Archive for June, 2011

A Brief Description of SCADA System Signals

Sunday, June 19th, 2011

It has been a while, I left this site without any updates. So, here I present a basic of a SCADA system signal. It’s basic and have to describe it a way earlier but, left out with an assumption every SCADA engineer should have known it. Anyway, as a general reading, this topic should be one of important information for public readers.

basic circuit of instrument signal tappingThe most common current signal standard in modern use is the 4 to 20 milliamp (4-20 mA) loop, with 4 milliamps representing 0 percent of measurement, 20 milliamps representing 100 percent, 12 milliamps representing 50 percent, and so on. The use of a 4 mA reading to indicate zero is known as “live zero.” This helps to distinguish a zero reading from a “dead signal” or non-functioning equipment. The range of readings possible for a properly functioning system then is only 16 mA (20 mA – 4 mA). This 16 mA range is known as the “live signal.” The actual reading being recorded in mA is called the “process variable.” The PV indicates a percentage of a particular measurement being monitored. The table below shows the relationship between various PVs and their corresponding percentages.

signal comparison tableA convenient feature of the 4-20 mA standard is the ease in converting these signals to 1-5 volt indicating instruments, as the table on the left shows. A simple 250-ohm precision resistor connected in series with the circuit will produce arange of readings from 1 volt of drop at 4 milliamps to 5 volts of drop at 20 milliamps. The current loop scale of 4-20 milliamps has not always been the standard for currentinstruments. In the past, 10-50 milliamp signals were used more frequently. That standard has since become obsolete. The main reason for the eventual supremacy of the 4-20 milliamp loop was safety. Lower circuit voltages and lower current levels (compared to 10-50 mA systems) mean less chance for electrical shock injuries and/or the generation of sparks capable of igniting flammable environments in certain industrial applications.

An Overview of  SCADA Signal Calculations

Definition of terms

Dead signal: A reading from a non-functioning system that can be mistaken for a measurement.
Live signal:
The range of possible process variables. In a 4 – 20 mA system, any signal below 4 mA or above 20 mA indicates malfunctioning equipment. The range of useable signals is between 4 & 20 mA. Therefore, the live signal = 16 mA.
Live zero:
A reading other than zero used to indicate zero so that a zero reading can be distinguished from a dead signal. In 4 – 20 mA systems, the live zero = 4 mA.
Process variable. The signal reading, in mA, that represents a percentage of a particular measurement.

One of the more common uses for SCADA systems is the monitoring of storage levels. Formulas for making these calculations include:

mA to feet conversion

So, given a SWH of 30 ft, and board reading of 14.67 mA, the water level would be calculated as follows:

water level calculation

Kindly, do enjoy to use or share this information. Have a nice day.

Reference: Water Opcert School