Noise

The four major types of noise effecting instrumentation are: static, magnetic, cross talk and common mode.

1. Static noise is created by the electric field radiated by a voltage source when coupled capacitively into the instrumentation circuit. The most efficient means to combat static noise is to place the circuit inside a total coverage shield, thus isolating the wires from outside influence. A grounded shield interrupts the static noise and transports it to ground. It is important that the shield be grounded in order to reduce static noise as an ungrounded (floating) shield will not.
2. Magnetic noise is produced by current flowing through other conductors and electrical equipment such as generators, motors, etc. When current flows through such equipment, a magnetic field is emitted around the conductor. As this field passes through the space between the instrument circuit and the conductors, a current is created in the instrument circuit opposing the magnetic (transformer action). Noise is superimposed onto the signal in the instrument circuit due to this current. Twisting the wires in the instrument circuit causes the noise in the adjacent sections of the wire to be cancelled. This recommendation is the most effective, and yet least expensive, means of reducing magnetic noise problems.
3. Cross talk noise is encountered when the instrument signals of more than one AC circuit is carried within the same cable. This is particularly true with pulse type of signals. The inclination is for a signal to be coupled from one pair to another within the cable creating noise. It is recommended that individual shielding and isolation of each pair will protect against noise picked up from adjacent pairs, as well as reducing noise radiated by the pair itself.
4. Common mode is encountered when grounds are at different potentials in a process plant causing current to flow between them and a resultant noise. A high common mode rejection ratio receiver will control common mode interference. However, a second type of common mode noise will occur even when using a high quality receiver and becomes a definite concern in thermocouple extension circuits. Most thermocouples used are grounded type where it is connected physically and electrically to the process well in which it is located. Therefore, when a thermocouple circuit shield (or any nearby metallic object, such as conduit, tray, building frames, etc.) is at a different potential than the thermocouple, a charging current will flow in the extension wires causing a superimposed common mode noise on the signal. To prevent common mode interference, it is recommended that the shield circuit be used and grounded only at the thermocouple. If multi-pair cables are used with sensors, then each circuit must be individually shielded and isolated. The shield circuit then can be maintained at the individual sensor ground potential right back to the control room.

Modern Instrumentation

Constant changes are taking place in controlling, recording and indication equipment used in industry. Most instruments fall into one of three categories: potentiometric (and bridge), current element and A to D converters, computers.

1. Potentiometric (and Bridge) types have a conditioned signal sent through a mechanical or electronic chopper to compare to null (or a standard voltage). The output voltage is then amplified so it can drive a chart recorder pen or indicator.
2. A current flowing in the instrument circuit is converted to torque to move a chart recorder pen or indicator in a current element type of instrument. This style of instrument never uses an electronic circuit. This classification includes pyrometers.
3. A to D Converters take an analog signal which is conditioned, amplified and converted to a digital signal for input into a computer or telemetry transmission. Additionally, this signal can be amplified and fed into a recorder or used for time sampling which can experience significant errors due to noise.

Industrial Installation Hints

Electrical interference should always be taken into consideration when selecting wire and cable for a plant. But mechanical factors should also be weighed as well. For example, a cable being installed in an open tray should be provided with armor to protect against damage from crushing or falling objects. Armor can safeguard cable when buried underground from rodents or shifting ground.

Armor can also provide strain relief for tensile forces when cables are suspended between two points. Served wire type armor can also be used for tensile, crush and impact resistance.

Precautions should be taken when possible during installation and engineering to reduce the effects of noise by locating the cables away from noise sources. Equipment such as power cables, generators, motors, and arc producing equipment greatly increase the chance of noise on cables. Cross talk can be reduced by transmitting only signals of the same relative magnitude and excluding any higher level signals on the same cable.

Data transmission cable should be shielded to reduce noise pickup from nearby instrument circuits. Power and control cables should be twisted to reduce magnetic interference in nearby instrument circuits. In general, locate the cables away from all noise sources as magnetic and static fields reduce rapidly as distance from the source increases.

Typical Rules for Shielded Cables

1. A shielded circuit should be grounded at the point which the instrument circuit is grounded and isolated from all other grounds to prevent common mode noise. For example a grounded thermocouple should have its extension wire shield grounded at the couple itself. The shield should not be grounded at the junction box, control room or any other shield in the cable. Rather, connect the pair shield in the cable directly to the single pair shield attached to the sensor.
2. Be sure all shields are grounded because an ungrounded shield provides no screening of noise.
3. A shield should be grounded only at one point.