Yeah, I know there are more important things,
But don't forget to remember me.

("Don't Forget To Remember Me," by M.Hayes, K. Lovelace, A. Gorley, sung by Carrie Underwood, 2006)

 

PTOA SEGMENT #101: INSTRUMENT TECH MUST-KNOWS: USES OF LIQUID-IN-GLASS TEMPERATURE MEASUREMENT

PTOA Segment #101 demonstrated how "Instrument Techie" PTOA Readers and Students can apply what they had just learned about the sources of measurement error to assess whether or not a temperature-measuring technology would be suitable for a proposed temperature-measuring service.

The sources of measurement error considered during this exercise were:

• (Inherent) Instrument Error.
• (Measurement) Response Lag.
• Instrument Reliability.

The example used in this exercise was the liquid-in-glass thermometer technology; however, the assessment process detailed in this PTOA Segment #101 could be universally applied to assess the suitability of any process variable measuring instrument.

Regarding Inherent Instrument Error, "Instrument Techie" PTOA Readers and Students were challenged to calculate how many degrees of measuring error results when a manufacturer states the accuracy of a liquid-in-glass thermometer to be "1% of its 300 °C (540 °F) span."

 

 

Your Mentor presumes that the future-most-successful "Instrument Techie" PTOA Readers and Students did the math that verified the example liquid-in-glass thermometer had an inherent instrument measurement error of ± 3 °C (5.4 °F).

Your Mentor predicts that the future-most-successful "Instrument-Techie" PTOA Readers and Students did the math because they have "picked up what Your Mentor has put down:"

The successful application and use of this liquid-in-glass thermometer cannot be determined until the stated % inherent instrument error has been converted into a process variable equivalent (in this example, degrees).

While considering the portion of measurement error caused by  Measurement Response Lag, "Instrument Techie" PTOA Readers and Students directly applied what they had learned in the PTOA Heat Transfer Focus Study Area.

"Instrument Techie" PTOA Readers and Students expertly understood that the time delay required for conduction and convection heat transfer to occur between the process stream having its temperature measured and the expansion or contraction of the working fluid in the thermometer was the source of Measurement Response Lag error.

Furthermore ... because of their fundamental understanding of conduction and convection heat transfer ...

"Instrument Techie" PTOA Readers and Students were not surprised to learn that the depth to which the bulb of the liquid-in-glass thermometer is immersed in the process fluid or stream directly impacts the measurement error that is attributable to Response Lag.

At this point in their assessment, what remained to be considered was the Reliability of the liquid-in-glass technology.

Applying their knowledge of the physical properties of fluids,  "Instrument Techie" PTOA Readers and Students understood why the liquid-in-glass thermometer was able to consistently repeat temperature measurements and therefore did not require calibration.

Otherwise stated ...

Liquid-in-glass thermometers do not need to be calibrated because of the reliable relationship between changing temperatures and the expansion/contraction of the thermometer's working fluid.

"Instrument Techie" PTOA Readers and Students determined the liquid-in-glass technology to be simple and straight forward to use ...  as long as the meniscus was read on the bottom and the scale divisions were easy to read.

Another realized benefit of liquid-in-glass technology was the ability to detect and measure a temperature without electrical power.

Next, "Instrument Techie" PTOA Readers and Students considered the limitations of liquid-in-glass thermometers that might impact instrument Reliability.

They learned why mercury is the most popular working fluid in thermometers and why the temperature-measuring range of mercury-filled thermometers is limited to -200 to 620 °C (-328 to 1148 °F).

The biggest limitation for liquid-in-glass technology was determined to be its inability to be made automatic  ... a human being would always be required to determine the temperature measurement.

Ergo, the use of liquid-in-glass technology was concluded to be limited to local temperature measurement applications only ... for example determining the temperature of a sampled process stream and when wet and dry bulb thermometers are used to optimize Cooling Tower operations.

 

 

 

PTOA SEGMENT #102: INSTRUMENT TECH MUST-KNOWS: USES OF FLUID-FILLED TEMP MEASUREMENT (Part 1)

 

The PTOA Department of Redundancy Department decreed that the best way to learn the instrument error assessment process demonstrated in the previous PTOA Segment #101 for liquid-in-glass thermometers was to repeat the procedure for fluid-filled systems which had been introduced in PTOA Segment #98.

"Instrument Techie" PTOA Readers and Students were reminded that fluids can be liquids (like mercury), gases (like nitrogen), and on-the-verge-of-boiling mixtures of liquids and gases (aka vapors) ... all of which predictably expand and contract with changes in temperature.

They were also reminded that the components of a fluid-filled system include a bulb, a capillary and a helical bourdon tube which interfaces with a pointer on a dial face.

"Instrument Techie"  PTOA Readers and Students quickly became aware that fluid-filled systems would be more difficult to analyze than liquid-in-glass thermometers because the inherent instrument error of fluid-filled systems was stated as a range that varied between 0.5% of span up to 2.0% of span ... dependent upon:

• the chosen working fluid.
• bulb type.
• capillary length.
• Whether or not the helical bourdon tube had been compensated for ambient temperature changes (which are not related to changes in the temperature of the process fluid having its temperature measured).

"Instrument Techie" PTOA Readers and Students learned that the range of successful temperature measurement for fluid-filled systems was -185 °C to 650 °C.

Combining this information with the % error range stated above made it possible to convert the inherent instrument measurement error into degree equivalents of  ± 4 °C to almost ± 17°C ( aka ±7 °F to ± 31 °F).

That's quite a range of inherent instrument measurement error!

A temperature-measuring application that can tolerate a ± 4 °C (±7 °F) temperature variance without significantly impacting final product quality may not be as successful if the variance in temperature were allowed to be  ± 17°C (± 31 °F).

"Instrument Techie" PTOA Readers and Students learned that two steps can be taken to reduce the inherent instrument error of fluid-filled systems:

• Vapor pressure type fluid-filled systems must be used to detect and measure temperatures that lie well within the linear relationship range between vapor pressure and temperature.
• The movement of the helical bourdon tube caused by ambient temperature changes ... not changes in the process stream temperature ... must be "fully compensated" (aka offset) when the capillary is long or at least "case compensated" when the capillary is short.

Can we talk?

Understanding "compensation" is crucial when working with fluid-filled temperature measuring systems and well worth reiterating in this review.

"Instrument Techie" PTOA Readers and Students learned that "compensation" means that the movement from a complementary bourdon tube is subtracted from the movement of the measuring bourdon tube ... the end result generates a more accurate temperature reading.

 

The second fluid-filled system phenomenon "Instrument Techie" PTOA Readers and Students must thoroughly understand is the relatively large contribution to total Instrument Measurement Error that is caused by Measurement Response Lag.

The significant time that it takes for conduction and convection heat transfer to occur between a change in the process stream temperature and the expansion or contraction of the working fluid in the bulb ... and thence movement of the bourdon tube ... is a major contributor to the total instrument measurement error observed in fluid-filled systems.

"Instrument Techie" PTOA Readers and Students learned at the conclusion of PTOA Segment #102 that the choice of bulb style can reduce the contribution to total instrument error that is caused by Measurement Response Lag.

Specifically, the large surface area of the Capillary Bulb style enhances conductive heat transfer and reduces Measurement Response Lag in fluid-filled systems.

The instrument error assessment process for fluid-filled systems and determination of best industrial uses would conclude in the following PTOA Segment #103.

©2016 PTOA Segment 00124
PTOA Deja Vu Review 4-4

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