Or maybe when you're older
And you're thinking back
Well you might recall
Now did I act carefully, did I do right?

("Do You Recall?," by Genesis, 1982)

PTOA SEGMENT #105: INSTRUMENT TECH MUST-KNOWS: USES OF BIMETALLIC STRIPS

PTOA Segment #105 once again painstakingly showed "Instrument Techie" PTOA Readers and Students how to use the instrument error assessment process to determine the optimal use of bimetallic technology for the purposes of detecting and measuring the process variable Temperature.

The assessment process had been previously used to evaluate liquid-in-glass and fluid-filled system technologies and included considering the factors that impact overall measurement error and repeatability, measurement response lag, and the reliability of each instrument.

"Instrument Techie" PTOA Readers and Students learned that the capability of bimetallic strip technology to accurately infer a measured temperature depends upon selecting two metals with good conductivity characteristics and widely different thermal expansion rates."Instrument Techie" PTOA Readers and Students learned that bimetallic technology can accurately detect and measure temperatures within the range of -65 °C to 430°C (-85 °F to 806°F) and has a typical "inherent measurement error" of  ±1% of span.

"Instrument-Techie" PTOA Readers and Students learned that bimetallic strip technology is very repeatable because it is based upon the one-to-one (aka linear) physical relationship that exists between the expansion and contraction rate of metals upon exposure to increasing or decreasing temperatures.

"Instrument Techie" PTOA Readers remembered that fluid-filled system technology was also based upon a linear, one-to-one relationship between fluid density changes that occur with temperature changes.

Also like fluid-filled systems, "Instrument Techie" PTOA Readers and Students learned that bimetallic strip technology was dependably rugged and simple technology because no electrical power was required to generate a measured temperature in a format human beings could understand.

Yet bimetallic strip technology had a much faster measurement response time (aka much lower measurement response lag) than fluid-filled systems  ...

... and that's because the deflection movement of the bimetallic strip is increased with the square of its length.

Thus, the length of a bimetallic strip is increased by helically winding the conjoined strips which greatly increases their sensitivity of temperature measurement.

 

"Instrument Techie" PTOA Readers and Students learned that this significant decrease in response measurement lag made bimetallic strips the go-to technology for local temperature detection and measurement ...

... which explained why bimetallic strip TIs are found all over a processing facility ...

 

 

... and  hopefully all bimetallic TIs are properly inserted into thermowells that have been submerged 2 to 3 inches into the media that is having its temperature measured ...

otherwise the temperature indicated will be bogus!

"Instrument Techie" PTOA Readers and Students learned that bimetallic technology can be enhanced to include local temperature recording (albeit with some sacrifice in response time) and had already learned that the technology could be used for simple on-off control.

There were two instructional bonus gems tucked into PTOA Segment #105:

First, yet another naturally occurring linear relationship was shown to be exploited by mankind and transformed into a useful technology that makes it possible to infer a process temperature in a format that human beings can understand.

This gem was tucked into the verbiage that explained why bimetallic strip technology generates repeatable temperature measurements.

Secondly, the hard work that PTOA Readers and Students devoted to learning the principles of heat transfer was applied to understanding the sources of measurement response lag that can occur with bimetallic strip technology.

 

PTOA SEGMENT #106: WE GONNA ROCK DOWN TO ELECTRIC AVENUE

PTOA Readers and Students had probably noticed that all the temperature detecting/measuring technologies up to this point were limited to local temperature measuring and indicating ... meaning the TIs were out with the pumps and the pipes and monitored by the Outside Process Operator.

That's because these instruments did not have any capability to generate a standard signal that could be successfully transmitted into a control room without distorting the inferred temperature measurement.

 

In PTOA Segment #106 PTOA Readers and Students were introduced to electrical detecting and measuring technologies that are likewise situated out in the plant area by the pumps and the pipes ...

yet their standard signal electrical outputs are rapidly transmitted to the cozy control room that is usually several hundred feet away.

 

 

PTOA Readers and Students learned that:

Thermocouples generate a standard electrical current signal output which is measured in milliamps.

RTDs (Resistance-Temperature Detectors) and Thermistors cannot generate a standard signal output; rather they both generate an electrical resistance output measured in ohms which is thence converted into a standard signal form of millivolts.

PTOA Segment #106 then launched into introducing the three mathematical expressions that define Ohm's Law ...

 

 

I = V/R     V (or E) = I*R     and  R = V/I

... which made it possible for PTOA Readers and Students to easily understand the direct and inverse relationships that exist between electrical Current (I, measured in Amps or milliAmps), Voltage (V and sometimes E, measured in Volts or milliVolts), and electrical Resistance (R, measured in Ohms).

The variations of Ohm's Law presented in PTOA Segment #106 showed PTOA Readers and Students how human beings have successfully modelled the "Electrical Transport Phenomena" and can predict how electrical energy will flow throughout an electrical circuit.

Yes Indeedo!

Just like the mathematical expressions for heat transfer that were presented in the PTOA Heat Transfer Focus Study Area, Ohm's Law is likewise a simple mathematical expression that can define Current (I), Voltage (V or E), and electrical Resistance (R) and also predicts how electrical energy can be forced to flow through circuits that human beings design.

A small postscript in PTOA Segment #106 informed PTOA Readers and Students that a common trait and requirement between the flow of electrical current (I) and the flow of heat (q or Q) is the existence of a driving force:

All PTOA Readers and Student who are reading the PTOA Segments in the intended sequential order by now could write a thesis that explains why:

Heat (q or Q) will not transfer via conduction, convection or radiation without a Temperature Differential (ΔT).

Likewise ...

Electrical current (I) will not flow through a circuit without the existence of a Voltage Differential (ΔV) which is also sometimes called "Potential Difference" and sometime just verbally abbreviated to the expression "potential."

PTOA Readers and Students who may be fretting about the wonky sounding terminology should chillax.

All's Your Mentor is trying to do here is show how the concepts that you have worked hard to learn by heart are starting to show up again in slightly altered states.

Pattern recognition is one of the things that separates humans from other species.

The above paragraphs just mean that ... in the absence of a difference in temperature ... heat transfer will not occur and everything will stay at the same temperature.

Hey, you already know that!

Likewise, in the absence of a voltage difference or potential difference, electrical current will not flow no matter what.

Fortunately batteries can be placed in a circuit to boost up their voltage and provide the electromotive force that keeps electrons moving!

Otherwise, the circuitry can be modified to create a potential difference between two points within a circuit ... that modus operandi also keeps electrons moving through the circuit as desired.

To directly link how the theory of Ohm's Law is applied in the real world, PTOA Segment #106 ended with an introduction to the structure and operating theory that supports thermocouple technology.

Like bimetallic strips, PTOA Readers and Students learned that thermocouples are made from two dissimilar metals that have a wide variance in thermal expansion rate.

However ... unlike bimetallic strips ...

the two thermocouple metals are fabricated into metal wires that are joined just at their ends.

PTOA Readers and Students learned that thermocouples work because when two dissimilar metals are joined at their ends and one end is exposed to heat, a milliVoltage output that correlates to a temperature is generated.

©2016 PTOA Segment 00126
PTOA Deja Vu Review 4-6

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