I used to be half empty,
But now I'm half full.
I'm half full, I'm half full
You woke me up.

("Half-Empty Jam" aka "Half-Full Jam," by Marillion, 2008)

 

THE COMMON LIQUID-IN-GLASS THERMOMETER

The sick dude featured in the photo on the right has a fever.

His temperature is being detected and measured with a common liquid-in-glass thermometer that all PTOA Readers and Students will recognize.

PTOA Readers and Students also learned how liquid-in-glass thermometers are used by Outside Process Operators to measure the wet and dry bulb temperatures of air.

 

The PTOA Segment #77 entitled "It's All Relative" showed how wet and dry bulb temperatures can be used to determine the Relative Humidity of air, a parameter that can be used to optimize the operation of a cooling tower.

 

LIQUID-IN-GLASS THERMOMETER COMPONENTS

The temperature-sensing components of a liquid-in-glass thermometers are:

• a fluid-filled Bulb that is attached to
• a Capillary Tube.

The bulb contains the working liquid that is going to expand (when heated) or contract (when cooled) which changes the height of the column of liquid in the capillary tube.

The bulb is not usually as big as the big red bulb shown in the picture of a bulb and capillary at the left.

The working liquid in the bulb on the left is an alcohol that boils at such a low temperature that it can be used in hand-boiler toys like the one shown on the right.

The bulb-capillary are encased in a glass stem that has temperature markings (called graduations) engraved on it as shown to the right.

The working liquid in most liquid-in-glass thermometers is mercury because mercury very conveniently expands just 1.8% for each 1 °C increase in heat absorbed; ergo, a much shorter capillary and stem can be used to detect and measure most temperatures encountered on the earth's surface.

Process Operators must be careful when handling liquid-in-glass thermometers that are filled with mercury.

Mercury that spills out of a broken thermometer is hard to resist playing with because of its natural tendency to form roly-poly liquid balls at room temperature.

Always remember and never forget that mercury is highly toxic and will find a way to target human soft tissues when touched. 

Mercury is so toxic that its disposal is subject to all the regulations required of liquid hazardous wastes.

 

HOW LIQUID-IN-GLASS THERMOMETERS WORK

All fluid-filled temperature measurement instruments work on the same principle: thermal expansion causes a change in volume/density or length.

Liquid-in-glass thermometers work because ... as any PTOA Reader or Student who reads the PTOA Segments in the intended sequential order can tell ya ...

An increase in heat transferred into a liquid →

excites, agitates the liquid molecules so they bump and jive into each other and want more room →

resulting in an expansion of the volume that contains the liquid molecules.

The photo to the left exaggerates how the molecules of the working liquid in a liquid-in-glass thermometer become excited when warmed and expand their volume by moving up into the capillary of the thermometer.

A future PTOA segment will explain in detail that when the same number and type of molecules are contained in different volumes, this is also known as a change in density.

Ergo, when a change in temperature changes volume, it also changes the physical property known as "density."

The graphic to the right is less exaggerated than the one above.

In the real world, the molecules of the working liquid appear to gradually fill up the capillary as more and more heat is transferred into the fluid.

 

In summary, as the working liquid absorbs more heat, the liquid expands and increases the height of the liquid in the capillary.

As the working liquid becomes cooler, the molecules condense back together and the height of the liquid in the capillary decreases.

 

LIQUID-FILLED TEMPERATURE MEASUREMENT INSTRUMENTS FOUND IN THE PROCESS INDUSTRIES

All PTOA Readers and Students will recognize that liquid-in-glass thermometers are very helpful for detecting and measuring local temperatures like fevers or dry-bulb/wet-bulb air temperatures.

With just a little tweaking, this physical property of thermal expansion can be used to translate molecular movement into process stream temperatures that human beings called Process Operators can understand.

When a bourdon tube is attached to the end of an industrial-strength bulb-capillary ... voila! ... local industrial process temperatures can be indicated to anyone who can learn how to read a dial.

The picture above is a cutaway illustration of a process industry TI that works on the principle of thermal expansion and density changes.

The bulb (shown in a protective sheath) is immersed into the process stream that needs to have its temperature measured.

The bulb might be filled with working liquids called xylene or toluene or maybe even with mercury which was described above.

When heated, the working liquid expands through the capillary and causes movement in the attached bourdon tube.

