Tuesday, January 24, 2017

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Discrete Process Measurement

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.

“Normal” status of a switch

Perhaps the most confusing aspect of discrete sensors is the definition of a sensor’s normal status. Electrical switch contacts are typically classified as either normally-open or normally-closed, referring to the open or closed status of the contacts under “normal” conditions. But what exactly defines “normal” for a switch? The answer is not complex, but it is often misunderstood.

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 push button 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):

simple circuit showing normally open contacts controlling a lamp.

 

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. :

 

circuit showing normally-closed contacts controlling a lamp

This seems rather simple, don’t you think? What could possibly be confusing about the “normal” status of a switch? The confusion becomes evident, though, when you consider the case of a different kind of discrete sensor such as a 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:

 

A low coolant flow alarm circuit

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)!

Students often wonder why process switch contacts are labeled according to this convention of “minimum stimulus” instead of according to the typical status of the process in which the switch is used. The answer to this question is that the manufacturer of the sensor has no idea whatsoever as to your intended use. The manufacturer of the switch does not know and does not care whether you intend to use their flow switch as a low-flow alarm or as a high-flow alarm. In other words, the manufacturer cannot predict what the typical status of your process will be, and so the definition of “normal” status for the switch must be founded on some common criterion unrelated to your particular application. That common criterion is the status of minimum stimulus: when the sensor is exposed to the least amount of stimulation from the process it senses.

Here is a listing of “normal” definitions for various discrete sensor types:

Hand switch: no one pressing the switch

Limit switch: target not contacting the switch

Proximity switch: target far away

Pressure switch: low pressure (or even a vacuum)

Level switch: low level (empty)

Temperature switch: low temperature (cold)

Flow switch: low flow rate (fluid stopped)

These are the conditions represented by the switch statuses shown in a schematic diagram. These may very well not be the statuses of the switches when they are exposed to typical operating conditions in the process.



Click here to continue to the next page, Hand Switches

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