But they're cousins,
Identical cousins all the way.
One pair of matching bookends,
Different as night and day.

("The Patty Duke Show Theme Song", by Sid Ramin, 1963)

Every card-carrying PTOA Reader and Student can glance at the above graph and immediately realize:

The Thermistor and RTD technologies are alike because both of them have a predictable relationship between Resistance and Temperature.

The Thermistor and RTD technologies are totally different because the thermistor shown has a negative relationship (aka "inverse relationship") between Resistance and Temperature... meaning  the resistance decreases as the temperature increases.

Thermistors that have an inverse relationship with temperature are known as "NTC thermistors."

So the NTC Thermistor and RTD are functionally identical cousins ... alike yet totally different!

 

MEET RTD's COUSIN: THE THERMISTOR

Thermistors are semiconductors, not pure metals like RTDs.

The most commonly used RTD in industry is made of painstakingly pure platinum.

Platinum is one of the metals that appears on the Chart of Stuff (aka the "Periodic Table of the Elements") from which everything on earth is made out of.

The chemical symbol for platinum is Pt.

The Chart of Stuff (first introduced in PTOA Segment #60 entitled "I am the Walrus") also includes the metals manganese (Mn), nickel (Ni), cobalt (Co), and iron (Fe) which are used to fabricate thermistors.

Perhaps it is a little easier to identify the metals that are used to manufacture temperature-measuring devices on the more scientifically labelled "Periodic Table of the Elements" below: 

When human beings intentionally combine these metals with oxygen (also found on the chart as the letter "O") wouldn't you know that logically named metal-oxides are formed!

These metal oxides are also called semiconductors.

Man-made semiconductor thermistors are intentionally designed to have a significant change in electrical resistance output upon sensing a change in temperature.

As stated above:

The NTC resistor is a temperature-measuring device that outputs less electrical resistance as the temperature increases (and vice versa).

A little more tinkering by human beings makes it possible to fabricate PTC resistors which function like RTDs:

The PTC resistor is a temperature-measuring device that outputs more electrical resistance as the temperature increases (and vice versa).

 

Thermistors are dinky.

Thermistors can be manufactured small enough to fit comfortably on the head of a dime as shown to the right.

All PTOA Readers and Students know by now that heat transfer through such a small device would occur rapidly and result in a very fast temperature measurement response time.

Several more differences between a thermistor and a RTD become apparent after glancing at the Resistance versus Temperature graph shown to the left:

 

Comparison of Temperature Ranges

The temperature range that a thermistor can measure is significantly limited compared to the range for the RTD or the thermocouple.

Thermistors are limited to measuring temperatures in the range of -260 °C to +315 °C (-436 °F to 599 °F).

PTOA Readers and Students that are reading the PTOA Segments in the intended sequential order already learned the following about electrical temperature-measuring devices:

From PTOA Segment # 114: Platinum RTDs can measure temperatures -259 °C to 631 °C (-434 °F to 1168 °F)

From PTOA Segment #112: Various thermocouple calibrations can measure temperatures ranging from -295 °C to 2316 °C (-499 °F to 4201 °F).

 

Thermistors can detect SMALL temperature changes

As was just stated above ...

Both a thermistor and a RTD can measure temperatures between -259 and 315 °C ( -434 to 599 °F).

However ... within this shared temperature range ...the resistance output of a thermistor is much greater than that for a RTD.

Otherwise stated ...

The increased measurement sensitivity (aka: greater resistance change per degree change in temperature) means the thermistor can detect small temperature changes.

Wow! Just look at that graph to the left!

See how the thermistor curve plunges steeply downward as the temperature increases?

In the real world that steep decline translates into a significant decrease in resistance output as the temperature just slightly increases.

Who amongst us PTOA Readers and Students remembers that PTOA Segment #112 informed us that thermocouples cannot be used unless there is at least a 33 °C temperature change?

