I remember the night
I remember the sound
I remember the light 

("You and Me," by Sara Watkins, 2012)

PTOA SEGMENT #117: INSTRUMENT TECH MUST-KNOWS ABOUT THERMISTORS

 

Both Your Mentor and You Tube instructor Jacob Dykstra mention multiple times in  PTOA Segment #117 that the typical "Instrument Techie" PTOA Reader or Student will not ever be responsible for determining which of the many types of thermistors is best for the desired application.

In the event Your Mentor and Jacob Dykstra are dead wrong ...

and future "Instrument Techie" PTOA Readers and Students essentially become "advanced thermistor application technicians" ...

then the below criteria are what you ...

the Thermistor Expert ...

will use to match the optimal thermistor for each temperature-measuring application:

• The required temperature measurement accuracy and how the self-heating error will be eliminated if necessary.
• The required temperature-measurement reliability.
• The required measurement response time.
• The ease of correlating the thermistor output to a temperature measurement.

The rest of the "Instrument Techie" PTOA Readers and Students just needed to learn the following interesting facts about thermistors which were presented in PTOA Segment #117:

Thermistors have a predictable ... but not linear... relationship with temperature.

 

 

 

The "predictable" relationship between the thermistor's output of electrical resistance (measured in ohms =Ω ) and the temperature being measured ...

... was easier to observe once the temperature scale of the combined thermistor-thermocouple-RTD graph was limited to just the thermistor's range of temperature measurement.

Once limited to the thermistor's temperature range, the electrical resistance output and corresponding temperature revealed the thermistor's "characteristic curve."

"Instrument Techie" PTOA Readers and Students learned that when the data that make up the "characteristic curve" are fed into the Steinhart-Hart equation, the relationship becomes much more linear and ergo useful as a means to detect and measure temperature.

A thermistor's "measurement sensitivity" was a defined as the electrical resistance output from the thermistor that correlates to a detected temperature.

Ergo, the units of a thermistor's "measurement sensitivity" are  Ω/°C.

The concept of "measurement sensitivity" made more sense after viewing the "measurement sensitivity table" for a typical 10 kΩ NTC thermistor shown below:

• 5600 Ω /°C output at -20 °C
• 439 Ω /°C output at 25 °C
• 137 Ω /°C output at 50 °C

"The measurement sensitivity table" revealed that the ohms output of the NTC thermistor is greatest at the lowest temperatures on the characteristic curve (where the curve is steepest).

The "measurement sensitivity table" also showed that the thermistor's "measurement sensitivity" declined rapidly over the relatively small temperature range that the thermistor measured.

The above conclusions were also observed real-time in Jacob Dykstra's Thermistor and Multimeter You Tube.

Jacob's video demonstrated how the electrical resistance output of a typical 10 kΩ NTC thermistor plummeted steeply downward as the thermistor was exposed to the heat of a flame.

Super smart "Instrument Techie" PTOA Readers and Students correctly deduced that the accuracy of temperature measurements would likewise decrease at the higher temperatures because there would be less ohms output per degree temperature measured.

(Just re-read the above paragraph while looking at the below graph).

Therefore, "Instrument Techie" PTOA Readers and Students would understand that in some applications a 2-lead wire Wheatstone Bridge might be necessary to improve the accuracy of a thermistor's temperature measurement throughout the temperature range being measured.

Truth be known ... like Your Mentor and Jacob Dykstra keep telling ya ...

in the real world ...

"Instrument Techies" will not be worried about whether a bridge is in the circuitry or not ...

 

...because s/he will be using the magical device known as a "multimeter" to test the output of various instruments.

The PTOA does not advocate the use of any particular manufacture of multimeter ...

but since we are all living in the real world ...

 

the Fluke brand multimeter used by Jacob Dykstra in his You Tube  is a standard device that can test the output of several instruments. 

Incidentally...

Your Mentor high-fives all "Instrument Techie" PTOA Readers and Students who read the PTOA Segments in the intended sequential order because they automatically understood what Jacob Dykstra meant when he placed his hand on the thermistor so that it could act "like a heat sink" to lower the detected temperature .... and hence increase the resistance output ... of the thermistor.

This PTOA Segment concluded with the following list of factors that could cause self-heating detection and measurement error in a thermistor:

• The size of the thermistor.
• The thermistor's protective covering.
• The measurements of the two lead wires.

 

PTOA SEGMENT #118: NO TOUCHING!

PTOA Readers and Students learned that pyrometry makes it possible to measure surface temperatures from a distance.

 

No PTOA Readers or Student who has read the PTOA Segments in the intended sequential order was surprised to learn that heat transfer via radiation makes optical and IR pyrometers function.

PTOA Readers and Students also learned that most emitted radiation from a surface occurs in the visible and infrared portions of the electromagnetic wave spectrum ...  the spectral range of pyrometers!

PTOA Readers and Students learned the following similarities and differences between Optical and Infrared (IR) Pyrometers:

• Optical Pyrometers are sometimes called "narrow band" pyrometers because they only work in the visible spectral range (.35 - 75 µ).
• IR pyrometers are sometimes called "broad band pyrometers" because they operate within both the visible and IR spectral range (.35 µ -14 µ).

 

• To infer a temperature, Optical Pyrometers compare the intensity of emitted surface radiation to the intensity of a platinum reference filament enclosed within the instrument.
• To infer a temperature, IR pyrometers focus the detected surface radiation onto a thermal element which might be a thermocouple, RTD, thermistor or perhaps photo cell. PTOA Readers and Students already competently understand how an electrical output from the thermocouple, RTD, or thermistor can be generated.

 

• Manual Optical Pyrometers are used by Process Operators to determine surface temperatures .... like the temperature of the tubes in a fired heater. When the Process Operator adjusts the knob, s/he is adjusting the intensity of the circuit exciting the platinum filament.
• Automatic Optical Pyrometers automatically match the glow of the platinum filament to the detected radiation intensity and can be used for temperature recording as well as temperature indicating.

The accuracy of temperature measurements for both the Optical and IR Pyrometer is highly dependent upon the instrument positioning:

• The Optical Pyrometer must be pointed so that the surface of interest is clearly visible; the filament brightness must be easy to match to just the light emitted from the surface.
• The IR Pyrometer will be not be able to distinguish radiation from the surface having its temperature measured from the random radiation emitted by dust and particles in the area. To generate an accurate temperature measurement, the surface of interest must be in direct line of sight of the pyrometer and the instrument must be pointed so that the lens is focused on and filled by the surface that is having its temperature measured (laser guides can help).
• Bandpass Pyrometers are a type of IR Pyrometer that has reduced emissivity error because it uses a more specific wavelength range.

 

©2016 PTOA Segment 00131
PTOA Deja Vu Review 4-11

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