Low
Medium
High |
Class
III
Class III
Class III |
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Current
Systems 1-5 mA
4-20 mA
10-50 mA |
Force
balance.
P/I transducers,
differential pressure
flow meters |
Magnetic |
Low
Medium
High |
Class
I
Class II
Class III |
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