Don't you think it's time
You remember me

("Remember Me," by Fleetwood Mac, 1973)

 PTOA SEGMENT #103: INSTRUMENT TECH MUST-KNOWS: USES OF FLUID-FILLED TEMP MEASUREMENT (Part 2)

PTOA Segment #103 concluded the instrument error assessment process for fluid-filled systems that had begun in PTOA Segment #102.

PTOA Segment #102 had featured contributions to measurement error from the following sources:

• The manufacturer's stated "inherent instrument error."
• Error caused by ambient conditions that will definitely impact the accuracy of the temperature measurement if not compensated with offsetting modifications.
• The error due to "measurement response lag" caused by the time required to conduct and convect heat through the instrument's bulb.

The STRATEGICALLY IMPORTANT information in PTOA Segment #103 was all the nouns and verbs devoted to explaining the function and design of bourdon tubes as well as how the proper choice of bourdon tube style will improve the sensitivity and accuracy of process variable measurement.

PTOA Readers and Students learned that bourdon tubes are THE DEVICES that perform the function of transducing molecular movement into a mechanical movement.

The mechanical movement (aka deflection) is linked to a pointer and dial that translates the temperature measurement into a reading that human beings can understand.

An animated graphic by that was once available at pumpfundamentals. com and shown below clearly illustrated how the molecular movement that causes pressure to build up in a C-shaped bourdon tube creates a deflection of the tube tip that is linked to a dial and pointer that human beings called Process Operators easily understand.

"Instrument Techie" PTOA Readers and Students learned that the deflection of a spiral-wound bourdon tube is more sensitive and therefore yields a more accurate measurement than the measurement of a C-shaped bourdon tube.

Even better ...helical-wound bourdon tubes have the most sensitivity of movement and thus yield the most accurate measurements.

 

"Instrument Techie" PTOA Readers and Students learned that stacked, helical bourdon tubes are the transducers found in instruments that use fluid-filled temperature measuring technology.

This PTOA Segment #103 concluded with assessing the Reliability of fluid-filled systems which led directly into considering the benefits and limitations of the technology.

"Instrument Techie" PTOA Readers and Students identified these benefits of fluid filled systems:

• No electrical power requirements are needed thus fluid-filled system technology is intrinsically safe.
• Because the fluid-filled system technology depends only upon the reliable physical relationship between temperature changes and the simultaneous expansion/contraction of the working fluid, fluid-filled systems are more reliable than temperature-measuring instruments that require electricity to function.

 

"Instrument Techie" PTOA Readers and Students had learned in PTOA Segment #102 that the error caused by the measurement response lag of the bulb can be reduced by the proper choice of bulb style ...

and PTOA Readers and Students learned in PTOA Segment #103 that the measurement response lag of the bourdon tube can also be reduced by selecting the best bourdon tube style for the service of the instrument.

Unfortunately, PTOA Segment #103 revealed the major limitation to fluid filled technology that cannot be remedied:

The length of the capillary between the bulb and bourdon tube is limited to 100 feet maximum because expansion or contraction of the working fluid through a capillary is a slow process.

Since capillary lengths exceeding 100 feet result in useless temperature indications, "Instrument Techie" PTOA Readers and Students deduced that fluid-filled systems would always be limited to local temperature detection, measurement, recording and control.

Furthermore, liquid-filled technology would also be limited to process services in which the process temperature is not expected to significantly vary.

At the beginning of PTOA Segment #103, "Instrument Techie" PTOA Readers and Students were reminded what a local temperature controller/recorder that uses fluid-filled technology looked like.

"Instrument Techie" Readers and Students were also reminded that the ISA standard symbol for a capillary system would be depicted on a P&ID as a line with equidistant Xs.

The local fluid-filled temperature controller shown in the picture above is more than likely the only application of a capillary system "Instrument Techie" PTOA Readers and Students will stumble upon in their careers.

 

PTOA SEGMENT #104: BEND ME, SHAPE ME, ANY WAY YOU WANT ME!

PTOA Segment #104 featured bimetallic strips, a temperature-measuring technology that works because of the reliable relationship between changing temperatures and the rate that different metals will simultaneously expand and contract.

PTOA Readers and Students learned that the deflection caused by two conjoined metals (which expand and contract at different rates as temperature changes) can be linked to a pointer and a dial ...

otherwise stated ...

the molecular movement and agitation that conducts heat into the two different metals at different rates is transduced into a mechanical movement that correlates to a temperature indication shown by a pointer on a dial face.

PTOA Readers and Students learned the success of bimetallic strip instruments depends upon:

• The choice of the two metals; the two metals chosen for the instrument should have widely different rates of thermal expansion.
• The length of the two metal strips; the increased length of a spiral-wound bimetallic strip increases the sensitivity and accuracy of the measurement and the helically-wound bimetallic strip technology has the best measurement sensitivity and accuracy.  Spiral-wound bimetallic strips are used in household thermostats and helically-wound bimetallic strips are used for local process temperature measurement with TIs.
• The thickness of the two metal strips; a thicker bimetallic strip will sacrifice sensitivity and accuracy of measurement because of the increased measurement response lag that results.

PTOA Readers and Students were not surprised to learn that the bimetallic strip must be protected in a thermowell which is inserted into the process stream that is having its temperature detected and measured.

The measurement lag that is required for conduction and convection heat transfer to occur from the process stream through the wall of the thermowell and hence into the bimetallic strip is reduced by fabricating the thermowell out of highly conductive material and packing silicone gel between the interior of the well and the strip.

PTOA Readers and Students learned that bimetallic strip technology can also be used for simple on-off temperature control.

Household oil stoves use this technology to control temperatures.

 

©2016 PTOA Segment 00125
PTOA Deja Vu Review 4-5

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