You're growing, you're growing,
You're growing in and out.
You're growing, you're growing,
You're growing all about.

("You're Growing," by Fred M. Rogers, 1967)

In this PTOA Segment #105, PTOA Readers and Students will determine the best industrial applications of bimetallic technology for measuring process stream temperatures.

PTOA Readers and Students learned in PTOA Segment #104 that the construction of a temperature-measuring bimetallic strip is significantly impacted by:

• The choice of the two conjoined metals.
• The length of the metal strips.
• The thickness of the metal strips.

This PTOA Segment #105 will reveal how the above criteria impact the accuracy, response time, and reliability of bimetallic TE/TIs (Temperature Measuring Elements/Temperature Indicators) found in the processing industries.

 

INSTRUMENT MEASUREMENT ERROR
AND REPEATABILTY OF BIMETALLIC STRIPS

Accuracy/Error of Bimetallic Strips

The effective temperature-measuring range for bimetallic strip thermometers is -65 °C to 430°C (-85°F to 806°F) with an accuracy of ± 1% of span.

Instrument-Techie PTOA Readers and Students should know how to verify that the above declaration of accuracy translates into the following real-world meaning:

The actual process stream temperature may vary from the observed/recorded temperature by ± 5 °C (±9 °F).

Laboratory-grade bimetallic thermometers can improve the accuracy to ± 0.5% of scale.

 

Repeatability of Measurements

The relationship of a temperature increase/decrease to the expansion/contraction of a metal is a "one-to-one correspondence."

That means for every increase/decrease in temperature, the amount of increase/contraction in the length of the bimetallic strip is predictable.

The below graph illustrates what happened when a bar of Copper metal was heated.

Temperatures were graphed on the X axis and the length of the bar was graphed on the Y axis.

A one-to-one relationship is called a linear relationship.

Graphing a linear relationship will always result in a straight line like those shown above and to the left.

The steepness of the line hints at how fast the change on the Y slope (in this case paraphrased "Expansion of Metal Bar") will occur while the number on the X axis grows (in this case "Temperature in °C").

Measurement Response Lag Associated with Bimetallic Strip Technology

The response time of bimetallic strip temperature-sensing elements is greatly impacted by the choice of conjoined metals.

The easiest way to produced the desired twisting action of the bimetallic strip (aka "torque") upon heating is to choose two metals with a wide variance in thermal expansion rates.

In PTOA Segment #96 entitled "Temperature Measurement Must Knows (Part 2)," PTOA Readers and Students learned that Brass expands 12 times faster than an alloy metal called Invar.

At 1.6, Invar has the lowest coefficient of thermal expansion listed on the

TABLE OF LINEAR THERMAL EXPANSION COEFFICIENTS

thankfully provided by EngineeringToolbox.com.

The table suggests that a Copper/Invar bimetallic strip would not provide as much torque as Brass/Invar bimetallic strip when heated because:

Copper/Invar = 16.6/1.5 = 11.0

If torque were the only consideration, the Copper/Invar strip would be an inferior choice of metals compare to the Brass/Invar strip.

However ...

By now, all PTOA Readers and Students who read the PTOA Segments in the intended sequential order could give their own Powerpoint presentation illustrating how ... way before the pointer can move on the dial ... heat transfer at the molecular level must happen.

For a bimetallic TE/TI, measurement response lag happens because:

• The process stream temperature being measured must be conducted through the wall of the thermowell →
• The heat on the inside wall of the thermowell must be convected to the exterior surface of the helical bimetallic strip →
• The heat absorbed at the exterior surface of the helical bimetal strip must be conducted through the strip →
• THEN ... and only then ... can the mechanical movement change the position of the pointer on the scale of the dial!

Therefore, the optimal choice for a bimetallic strip metal must have both a high coefficient of thermal expansion and a high conductivity coefficient.

The TABLE OF THERMAL CONDUCTIVITY FACTORS provided by EngineeringToolbox.com indicates that Copper has a k=400 compared to k=109 for Brass.

Copper can conduct heat almost 4 times faster than brass.

The superior heat transfer properties of Copper make it the optimal choice for bimetallic technology when mashed up with Invar.

Incidentally ...

In some applications the spaces between the  the bimetallic strip and interior wall of the thermowell is filled with a gel that enhances heat transfer properties.

 

RELIABILITY OF BIMETALLIC STRIP TECHNOLOGY

In PTOA Segment #100, PTOA Readers and Students learned that Instrument Reliability is determined by considering the instrument's  functionality, physical limitations, and working environment.

Functionality of Bimetallic Helical Strips

The functionality of a bimetallic helical TE/TI is determined by matching its accuracy, range of operation, and power requirements with the intended industrial use.

Accuracy/Repeatability of Bimetallic TE/TIs
Used in Industrial Applications

As mentioned above, the accuracy of  bimetallic strips is ± 5°C (9 °F) over a temperature measuring range of  -65 °C to 430°C (-85°F to 806°F).

The accuracy will be maintained if the TI is properly handled.

As long as the bimetallic strip is in good working order, the repeatability of bimetallic strip technology will be sustained because of the unchanging, linear relationship between temperature and the thermal expansion rate of metals described previously in this PTOA Segment #105.

Range of Operation

The  -65 °C to 430°C (-85 °F to 806°F) range of bimetallic strips does not attain the high temperature limits of fluid-filled systems.

