Beyond my control, we all need control
I need control, we all need control,

("Mr. Roboto," by Styx, 1983)

This PTOA Segment is RTD - Two because it is the second PTOA Segment that features RTDs ... get it?

But don't confuse RTD - Two with everybody's favorite robot from Star Wars, R2D2.

Spoiler alert!

Sometimes the "robot" R2D2 is literally "manned" by a human being as shown in the picture to the right.

 

 

The remote-controlled versions of the R2D2 robot no doubt require temperature controllers to insure the electronics inside do not overheat.

Maybe RTDs are used in R2D2!

 

As PTOA Readers and Students recently learned in PTOA Segment #113, an RTD is a temperature measuring device that generates an output of electrical resistance in ohms (symbol = Ω).

The ohms output can then be correlated to temperature because the relationship between electrical resistance and temperature is very linear ... if the RTD is fabricated from certain pure metals!

 

TWO CRUCIAL RTD LINEAR RELATIONSHIPS

By now all PTOA Readers and Students realize that mankind has recognized and made math models of actual linear relationships that exist in nature and the cosmos.

Also PTOA Readers and Students have realized that these  linear relationships can be put to work to do useful things ... like measure process temperatures.

Doesn't it make you wonder why your math teachers spent so much time having you solve linear equations like these? Small wonder you don't think you like science, technology, engineering, or math!

Ok ... done spewing ...

Back to work ...

Here's the latest bulletin from the PTOA Department of Redundancy Department:

The RTD would not be a useful temperature-measuring device were it not for the fact that the circuit flowing through the wire generates an electrical resistance output that can be correlated with temperature because of the one-to-one relationship (aka linear relationship) that exists between resistance (Y axis) and temperature (X axis) as is shown in the below graph.

PTOA Readers and Students are becoming experts at interpreting graphs that illustrate linear relationships.

Your Mentor probably does not even have to point out that the steeper slope of the graphed line representing the Platinum RTD  translates into a greater output in ohms (Y axis) compared to the output from the Copper or Nickel RTDs.

Ergo, the most accurate temperature measuring RTD is made of platinum.

But guess what?

An output of resistance in ohms is not particularly useful with respect to generating a standard signal that can be transmitted into a DCS device or otherwise brought into the control board in the control room.

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

For this reason, an electrical circuit called a Wheatstone Bridge is incorporated into the circuit of platinum RTDs.

The Wheatstone Bridge circuit measures a change in resistance (aka difference in ohm output) and converts the output into a voltage output.

Naturally, the relationship between voltage output from the Wheatstone Bridge and temperature measurement must also be very linear.

The below graph illustrates the linear relationship between the voltage output from the Wheatstone Bridge (Y axis) and  temperature (X axis).

That's what I'm talking about!

Boy Howdy is the relationship straight-line linear ... and not just over the short temperature range from 0 to 60 °C shown in the above graph.

Platinum RTDs are so accurate that they are used to calibrate other temperature measuring devices that are used to measure temperature in the -259 to 631 °C temperature range (aka -434 °F to 1167.8 °F).

Platinum RTDs are so accurate and reliable that they are used to measure the primary coolant temperature at nuclear power plants.

And that's an important job because nuclear reactions are extremely exothermic.

Also ...

The RTD's measured/inferred temperature of the primary coolant is tied into crucial control loops and logic devices that constantly calculate plant performance and vigilantly monitor safety systems.

That's a lot of automatic-control decision making!

RTD technology is up to the task.

Within the coolant temperature range of  520-550 °F the accuracy of the RTD is a mere one half of one degree Fahrenheit!

And the RTD will repeatedly and reliably infer accurate temperature measurements ... even if the process cycles up and down through hot and cold temperature ranges.

 

RTD AND THERMOCOUPLE SIMILARITIES

RTDs and thermocouples are both electrical devices that are used to measure process temperatures at a processing facility.

Both the thermocouple and RTD have electrical outputs that can be converted into standard signals and thence transmitted into a DCS component in the field or into the control room.

Control system architecture will vary from plant to plant ... but some device in the DCS will do the math that correlates the standard signal into a temperature that human beings can understand.

Both thermocouple and RTD technologies rely on a linear relationship with temperature to infer a process temperature from a millivolt output (thermocouple) or change in detected ohms which are thence converted into volts (RTD).

Rigid thermocouples and RTDs physically look similar.  Both are protected by thermowells which are attached to connector heads.

 

At the time this PTOA Segment #114 was written, the connector heads of RTDs were noticeably larger than those used for thermocouple installations and the purchase price of a platinum RTD was more costly than base metal thermocouple calibrations.

 

RTD AND THERMOCOUPLE DIFFERENCES

The thermocouple has a hot junction created by joining two separate wires.

In contrast:

The RTD has a single helically coiled wire.

The electrical circuit flows into the RTD → around and around the helical wire → out of the RTD.

Some thermocouples have exposed junctions.

In contrast:

All RTDs must be protected in a sheath and typically a thermowell, too.  RTDs are not as rugged as thermocouples.

 

The output of a thermocouple is millivolts which are thence amplified and converted into standard signals.

In contrast:

The output of an RTD is ohms which are not a transmittable standard signal.

The RTD must be incorporated into the electrical circuit of a Wheatstone Bridge so that a change in ohms is detected and converted into a voltage standard signal which is the output of the bridge circuitry.

Some thermocouple calibrations have been improved by using man-made metal alloys like "Constantan" and "Nicrosil."

In contrast:

RTDs must be made of pure platinum or pure nickel or pure copper.

Only these pure metals produce a noteworthy change in electrical resistance without having other undesirable characteristics.

The thermocouple installation must be constantly vigilant for undesirable emf ... which can be offset with CJC and reduced with the use of the properly paired extension wire.

In contrast:

The RTD installation must provide a constant DC current to the device.

The DC current is used as a reference point from which the change in voltage from the Wheatstone Bridge can be measured and a change in temperature subsequently inferred.

Truth be known, there are many Outside Process Operators that are blissfully unaware whether or not the hardware they pass by several times a day protects a thermocouple or an RTD ... and no doubt some Control Board Operators are likewise unaware.

The best Control Board Operators are aware that one of the main differences between the RTDs and thermocouples is measurement response time. RTDs are good ... but not as fast as thermocouples.

PTOA Readers and Students learned in this PTOA Segment that the output of the RTD in ohms has to be balanced with the Wheatstone Bridge so that the bridge can generate an output in volts that is transmitted as a standard signal into a logic device that electronically performs the math which then generates a standard signal form of the temperature that is eventually converted into a HMI format that makes sense to the Control Board Operator.

Logically, all of the above takes time.

A Control Board Operator must get a feel for the response lag of RTDs so that a change in desired temperature (aka "a change in setpoint temperature") is allowed sufficient processing time to be acted upon prior to making another adjustment to the setpoint.

TAKE HOME MESSAGES: RTD Technology is a lot like thermocouple technology with respect to:

• Both have a fundamental dependence upon a linear relationship between respective electrical output and temperature.
• The hardware of rigid thermocouples and RTDs physically look similar.

Platinum RTDs rely on the incorporation of a Wheatstone Bridge in the electrical circuitry to work. The bridge converts the resistance output in ohms into a difference in ohms which is then converted into a voltage output from the bridge.

A DC current must be supplied to the RTD to provide a basis of resistance to the Wheatstone Bridge so that it can detect a change in resistance.

Platinum RTDs are extremely accurate, repeatable and stable devices that measure temperature.

The response time of RTDs is good, but not as fast as that for thermocouples.

©2016 PTOA Segment 00114
PTOA Process Variable Temperature Focus Study Area
PTOA Process Industry Automation Focus Study Area

 

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