Introduction
Analog signals play a pivotal role in the realm of instrumentation. They serve as the backbone of communication between various devices and systems. The significance of analog signals lies in their ability to provide a continuous and varying representation of real-world variables. These could be temperature, pressure, or fluid levels. The understanding of this is essential for an Instrumentation apprentice.
Unlike digital signals, which convey information discretely as on or off, analog signals offer a spectrum of values. This allows for precise measurement and control. The widespread adoption of analog signals, especially the 4-20 mA signal, stems from their reliability, accuracy, and resilience in industrial environments.
How Does it work?
For example let’s consider an ultrasonic level sensor within a fire water tank. We’ll say the tank has a depth of 50 ft. We want to find out how our sensor communicates with the transmitter. This is how we will get our level reading. This sensor communicates its readings through a 4-20 mA analog signal. 4 mA signifies the zero level (e.g., empty tank), and 20 mA represents the full scale (50 ft.).
We call the difference between the zero and the high end value the span. We use these numbers to calculate our process variable when it is between 4 and 20 mA.
4 mA almost always represents the lower end of the scale and 20 mA will almost always represent the high end on the scale. Having an understanding of this is critical for troubleshooting.
Let’s say that you are looking into the tank and it appears to be empty but you are reading 12 mA(50%) from the sensor. Now understanding 4-20 mA you will know that something is not right with the sensor.
While 4-20 mA is the prevalent analog signal, other signals like 3-15 PSI or 1-5 V might be encountered as well. You’ll deal with 3-15 PSI typically when dealing with pneumatic equipment such as control valves or pneumatic controllers.
For more information on Analog signals I would recommend checking out Chapter 29 in the Instrumentation and Process Control text book. Additionally out our article on control loops as well if you are looking for more beginner content.
Finally in our next article, we will go over conversions from our analog signal to our process variable.