In PTOA Segment #102, PTOA Readers and Students learned the effective range of measurement for fluid-filled systems was -185 °C to 650 °C (-301 to 1202 °F).

However, the short measurement response lag of bimetallic strips is superior compared to the long response lag of fluid-filled systems.

The quick response time of the bimetallic strip is due to the easy torque movement that is created as described earlier in this PTOA Segment #105 and which results in a deflection that moves the pointer.

The deflection movement of the strip increases with the square of the strip length.

Say what?

If the length of the bimetallic strip is doubled (multiplied by 2), the deflection of the strip...ergo the sensitivity of the pointer movement and accuracy of the reading ... will be multiplied by four (2 X 2).

If the length of the strip is tripled (multiplied by 3), the deflection movement will increase by a factor of nine (3 X 3).

Etcetera. Etcetera. Etcetera.

The above statement means that helical winding of strips both increases measuring sensitivity and deflection as well as makes a more physically compact bimetallic strip.

Physical Limitations of Bimetallic Strips

Yet there are physical limits to how long the bimetallic strips can be manufactured because there are physical limitations on the diameter of the process piping which contains the process stream that needs to have its temperature measured.

Accurate temperature measurement requires that the bimetallic strip TE (enclosed in a thermowell) be submerged 2 to 3 inches in the process stream that is having its temperature determined.

Optimizing the sizing of the TE (the bimetallic strip) and positioning of the TI (the pointer and dial face) in the pipe is part of the facility design process.

 

Power Requirements

There are no power requirements for the bimetallic strip TE/TI because the device works due to a physical property ... the relationship between temperature and linear thermal expansion tendencies of metals.

The bimetallic strip (TE) can generate a local temperature indication (TI) without any electricity; therefore bimetallic TE/TIs are intrinsically safe devices.

 

Recording and On-Off Control is Possible with Bimetallic Technology

The bimetallic strip can be manufactured wider and thicker which makes the deflection movement sufficiently strong to move a pen on a local recording chart.

Some means to power the chart recorder drive would need to be supplied.

Unfortunately, no technology exists to amplify and relay the mechanical signal generated by the deflection of a thicker,wider bimetallic strip into the control room without losing accuracy to the point of being meaningless.

Nevertheless, the stand-alone local mechanical Temperature Recorder (TR) can be nifty for some local temperature monitoring applications way out in the boonies.

PTOA Readers and Students learned in the previous PTOA Segment #104 that bimetallic technology can be used as a mechanical means for On-Off control applications.

On-Off mechanical control is a giant leap that significantly extends the usefulness of  bimetallic technology, greatly surpassing mere temperature indication.

 

SUMMARY OF BIMETALLIC TECHNOLOGY BENEFITS

Temperature measuring elements (TEs) that use bimetallic strip technology can provide accurate, repeatable and fast local temperature indication (TI), and yes ... even be beefed up to make it possible to perform local temperature recording (TR).

Bimetallic TE/TIs are inexpensive and can be manufactured to be relatively rugged and able to withstand the surrounding industrial  environment.

The dial faces on TIs are easy for Outside Process Operators to understand which greatly reduces online measurement error due to misreading.

The dependable relationship between temperature and the thermal expansion of metals guarantees that the bimetallic temperature measuring element will not need constant recalibration, ergo the chance of online miscalibration error is minimal.

Bimetallic TE/TIs require no power, thus interference from direct electrical connections is not a problem for these devices.

 

SUMMARY OF BIMETALLIC TECHNOLOGY LIMITATIONS

The effective high temperature measurement range limit of 430 °C (806 °F) is not sufficiently high for many process manufacturing applications.

The bimetallic strip cannot convert molecular movement into a mechanical movement that can be transmitted into a control room without reducing measurement accuracy to the point of uselessness.

For this reason bimetallic technology is limited to local temperature indication and ... in some applications local temperature recording.

 

SUMMARY OF USES OF BIMETALLIC TECHNOLOGY IN THE PROCESS INDUSTRIES.

The bimetallic strip is a temperature sensing element (TE) used in a wide variety of process industry applications which is linked to a local temperature indicator (TI ... aka the pointer and dial) that an equally wide variety of Outside Process Operators monitor several times each day all over the world.

In all cases, the process streams being monitored must be within the temperature range -65 °C to 430°C (-85 °F to 806°F).

 

TAKE HOME MESSAGES: Bimetallic strip technology is accurate, repeatable, and reliable because of the well known linear relationship between temperature increases/decreases and the expansion/contraction rates of metals.

The following criteria greatly impact the effectiveness of bimetallic strip technology:

• The choice of the two conjoined metals.
• The length of the metal strips.
• The thickness of the metal strips

Both the rate of thermal expansion and the heat transfer properties of metals used in bimetallic strip technology impact the measurement accuracy.

The deflection movement of the bimetallic strip increases with the square of the strip length.

The range of temperature measurement for bimetallic technology is limited by the physical limitations of strip length and width matched to the physical pipes and vessels that store and transport the process streams that must have their temperatures monitored by Outside Process Operators.

Bimetallic technology can also be used to record local temperatures and to create a dependable, mechanical temperature on-off switch.

 

©2016 PTOA Segment 00105
PTOA Process Variable Temperature Focus Study Area
PTOA Process Industry Automation Focus Study Area

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