Guess what!

PTOA Readers and Students have already been introduced to this liquid-filled industrial strength temperature detecting/measuring/indicating and recording instrument... the dial face just looked different.

PTOA Segment #8 entitled "Give Me a "T" for Temperature" and PTOA Segment #14 entitled "I Just Gotta Get a Message to U" featured the above  local temperature measuring instrument that could be used with a fluid-filled capillary-bulb device to detect and measure a process stream temperature.

Hey!

Your Mentor warned ya that the PTOA Temperature Detection/Measurement/Indication Focus Study Area would return us to the beginning over and over again!

 

BOURDON TUBE DEFINITION AND OPERATION

The bourdon tube is the amazingly simple device that translates volume expansion (originating from a change in molecular activity) into a mechanical movement.

Remember how the expansions and contractions of the working fluid in a  liquid-in-glass thermometer are interpreted as temperatures by human beings via reading the etched scale graduations on the glass stem?

The mechanical movement of a bourdon tube is likewise linked to a scale that indicates the magnitude of a process variable measurement in terms that human beings can understand.

The picture to the right illustrates a simple C-shaped bourdon tube that would be found in a gauge that measures the Process Variable Pressure, not Temperature.

The C-shaped bourdon tube is the best model to illustrate the tip movement of all bourdon tubes:

When the pressure of the gas inside the C-shaped bourdon tube increases, the gas molecules become agitated and seek to expand their enclosure.

Because the C-shaped bourdon tube is fixed at one end, only the tip has the freedom to move.

The tip tries to uncurl as shown in the photo to the left.

This small mechanical movement (aka "displacement") must never be taken for granted because it is converting a change in molecular movement into a mechanical movement.

The mechanical movement is then linked to a scale with a pointer and can even be linked to a recording device that will locally record the history of the process variable that the Outside Process Operator can review later.

Your Mentor and the PTOA give sincere thanks to pumpfundamentals.com for creating the below animated graphic which clearly demonstrates how the mechanical movement of a bourdon tube translates molecular movement into process variable readings that human beings can understand.

TEMPERATURE MEASURING BOURDON TUBES

Ok.

Who noticed that the C-shaped, gas-filled bourdon tube that is used to measure and indicate pressure looked a lot different than the more helix-shaped bourdon tube used to measure temperature?

Different shape bourdon tubes may be filled with gas or liquid but all perform the same function ... they generate a mechanical movement from a change in molecular movement.

The below graphic shows a temperature measuring bourdon tube that is supposed to be drawn in the shape of a helix.

As the working liquid in the capillary is heated, the helix-shaped bourdon tube makes a movement to uncoil.

The linkage attached to the top of the bourdon tube coil causes the pointer on a dial to move which indicates the local temperature in terms that human beings can understand.

The next PTOA Segment #99 will include more detailed information about liquid filled temperature measuring/indicating instruments that only Instrument Techs need to know.

If you know that Process Operations is your bag, go get a cup of coffee!

TAKE HOME MESSAGES: Using the commonly recognized example of a liquid-in-glass thermometer, PTOA Segment #98 explained how volume expansion and density changes caused by heat transfer are translated into temperature readings that human beings understand.

PTOA Readers and Students examined the operating principle behind the common liquid-in-glass thermometer that uses mercury as the working fluid.

They noticed that scale graduations etched on a glass stem are used to translate volume/density changes of mercury contained in a bulb and capillary into measured and indicated local temperatures.

Yes, this PTOA Segment was too long. Your Mentor wanted to show PTOA Readers and Students that the technology that makes the common liquid-in-glass thermometer work directly applies to fluid-filled systems that are used to measure and indicate local process stream temperatures.

The bourdon tube is a simple yet effective technology that makes it possible to translate molecular movement into a mechanical movement.

The mechanical movement is then linked to scaled dials with pointers that indicate the magnitude of process variables (pressure or temperature) in terms that human beings can easily understand.

Attaching a helix-shaped bourdon tube to an industrial strength bulb-capillary system makes it possible to translate the volume/density changes of fluid-filled bulb-capillary systems into locally measured and displayed process stream temperatures.

©2016 PTOA Segment 00098
PTOA Process Variable Temperature Focus Study Area
PTOA Process Industry Automation Focus Study Area

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