Technical Notes - Signal Conditioners, Transmitters, Isolators, & Signal Convereters

Signal Conditioner Technical Notes regarding API Absolute Process Instruments Signal Conditioners, Signal Transmitters, Signal Isolators, Loop Power Supplies, Signal Converters.

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What is deadband?

DEFINITION
Deadband is the range through which an input can be varied without initiating an observable response.
Deadband is usually expressed in percent of span.

EXAMPLE
A 20% total deadband is applied to the setpoint of a monitored parameter. The relay will trip and reset to its untripped state as indicated in the following graph.

Ohm's Law & Wire Resistance - E = I x R

OHM'S LAW is the relationship between current, voltage and resistance. It states that current varies directly with voltage and inversely with resistance.

E (Electromotive Force or Voltage) is the electrical potential that exists between two points and is capable of producing a flow of current when a closed circuit is connected between the two points. The unit of measure for voltage is the volt (V). One volt will send one ampere of current through a resistance of one ohm.

Typical Solid Copper Wire Resistance Ohms per 1000 feet

Switching inductive loads - Relay Protection and EMI Suppression

When using API alarm module relays to switch inductive loads, maximum relay life and transient EMI suppression is achieved by using external protection. All external protection devices should be placed directly across the load and all leads lengths should be kept to a minimum length.

For AC inductive loads (see Figure 1), place a properly rated MOV across the load in parallel with a series RC snubber. A good RC snubber consists of a 0.1 μF polypropylene capacitor of sufficient voltage and a 47 Ohm ½ Watt carbon film resistor.

The effects of radio frequency interference (RFI) and electromagnetic interference (EMI)

Electrical interference, or noise, is an unwanted electrical signal that can cause intolerable error in, or complete disablement of an electronic control or measurement systems. Interference or electrical noise is broken down into two somewhat overlapping categories: Radio Frequency Interference (RFI) and Electromagnetic Interference (EMI). Some of the more commonly encountered sources of interference are:

What is a ground loop?

In a process control loop, a ground loop circuit can develop when each device's ground is tied to a different earth potential thereby allowing current to flow between the grounds by way of the process loop (Figure 1). Ground loops cause problems by adding or subtracting current or voltage from the process loop. This addition and/or subtraction causes the receiving device to be unable to differentiate between the wanted and unwanted signals and thus can't accurately reflect actual process signals.

Using Signal Isolator to Avoid Ground Loops

Signal Conditioning Terms & Definitions

Accuracy: The closeness of an indication or reading of a measurement
device to the actual value of the quantity being measured. Accuracy calculations
are based on the linearity, hysteresis, and repeatability characteristics of
the transducer/sensor and supporting electronics, the range of the transducer/
sensor, as well as the resolution being displayed. It is usually expressed as
a ±% of full scale output of the transducer/sensor/system.

Signal Conditioners General Information

While there are many different types of signal conditioners, their basic function is to change or alter signals so that different process devices can communicate with each other accurately. Signal conditioners are most commonly needed to link temperature, pressure, weighing, level and flow devices with indicators, recorders, and computerized process monitoring and control systems. Signal conditioners can also perform some other tasks for you as listed below: