Oh no, we gonna rock down to Electric Avenue
And then we'll take it higher
Oh we gonna rock down to Electric Avenue
And then we'll take it higher

("Electric Avenue," by Eddie Grant, 1982)

PTOA Readers and Students have recently learned how the process variable Temperature is measured via:

• Liquid-in-Glass Thermometers.
• Fluid-Filled Systems.
• Bimetallic Thermometers.

These techniques involve either volume expansion/contraction of a fluid or  distance expansion/contraction of a bimetallic strip.

Another common feature of the above temperature-measuring devices is that they are all limited to local temperature measurement; they do not have the capacity to send a standard signal that represents the measurement to the control room or even nearby DCS component.

PTOA Readers and Students learned all about the various types of standard signals way back in PTOA Segment #14 entitled "I Just Gotta Get a Message to U."

Fast signal transmission between the process units "out in the field" and the snug, weather-proof control room is made possible by devices that generate electrical signals.

Three common electrical temperature-measuring devices found in processing industries are:

• Thermocouples
• Resistance-Temperature Detectors (RTDs)
• Thermistors

Understanding what these devices are and how they work will require a little side trip ...

Time to rock down through electric avenue!

 

OHHHHMMMMM

PTOA Readers and Students who are reading the PTOA Segments in the intended sequential order will remember the following introduction to The Electricity Transport Phenomenon which was first mentioned way back in PTOA Segment #57:

Instrumentation Technicians and Electricians will need to understand The Electricity Transport Phenomenon at the gut level:

Electrical current cannot exist unless there is a change in voltage.

The current will always flow from the highest voltage to the lowest voltage.

The change in voltage (aka "voltage drop") is commonly called "Potential."

The more potential, the stronger the current.

Process Operators will not need to understand The Electricity Transport Phenomena at the gut level ...

and let me tell ya it gets really, really wonky ...

but understanding the basics will make it possible to understand how electrical temperature-measuring devices work.

Ohm's Law

A dude named Ohm came up with the mathematical expression that defined how

• Current (I, measured in Amps or milliAmps)
• Voltage/Electromotive Force  (V or E, measured in Volts or milliVolts)
• and Resistance (R, measured in Ohms)

are related.

The relationship ... called Ohm's Law ... is easy to remember when drawing a circle and cutting it up into large pieces of pie as shown below:

What Do The Definitions & Relationships Mean?

Determining Total Electromotive Force E
aka Voltage V

V=I*R

The picture to the right shows a snippet of an  electronic circuit that depicts  Ohm's Law... and simultaneously everyone's daily grind.

Current (I) is flowing from a place with high Electromotive Force (+) to lower Electromotive Force (-) and encounters Resistance along the way.

The amount of energy E (V in the above schematic ... not very helpful!) that it takes to get through the circuit is the product of the Current (I) and the Resistance (R).

When you wake up in the morning you have a lot of potential electromotive force and energy.

The amount of energy you have at all times is a matter of how you flow through your "To-Do List" and the resistances you encounter to deal with while in the course of flowing through your daily activities.

Think of the force that gets you through the day as an Electromotive Force (emf), or Voltage.

If you run out of the force, then you will just stop in your tracks (you have no more Current, I).

The magnitude of an Voltage/Electromotive Force (V or E) can be  measured by tapping both sides of the circuit before and after where the source of Resistance (R, in Ohms) restricts the flow of Current (I,in Amps).

To determine the magnitude of flowing Current (I, in Amps), divide EMF aka Voltage (E or V, in Volts)  by Resistance (R, in Ohms):

I = V/R

Man, the expression for Flowing Current sure looks a lot like:
Conduction Heat Rate = Temperature Differential / Thickness of Metal!

Gee! That means that an increase in Resistance  will decrease the amount of flowing Current.

Righteoo!

The more resistance experienced throughout the day translates into feeling less and less energetic, making it harder to move at the same rate you started your day with.

Gee that also means if I have more energy to start with ... like a recharged or new battery ... then the magnitude of flowing Current (I) will be greater assuming all the Resistances (R) stay the same.

Hey, that's right!

Give me a cup of java and I can move a lot faster no matter what gets in my way!

 

 

Ohm's Law can be rearranged to define and determine the magnitude of Resistance (R, measured in Ohms).

To determine the magnitude of Resistance, divide the total Voltage/Electromotive Force (E, in Volts) by the flowing Current (I, in Amps).

R= E/I

Hey, that works in the real world, too!

A person can get a sense of how much resistance they are feeling by considering their energy level (E) and how motivated they are to keep moving (I) and deal with the present situation to make it through to the end of day.

Likewise,

too much multitasking in one day (an increase in I compared to the energy available) builds up the resistance until a body is frazzled, and heated .... feeling the burn of burned out.

After running around the circuit each day, don't you feel like saying Ohhhmmmm?

Get to bed early and recharge your battery!

 

 

 

HOW I AND V,E ARE REALLY MEASURED

The schematic at the right shows a more complete circuit.

The Cell is the battery providing energy to the circuit.

The current (I) flows through the circuit through the ammeter, resister, and then rheostat.

 

THE THERMOCOUPLE

PTOA Readers and Students just learned that  bimetallic thermocouples work because the temperature and thermal expansion rates of metals are predictably linear, a one-to-one relationship.

Thermocouples work by attaching two dissimilar metals ... but just at the ends. 

The two metals directly contact each other at the HOT JUNCTION.

At the COLD JUNCTION both wires are kept at the same reference temperature.

The hot junction (pointy right-hand side of graphic)  is exposed to the heat of the process stream, of course shielded from direct contact with the flowing stream when necessary.

The difference in the temperature sensed at both junctions creates a small electromotive force ... so small that it is measured in milliVolts.

 

And ... wouldn't you know that the relationship between the emf generated and the measured temperature is a well known linear, one-to-one relationship because the two paired metals that make up the thermocouple are carefully chosen!

Namaste!

TO BE CONTINUED IN THE NEXT PTOA SEGMENT!

TAKE HOME MESSAGES: Three types of electrical instruments used to measure temperature are:

• Thermocouple
• Resistance-Temperature Detector (RTD)
• Thermistor

Ohm's Law defines the mathematical expressions used to define the relationship between:

• Electromotive Force/Voltage, measured in volts or milliVolts.
• Flow of Current, measured in Amps or milliAmps.
• Resistance, measured in Ohms.

The version of Ohm's Law that is directly related to Thermocouples is V=I*R because thermocouples generate a milliVoltage that can infer a process Temperature.

Ohm's law can be rearranged to determine Current (I) and Resistance (R) ... like in a Resistance-Temperature Detector (RTD).

Thermocouples work on the principle that when two dissimilar metals are joined at the ends and exposed to heat, a milliVoltage is generated that directly corresponds to temperature.

©2016 PTOA Segment 00106
PTOA Process Variable Temperature Focus Study Area
PTOA Process Industry Automation Focus Study Area

 

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