Talking 'bout do it, fluid
Talking 'bout do it, fluid
Talking 'bout do it, fluid
Talking 'bout do it, fluid

("Do It, Fluid," By Donald Byrd & the Blackbyrds, 1974)

In this PTOA Segment #102, PTOA Readers and Students will begin to assess the benefits and limitations of fluid-filled temperature measuring systems.

In PTOA Segment #98, PTOA Readers and Students learned that an industrial fluid-filled measuring device has a bulb, a capillary, and a helically wound bourdon tube.

The exercise will conclude in the next PTOA Segment #103 and through the process PTOA Readers and Students will expertly determine the best industrial use of fluid-filled temperature measurement technology within a processing facility.

 

"FLUID-FILLED"  CLARIFIED

PTOA Segment #3 entitled "Do You Know What I Mean?," clarified that the term "fluid" pertains to liquids, gases, and vapors.

Depending upon the service of the instrument, the working fluid in a fluid-filled temperature system could be:

• a liquid like mercury.
• a gas like nitrogen.
• an easy-to-boil liquid on the verge of vaporizing a bit more into a gas when heated or condensing a bit more back into a liquid when cooled, like toluene or an unreactive alcohol.

The working fluid listed last describes a  "vapor pressure type fluid-filled system."

The working fluids chosen for this application are selected for their vp vs temperature relationship over the range of temperatures that need to be measured.

The bulb in a vapor pressure type system is partially full of volatile liquid.

The vapor phase on top of the liquid level is saturated which means ...

... the slightest increase in temperature, the more liquid vaporizes into the vapor state and...

... the slightest decrease in temperature, the more of the vapor phase condenses back into the liquid phase.

PTOA Readers and Students who are reading the PTOA Segments in the intended sequential order are already savvy understanding the phrase "saturation of liquid and vapor" because saturation was covered when studying steam generation in PTOA Segment #26 entitled "Houston, You Fixed the Problem. "

 

THE WORKING PRINCIPLE OF ALL TYPES
OF FLUID-FILLED SYSTEMS

The mechanics of generating a measurable temperature is the same regardless of the choice for a working fluid in a fluid-filled sytem:

• The bulb is immersed in the process liquid or gas stream that is having its temperature measured →
• Heat is conducted through the wall of the bulb into the working fluid →
• The working fluid absorbs the heat and its volume increases to fill up the capillary →
• Once the capillary is full, the entrapped and hot fluid creates a force that moves the helical bourdon tube →
• The mechanical movement of the bourdon tube can be linked to a pointer on a dial which indicates the temperature in a format human beings can understand.

 

INHERENT INSTRUMENT MEASUREMENT ERROR OF FLUID-FILLED SYSTEMS

Accuracy/Error

A filled system can have an error of ± 0.5% of span up to 2.0% of span.

With an effective measurement range of -185 °C to 650 °C, the % error stated above translates into ± 4 °C to almost ±  17°C.

The amount of error depends upon the chosen working fluid, bulb type, capillary length, and whether or not the helix bourdon tube is compensated (a technical term that is explained below).

Just like liquid-in-glass thermometers, the most accurate fluid-filled system design would ideally have a large volume of working fluid in the bulb relative to the volume of fluid contained in the capillary.

Why? ...

"Conduction  Happens"

Heat transfer via conduction is an equal-opportunity phenomenon; the bulb is not the only piece of hardware ruled by the laws of heat transfer via conduction!

The wall of the capillary tube is also going to experience heat transferred into it via conduction when the surrounding processing ambient atmosphere is warmer than the capillary.

First by conduction and then by convection, heat is transferred into the working fluid.

Naturally, the warm working fluid expands inside the capillary and impacts the helically wound bourdon tube.

So the bourdon tube feels a force and moves even though the force is not created by a temperature change in the process fluid that is having its temperature measured.

The measurement error generated within the capillary and bourdon tube can be fully compensated using a technology that involves linking a duplicate bourdon tube which offsets the error-creating movement.

In some applications, the capillary is sufficiently short and thus less  error-prone; the error from just the bourdon tube movement can be more simply case compensated.

In either situation the term 'compensated' infers that the movement from a complementary bourdon tube is subtracted from the measuring bourdon tube to generate a more accurate temperature reading.

Inherent instrument error of vapor-pressure fluid-filled systems.

Vapor pressure fluid-filled systems are very sensitive to temperature measurement which is fantastico with respect to reducing instrument response measurement lag.

Unfortunately, as the above graph shows, the vapor pressure/temperature relationship is not linear throughout the complete temperature range.

Of the three fluids graphed above, the plotted data show that the red fluid has a linear (straight line) relationship between temperature and vp when temperatures are in the range 5 to 38 °C.

The blue fluid is has a linear vp/temperature relationship in the temperature range of 50 to 78 °C.

The black fluid data indicates it has a linear temperature/vp relationship from 80 to 100 °C.

Error in temperature measurement can occur when the process fluid's temperature creeps outside of the regions where the temperature/vp relationship is linear.

 

RESPONSE LAG
OF FLUID-FILLED SYSTEMS

Any card-carrying PTOA Reader or Student can tell ya by now that fluid-filled systems have a significant amount of response lag measurement error because of the time it takes for a change in the process fluid temperature to be transferred as conducted heat one way or the other through the wall of the bulb not to mention the heat transfer via convection that expands or contracts the working fluid, depending upon if the measured temperature is increasing or decreasing.

Happily, there are some bulb designs that can decrease the measurement lag by increasing the amount of surface area available for conduction heat transfer.

PTOA Readers and Students learned how increased surface area increases the rate of conduction heat transfer in PTOA Segment #62 entitled "Can't Touch This!"

 

Bulb Styles

Three types of bulbs in use are:

The sturdy Plain Bulb is inserted into a temperature well to indirectly sense the temperature changes of a process fluid.

Averaging Bulbs are long and can be inserted deep into a duct. This design structure allows more of the temperature sensing bulb to be in contact with the process fluid. The increased surface area exposed to temperature changes decreases response lag.

The Capillary Bulb style has a helix-wound capillary which greatly increases its surface area yet is more compact than the Averaging Bulb. And, of course, the additional surface area greatly reduces the response time of the system to generate a change in measured temperature.

TO BE CONTINUED ...

In the next PTOA Segment, PTOA Readers and Students will conclude their assessment and determine where liquid-filled temperature measurement systems work best in processing industries.

TAKE HOME MESSAGES: Industrial use fluid-filled temperature measurement systems work on the basis of temperature changes that create changes in the volume of the working fluid.

The components that make up the system are bulbs, capillaries, and bourdon tubes. These components work together to convert molecular movement into a reading human beings can understand.

The working fluid can be a liquid, or gas, or a liquid with a saturated vapor space above it (aka vapor pressure system).

Fully compensated and case compensated liquid filled systems reduce the measurement error that results when ambient conditions transfer heat via conduction into the capillary and bourdon tube. This source of heat transfer has nothing to do with measuring the temperature of the process fluid in which the bulb of the system is immersed.

Bulb designs with greater conduction heat transfer surface area reduce the measurement response lag error associated with fluid-filled temperature measurement.

The three bulb types in use are:

• Plain Bulb
• Averaging Bulb
• Capillary Bulb (not to be confused with the capillary tube used in the system).

©2016 PTOA Segment 00102
PTOA Process Variable Temperature Focus Study Area
PTOA Process Industry Automation Focus Study Area

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