The thermistor easily generates a signficant change in resistance over a temperature change of 33 °C.

Hey!

All of the above explained why thermistors are highly useful and responsive electrical temperature-measuring devices.

Thermistors are also very accurate ... albeit not as accurate as a 4-Lead Platinum RTD circuit.

 

COMMON THERMISTOR USES

Most Process Operators would have a difficult time identifying where thermistors are located in the plant site even though they are surrounded by them in ordinary daily life.

Process Operators and Control Board Operators are not as knowledgeable about thermistors because most automatic temperature control loops incorporate thermocouples or RTDs as the temperature sensing and measuring device.

Both NTC and PTC thermistors are primarily used for electronic temperature monitoring and electronic temperature indicating.

The wide array of thermistor uses are shown in the nearby charts.

Thermistors are preferred in these simple applications because they are much less expensive than RTDs; thermistors do not require all the complicated electrical circuitry that is needed to convert the resistance output of a RTD into a temperature that human beings can understand.

Common household uses of NTC thermistors include:

• Digital home thermostats.
• Consumer electronics ... like monitoring the temperature of battery packs while they are charging.
• Toasters, refrigerators, hair dryers, freezers & fire alarms.
• Protection of power supply circuits from damage after the main source of power has been restored.
• Maintaining the proper temperature to melt the plastic media for 3-D printers.

 

Thermistors are also found in automobiles.

 

IMPORTANT INDUSTRIAL USES OF THERMISTORS

Excessively High Temperature Protection

Two of the most important industrial uses of PTC thermistors are protection against excessively high temperatures and currents.

Incorporating PTC thermistors into motor windings makes it possible for these sensors to directly measure motor heat.

Connecting a control relay in series with the thermistor enables the motor to be protected from overheating, overloading, and insufficient cooling.

 

Excessively High Current

PTC thermistors can replace fuses used for overcurrent protection on motors and transformers.

Unlike a fuse, the thermistor does not have to be replaced after "tripping out." The thermistor just needs time to cool down.

 

Other Important Industrial Uses of Thermistors

• The Food Industry is particularly reliant upon thermistors to accurately measure temperature and prevent food borne illnesses.
• The Automotive Industry uses thermistors for measuring the temperature of engine coolant and oil.
• 

  A thermistor is incorporated into the isothermal block of this thermocouple to provide cold junction compensation.

  Thermistor technology is incorporated into temperature compensation circuitry ... like the cold junction temperature compensation that is required for a thermocouple to accurately measure temperature as was previously described in PTOA Segment #110.

• There are many industrial applications of thermistor technology in the Instrumentation and Communication, Medical Electronics, and Military/Aerospace Industries.

 

TAKE HOME MESSAGES:  Thermistors are electrical temperature measuring devices that change their resistance output with temperature ... just as RTDs do.

Like RTDs, the resistance output of PTC thermistors increases as temperature increases.

Unlike RTDs, the resistance output of NTC thermistors decreases as the temperature increases.

Unlike RTDs, thermistors are semiconductors fabricated from metal oxides specifically created to generate a responsive resistance/temperature relationship.

The measurement sensitivity of both types of thermistors exceeds that for RTDs; thermistors have a much greater change in resistance per degree change in temperature.

However, the Universe limits the creativity of mankind with respect to fabricating thermistors capable of measuring the full range of temperatures needed for industrial operations; thermistors are limited to measuring temperatures in the range of -260 °C to +315 °C (-436 °F to 599 °F).

Thermistors are used for electrically measuring and monitoring temperature; thermistors are not used to measure temperatures in analog-type automatic temperature control loops.

Thermistors have many temperature measuring and monitoring applications encountered during everyday life.

In the processing industries the important use of thermistors include protection from excessively high temperatures and excessively high electrical currents.

©2016 PTOA Segment 00116
PTOA Process Variable Temperature Focus Study Area
PTOA Process Industry Automation Focus Study Area

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