4-20 mA Signal

An “analog” electronic signal is a voltage or current whose magnitude represents some physical
measurement or control quantity. An instrument is often classified as being “analog” simply by virtue
of using an analog signal standard to communicate information, even if the internal construction and
design of the instrument may be mostly digital in nature. This is to distinguish such instruments
from those making use of no analog electronic signals at all (e.g. wireless or Fieldbus instruments).

The most popular form of signal transmission used in modern industrial instrumentation systems
(as of this writing) is the 4 to 20 milliamp DC standard. This is an analog signal standard, meaning
that the electric current is used to proportionately represent measurements or command signals.
Typically, a 4 milliamp current value represents 0% of scale, a 20 milliamp current value represents
100% of scale, and any current value in between 4 and 20 milliamps represents a commensurate
percentage in between 0% and 100%.

ON/OFF Valves

The word “discrete” means individual or distinct. In engineering, a “discrete” variable or
measurement refers to a true-or-false condition. Thus, a discrete control element is one that has but
a limited number of states (usually two: on and off).
An on/off valve is the fluid equivalent of an electrical switch: a device that either allows unimpeded
flow or acts to prevent flow altogether. These valves are often used for routing process fluid to
different locations, starting and stopping batch processes, and engaging automated safety (shutdown)
functions.
Valve styles commonly used for on/off service include ball, plug, butterfly (or disk), gate, and
globe. Large on/off valves are generally of such a design that the full-open position provides a
nearly unimpeded path for fluid to travel through. Ball, plug1, and gate valves provide just this
characteristic:

*A plug valve is very much like a ball valve, the difference being the shape of the rotating element. Rather than a spherical ball, the plug valve uses a truncated cone as the rotary element, a slot cut through the cone serving as the passageway for fluid. The conical shape of a plug valve’s rotating element allows it to wedge tightly into the “closed” (shut) position for exceptional sealing.

Industrial Instrumentation - Measurement

Instrumentation is the science of automated measurement and control. Applications of this science
abound in modern research, industry, and everyday living. From automobile engine control systems
to home thermostats to aircraft autopilots to the manufacture of pharmaceutical drugs, automation
surrounds us.

The first step, naturally, is measurement. If we can’t measure something, it is really pointless to
try to control it. This “something” usually takes one of the following forms in industry:

• Fluid pressure
• Fluid flow rate
• The temperature of an object
• Fluid volume stored in a vessel
• Chemical concentration
• Machine position, motion, or acceleration
• Physical dimension(s) of an object
• Count (inventory) of objects
• Electrical voltage, current, or resistance

Once we measure the quantity we are interested in, we usually transmit a signal representing
this quantity to an indicating or computing device where either human or automated action then
takes place. If the controlling action is automated, the computer sends a signal to a final controlling
device which then influences the quantity being measured.

Flow Switch

A flow switch is built to detect fluid flow through a pipe. In a schematic diagram, the switch
symbol appears to be a toggle switch with a “flag” hanging below. The schematic diagram, of course,
only shows the circuitry and not the pipe where the switch is physically mounted:

This particular flow switch is used to trigger an alarm light if coolant flow through the pipe ever falls to a dangerously low level, and the contacts are normally-closed as evidenced by the closed status in the diagram. Here is where things get confusing: even though this switch is designated as “normally-closed,” it will spend most of its lifetime being held in the open status by the presence of adequate coolant flow through the pipe. Only when the flow through the pipe slows down enough will this switch return to its “normal” status (remember, the condition of minimum stimulus?) and conduct electrical power to the lamp. In other words, the “normal” status of this switch (closed) is actually an abnormal status for the process it is sensing (low flow)!

Normal Status

The “normal” status for a switch is the status its electrical contacts are in under a condition of
minimum physical stimulus. For a momentary-contact pushbutton switch, this would be the status
of the switch contact when it is not being pressed. The “normal” status of any switch is the way
it is drawn in an electrical schematic. For instance, the following diagram shows a normally-open
pushbutton switch controlling a lamp on a 120 volt AC circuit (the “hot” and “neutral” poles of the
AC power source labeled L1 and L2, respectively):

We can tell this switch is a normally-open (NO) switch because it is drawn in an open position.
The lamp will energize only if someone presses the switch, holding its normally-open contacts in the
“closed” position. Normally-open switch contacts are sometimes referred to in the electrical industry
as form-A contacts.

If we had used a normally-closed pushbutton switch instead, the behavior would be exactly
opposite. The lamp would energize if the switch was left alone, but it would turn off if anyone
pressed the switch. Normally-closed switch contacts are sometimes referred to in the electrical
industry as form-B contacts. :

Discrete Sensor

The word “discrete” means individual or distinct. In engineering, a “discrete” variable or
measurement refers to a true-or-false condition. Thus, a discrete sensor is one that is only able
to indicate whether the measured variable is above or below a specified setpoint.

Discrete sensors typically take the form of switches, built to “trip” when the measured quantity
either exceeds or falls below a specified value. These devices are less sophisticated than so-called
continuous sensors capable of reporting an analog value, but they are quite useful in industry.
Many different types of discrete sensors exist, detecting variables such as position, fluid pressure,
material level, temperature, and fluid flow rate. The output of a discrete sensor is typically electrical
in nature, whether it be an active voltage signal or just resistive continuity between two terminals
on the device.

Electrical signal and control wiring

There is much to be said for neatness of assembly in electrical signal wiring. Even though the
electrons don’t “care” how neatly the wires are laid in place, human beings who must maintain the
system certainly do. Not only are neat installations easier to navigate and troubleshoot, but they
tend to inspire a similar standard of neatness when alterations are made.
Here we see 120 volt AC power distribution wiring. Note how the hoop-shaped “jumper” wires
are all cut to (nearly) the same length, and how each of the wire labels is oriented such that the
printing is easy to read:

This next photograph shows a great way to terminate multi-conductor signal cable to terminal
blocks. Each of the pairs was twisted together using a hand drill set to very slow speed. Note how
the end of the cable is wrapped in a short section of heat-shrink tubing for a neat appearance: