Accept nothing short of complete reversal
Dig deep

("Arguing with Thermometers," by Enter Shikari, 2012)

 

In PTOA Segment #100, PTOA Readers and Students learned that all instruments installed at a processing facility are selected after considering the sources of Instrument-Related Measurement Error which include contributions from:

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

Furthermore, PTOA Readers and Students learned that the contributing factors to Instrument Reliability are:

• Instrument Service Functionality.
• The Physical Limitations of the Instrument.
• The Physical Working Environment of the Instrument.

Each instrument installed at the processing facility will have benefits and limitations that can be considered while using the above lists as a guide.

PTOA Segment #98 introduced the technology of fluid-filled temperature-measuring instruments like:

• Liquid-in-glass thermometers.
• Industrial sized bulb-capillary-bourdon tube systems.

During this PTOA Segment #101, PTOA Readers and Students will use the above lists as guides to assess the benefits and limitations of liquid-in-glass thermometers.

The exercise will help PTOA Readers and Students determine the best use of liquid-in-glass temperature measurement technology in a processing facility.

PTOA Readers and Students will repeat the assessment template for a bulb-capillary-bourdon tube system in the next PTOA Segment.

 

INHERENT INSTRUMENT MEASUREMENT ERROR
OF LIQUID-IN-GLASS THERMOMETERS

To reduce error, the ideal architecture for all fluid-filled temperature measuring devices would be a really large bulb and extremely small capillary.

If thermometers were really made like that their fragility would cause them to not last long in the processing environment.

Some liquid-in-glass thermometers used in the laboratory of the processing facility might be manufactured to an accuracy of 0.01 °C (0.18 °F).

The more rugged liquid-in-glass thermometers used in the processing area have an accuracy of plus or minus one whole scale division.

The manufacturer's data sheet states that a thermometer with a 300 °C (540 °F) span ranging from -20 to 280 °C (-4 to 536 °F) has an accuracy stated as 1%.

PTOA Readers and Students that read the PTOA Segments in the intended sequential order already learned in PTOA Segment #100 how to quantify the stated "inherent instrument measurement error" into the real world understandable temperature range of ± 3 °C (5.4 °F).

Do-It-Yourself (DIY!): Do the math and make certain you agree with the above conversion of  % error into a temperature range! All Instrument Techs must know how to do this!

 

RESPONSE LAG
OF LIQUID-IN-GLASS THERMOMETERS

PTOA Readers and Students already know that a temperature measurement device based on volume expansion like a liquid-in-glass thermometer will have measurement response lag because the working fluid in the bulb and capillary cannot be heated until the after the glass bulb and capillary have already been heated.

PTOA Readers and Students were introduced to the compared thermal expansion rates of Stuff in PTOA Segment #96 entitled "Temperature Measurement Must Knows (Part 2).

How many PTOA Readers and Students have also figured out that the glass bulb and capillary actually expand at a different rate than the working fluid when heated?

When a liquid-in-glass thermometer is designed and manufactured, the differing rates of thermal expansion for the glass and working fluid determine the internal diameter of the capillary and the spacing of the hash marks that define the temperature scale.

So guess what? How deep the bulb is immersed in the fluid that is having its temperature measured can impact the temperature measurement, too.

 

RELIABILITY OF LIQUID-IN-GLASS THERMOMETERS

Instrument Service Functionality

PTOA Readers and Students learned in PTOA Segment #100 that functionality of the instrument refers to its range of operation, accuracy, and power requirements.

Liquid-in-glass thermometers do not require calibration; their measurement readings cannot drift as might happen in a temperature measuring device based on resistance.

A liquid-in-glass temperature measurement is also highly repeatable (as long as the sample doesn't have time to cool down).

The highly repeatable measurement is because the reliable, direct relationship between temperature changes that cause changes in the expansion/contraction of the working fluid.

No power requirements are required for a liquid-in-glass thermometer to accurately measure a temperature.

 

Liquid-in-glass thermometers are simple to use ... gaze and measure. Just make certain that you read the BOTTOM of the meniscus.

On the other hand, depending upon the scale, liquid-in-glass thermometers are not the easiest thing in the world to read and there's no way to create a digital output from the device.

 

Low and High Measurement Ranges

The range of liquid-in-glass technology for measuring temperature is  -200 to 620 °C (-328 to 1148 °F).

The 600-ish °C (1100-ish °F) upper temperature measuring limit of liquid-in-glass thermometers derives from the fact that higher temperatures impact the glass and deteriorate the accuracy of the temperature.

The Most Popular Working Fluid is mercury.

Mercury freezes at -39 °C which is almost the same temperature on the Fahrenheit scale.

Ergo, mercury is not a good choice for a working fluid in a liquid-in-glass thermometer located outdoors in the arctic or antarctic.

Replacing the working fluid with alcohol makes it possible for liquid-in-glass thermometers to detect and measure temperatures down to -200 °C (-328 °F).

The high temperature limit for using liquid-in-glass thermometers is established by the fact that mercury will begin to boil around 674 °F (357 °C).

 

The surface of the planet Venus is 864 °F (462 °C).

No liquid-in-glass thermometer will function there without adjustments.

The top area of a mercury thermometer capillary could be blanketed with nitrogen (and therefore pressurized) to keep mercury from boiling until temperatures reached  1100-ish °F (600-ish °C) maximum.

Physical Limitations/Considerations

The physical limitations with respect to the instrument's internal and external dimensions include whether or not the instrument needs special mountings or connections.

Liquid-in-glass thermometers are easy to break. A closed cabinet would help safely isolate a liquid-in-glass thermometer from processing activities.

As mentioned in PTOA Segment #98, mercury is a health hazard and also a hazardous waste that must be properly disposed of.

On the bright side, liquid-in-glass thermometers are inexpensive and portable.

The very top photo of this PTOA Segment shows a Vintner Technician measuring the temperature of a vat of grapes. The photo is also shown below.

The vintner must not be expecting the temperature of the grapes in the vat to vary widely.

The Vintner Technician can easily move the thermometer from one vat to the next.

Before automation, measuring the process stream temperature with a liquid-in-glass thermometer immersed in a sample was a common practice used to help guide the conversion of raw materials to products.

The samples of intermediate and final process streams were collected at "sample stations" piped in throughout the process.

Nowadays "stream sampling" is still performed but is used for providing quality control feedback to process operations.

The obvious limitations of the liquid-in-glass technology eliminate its use in continuous process temperature control.

 

SUMMARY OF LIQUID-IN-GLASS THERMOMETER
BENEFITS

For the temperature range -200 to 620 °C (-328 to 1148 °F) the benefits of liquid-in-glass thermometer technology are:

• Highly accurate and repeatable with no need for calibration or special training to properly use.
• Inexpensive and easily portable to any location in the processing facility.

 

SUMMARY OF LIQUID-IN-GLASS THERMOMETER
LIMITATIONS

The benefits of the liquid-in-glass technology for measuring process temperatures are significantly offset by the great limitation of not being able to automate the  temperature measurement.

 

GOOD USES FOR LIQUID-IN-GLASS TEMPERATURE MEASUREMENT DEVICES IN PROCESS INDUSTRIES

The inability of liquid-in-glass temperature measurement technology to be automated limits the uses of this technology to:

• Process stream analysis performed in a laboratory.
• Process applications performed by Outside Process Operators like measuring the wet and dry bulb air temperature to optimally operate the use of cooling water fans.

 

 

 

TAKE HOME MESSAGES: PTOA Readers and Students can determine the best use of instruments after considering the device's error, measurement lag, and reliability.

Liquid-in-glass thermometers have great accuracy and repeatability and are also inexpensive, portable, easy to use, and have no power requirements.

The industrial use of liquid-in-glass temperature measurement is limited because it is not possible to convert the liquid-in-glass temperature measurement into a mechanical movement that can be used for automatic temperature indication, recording, or control.

Liquid-in-glass temperature measurement is used for determining the temperature of a sampled stream or product at one point in time.

©2016 PTOA Segment 00101
PTOA Process Variable Temperature Focus Study Area
PTOA Process Industry Automation Focus Study Area

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