Lean on me when you're not strong
And I'll be your friend, I'll help you carry on.

("Lean On Me," by Bill Withers, 1972)

 

SELECTING THE RIGHT THERMISTOR FOR SERVICE:
NOT YOUR PROBLEM!

Thermistors can be fabricated into barrel-rod shapes, films, discs, and beads.

As PTOA Readers and Students recently learned in PTOA Segment #116, some thermistors are NTC and others ar PTC.

Thermistors work best in temperature-measurement and temperature-monitoring applications where the expected temperature range is known.

Just like the industrial application of RTDs, the resistance output of a themistor must be converted into a voltage output from a circuit and thence transmitted to a digitizer prior to being able to "talk with" a microprocessor that uses an algorithm to determine the change in resistance and correlate that finding into a temperature.

WHEW!

A lot of expertise is needed to make the resistance output of a thermistor generate a reliable and accurate temperature measurement!

There is a time and place to rely on professionals ... and selecting the right thermistor for the industrial temperature-measuring application is one of them!

Otherwise stated:

The Instrument Technician that is truly proficient understanding the intricacies of selecting the perfect thermistor for the desired temperature-measuring/monitoring service will undoubtedly be an employee of the electronic company that builds the instrument that incorporates the thermistor!

So in this case just relax and lean on the professionals; let them sweat the details of thermistor selection.

And yet ...

The official Process Technology curriculum being taught at colleges nationwide still insists that the following PTOA Instrument Tech Must-Knows about thermistor technology be given attention.

 

So mote it be!

OHHHHMMMM!

 

THE RESISTANCE-TEMPERATURE RELATIONSHIP OF A THERMISTOR IS NOT LINEAR!

Remember the graph shown to the right that compares the output of a themistor, a RTD, and a thermocouple  to the temperature being measured?

The graph shows that thermistor output of resistance in ohms (Ω) is predictable ... however, the relationship IS NOT linear.

Commonly available thermistors range from  250 Ω to 100 kΩ (aka 100,000 Ωs).

Each thermistor has a "characteristic curve" that clarifies the "Resistance Output-to-Temperature Measured" relationship for the device.

The scale on the characteristic curve below shows a resistance output range from 0 to a maximum of 50,000 Ω (aka 50 kΩ).

The temperature range is from -10 °C to 118 °C (14 °F to 244 °F).

The  characteristic curve for this thermistor clearly illustrates that the Resistance Output and Temperature Measured do not share a one-to-one, straight-line linear relationship.

Yet, the relationship is still predictable.

The resistance output from this themistor can be input to the Steinhart-Hart equation which would be programmed into a microprocessor of some type.

The output from the equation would reveal the temperature that correlates to an ever-changing value of resistance.

 

TEMPERATURE MEASUREMENT SENSITIVITY = MEASUREMENT ACCURACY

PTOA Readers and Students that read the PTOA Segments in the intended sequential order learned in PTOA Segment #116 that the real-world signficance of the plunging thermistor line in the nearby graph means that the semiconducting materials human beings used to fabricate a thermistor are intentionally crafted to yield a significant change in resistance output when there is just a small change in temperature.

The measurement sensitivity table for a typical 10 kΩ thermistor is as follows:

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

Wow!

That's a lot of ohms per °C change in temperature!

This sensitivity in temperature measurement makes it worth tolerating the required correction for nonlinearity because surely the measurement sensitivity must translate into increased accuracy with respect to the temperature measurement.

Yes indeedo ...  it does!

A thermistor fabricated to measure a temperature range of -30 °C to 125 °C (-22 °F to 257 °F) can have a measurement accuracy of 0.1 °C (0.18 °F).

A thermistor fabricated to measure a temperature range between -20 °C and 50 °C C (-4 °F to 122 °F) can have an accuracy of 0.01 °C (0.018 °F). This thermistor has a measurement accuracy that is ten times better than the accuracy for the thermistor described in the prior paragraph.

Trends related to measurement sensitivity and measurement accuracy become obvious upon  reviewing the data in the sensitivity table and the information in the above two paragraphs:

• The measurement sensitivity (aka Ω/°C) is greatest at the lowest temperatures on the characteristic curve (where the curve is steepest).
• The measurement sensitivity (aka Ω/°C) and measurement accuracy decline rapidly as the temperature being measured increases.

The professionals who select the best thermistor for temperature measuring and monitoring service must optimize the measuring sensitivity and measuring accuracy that is required.

 

TO WHEATSTONE OR NOT TO WHEATSTONE?

PTOA Readers and Students learned all about the purpose and structure of Wheatstone Bridge circuits in PTOA Segment # 115.

The increased measurement sensitivity of a thermistor means a Wheatstone Bridge circuit is not always necessary to translate the thermistor's resistance output into a temperature measurement.

Incorporating the thermistor into a bridge circuit will be necessary in applications that cannot allow the temperature measurement to be impacted by self-heating.

In applications where the thermistor must be incorporated into a Wheatstone Bridge circuit, the 2-lead bridge is probably sufficient to generate an accurate temperature measurement.

 

SIMPLE THERMISTOR CIRCUITS

Many useful applications of NTC thermistor technology do not require a Wheatstone Bridge circuit.

These simple applications of thermistor technology utilize the thermistor's high resistance output at ambient and colder temperatures in a variety of simple temperature-monitoring applications.

A typical simple thermistor circuit would include the following 3 things:

• 

  Typical NTC thermistor circuit has a power source and thermistor in series with a fixed resistor. A change in voltage is sent out of the frame to another electronic device in the circuit.

  A power supply.

• A thermistor in series with
• A fixed resistor.

All PTOA Readers and Students can apply what they learned in PTOA Segment #113 to predict that:

• An increase in temperature will cause a decrease in the thermistor's resistance.
• A change in the voltage drop across the fixed resistor will simultaneously occur.
• The voltage drop across the fixed resistor can be used to infer the thermistor's changed resistance.

A conventional digital multimeter is a device that can display the changes in resistance output from a thermistor.

Then (pre-established) calibration curves for that specific thermistor can be used to correlate the multimeter's resistance reading into a temperature.

Here's Jake to explain how a multimeter can be used to measure the resistance changes of a stand-alone disc thermistor that is not incorporated into a circuit.

Hang tight until about 3 minutes into the video.

Jacob Dykstra's Thermistor & Multimeter Video

 

SELF-HEATING

As stated above, the accuracy of the thermistor temperature measurement will be impacted by self-heating.

The goal of themistor design is to measure but not be influenced by a change in environmental temperature.

Factors that impact the self-heating of a thermistor include:

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

Eliminating self-heating is yet one more reason to leave thermistor selection up to the experts!

IN SUMMARY

The selection of thermistors used in process industry applications is best left up to the experts.

The experts will determine the optimal thermistor based upon:

• The required temperature measurement accuracy and elimination of self-heating error.
• The required temperature-measurement reliability.
• A fast measurement response time.
• The ease of correlating the thermistor output to a temperature measurement.

 

TAKE HOME MESSAGES: Selection of the optimal thermistor for the industrial process application is best left up to professionals in the field of electrical circuit design.

The Characteristic Curve for a themistor illustrates the thermistor's relationship between the Resistance Output and Measured Temperature.

The characteristic curve for thermistor is not linear but is still predictable.

The Steinhart-Hart equation is incorporated into a microprocessor and used to predict the temperature that correlates to a thermistor's change in resistance.

The measurement sensitivity of a thermistor is the Ω /°C observed at a specified temperature on the characteristic curve.

The measurement sensitivity and accuracy of a thermistor decline rapidly as the temperature increases.

Many simple applications of NTC thermistors do not require use of bridge circuits and microprocessors to generate a temperature measurement.

Simple circuit applications that incorporate thermistor technology at a minimum must include a power source and a fixed resistor in series with the thermistor.

Self-heating will impact the accuracy of a thermistor's temperature measurement.

Thermistor applications that must be incorporated into Wheatstone Bridge circuits can achieve accuracy with a 2-lead wire bridge.

Properly designed thermistors can quickly, reliably, and accurately measure and monitor temperatures.

©2016 PTOA Segment 00117
PTOA Process Variable Temperature Focus Study Area
PTOA Process Industry Automation Focus Study Area

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