How Temperature Probes and Thermistors Work

How medical equipment turns temperature into an electrical signal and why probe resistance, connectors, calibration, and sensor location matter

Temperature measurement looks simple.

Published August 31, 2026 · Revised September 6, 2026

Back to Biomed Basics

What This Page Explains

This page covers:

The Simple Version

A thermistor changes resistance as its temperature changes. The device measures that resistance directly or as part of a voltage-divider circuit, then uses a stored response curve to convert the electrical value into degrees. The display is therefore the end of a chain that includes the sensing element, cable, connector, input circuit, conversion table, and calibration.

A wrong reading may come from the probe being at the wrong physical temperature, poor thermal contact, an open or shorted cable, corrosion, an incompatible probe curve, input-circuit error, or calibration. Compare the complete channel with a traceable temperature reference or approved simulator before deciding which part failed.

Worked Example: Probe Reads 2°C High

Place the probe and reference in the manufacturer-specified medium, depth, and stabilization conditions. If a compatible simulator makes the monitor read correctly, focus on the probe, cable, and connector. If both the real probe and simulator read high, investigate the device input and calibration.

Measure probe resistance only when the service procedure permits and compare it at a known temperature with the correct curve. After repair, verify multiple points across the required range; one room-temperature check does not prove performance at clinical temperatures.

What Is a Thermistor?

A thermistor is a resistor whose resistance changes significantly with temperature.

The word comes from:

Thermal resistor.

Two Common Types

Thermistors are broadly divided into:

NTC Thermistor

NTC means:

Negative Temperature Coefficient.

As temperature increases:

Resistance decreases.

Simple NTC Example

Cold:

High resistance.

Warm:

Lower resistance.

Hot:

Even lower resistance.

This is extremely common in temperature sensing.

PTC Thermistor

PTC means:

Positive Temperature Coefficient.

As temperature increases:

Resistance increases.

PTC devices are also used for:

depending on design.

Do Not Assume Every Temperature Sensor Is NTC

Check the actual design.

Other technologies can include:

Thermistor Resistance Is Not Linear

This is important.

A thermistor usually does not change resistance at the same rate across the entire temperature range.

Example:

The resistance change between:

20°C and 21°C

may not equal the change between:

40°C and 41°C.

Conversion Table

Manufacturers may provide a table showing:

Temperature → Expected Resistance.

That can be extremely useful for troubleshooting.

Example

At a specific temperature:

Expected thermistor resistance:

10 kΩ.

Measured:

10.1 kΩ.

That may be reasonable depending on tolerance.

Voltage Divider

The thermistor is often part of a:

Voltage divider.

Basic Concept

Known resistor

+

Thermistor

are connected together.

As thermistor resistance changes:

The voltage at the measurement point changes.

Processor Reads Voltage

The device's analog circuitry measures that voltage.

Then:

ADC

converts it into a digital value.

ADC

ADC means:

Analog-to-Digital Converter.

It converts a continuously varying voltage into a number the processor can use.

Conversion Chain

Think:

Temperature

↓

Resistance

↓

Voltage

↓

ADC value

↓

Software conversion

↓

Displayed temperature

A failure anywhere in that chain can create the wrong reading.

Patient Temperature Probe

A patient probe may contain the thermistor at its tip.

The cable carries the electrical signal back to the monitor.

Probe Tip Matters

The sensing element must be thermally connected to what you are trying to measure.

If the probe is poorly positioned:

The electronics may be perfect while the clinical temperature appears wrong.

Sensor Location

Temperature is always measured:

Somewhere.

That location matters.

Examples:

These temperatures are not interchangeable.

Skin Temperature vs Core Temperature

A skin probe can be functioning perfectly while showing a different value from a core temperature measurement.

That is not necessarily a device failure.

Thermal Lag

Temperature sensors do not respond instantly.

They need time to reach thermal equilibrium with what they are measuring.

Example

Probe starts at room temperature:

22°C.

Placed into:

37°C reference bath.

Reading may climb:

22 28 32 35 36.5 37

over time.

That delay is:

Thermal response time.

Do Not Compare Too Quickly

If you compare a probe to a reference before it stabilizes:

You may falsely conclude it is inaccurate.

Probe Construction

Temperature probes may include:

Any part can fail.

Open Circuit

If a conductor breaks:

The thermistor circuit becomes open.

What Does Open Look Like?

Depends on circuit design.

The device may show:

Short Circuit

If the probe conductors short together:

Resistance may become near zero.

The device may interpret that as an extreme temperature.

Open vs Short

With an NTC system:

Very high resistance may look like:

Very cold.

Very low resistance may look like:

Very hot.

But do not assume exact behavior without documentation.

Probe Recognition

Some temperature probes include more than the sensing element.

They may also use:

to tell the monitor what type of probe is connected.

Compatible Does Not Mean Identical

Two probes may physically fit but use different:

That can produce wrong readings.

Connector Problems

The probe connector can create:

Added Resistance

Extra resistance from:

can shift the measured temperature.

Cable Flexing

A broken conductor may only open when the cable bends.

Static resistance test:

Pass.

Flex test:

Fails.

Known-Good Probe

Substitution is useful.

Original probe:

Reads 4°C high.

Known-good probe:

Correct.

Failure follows original probe.

Strong evidence.

Failure Stays With Monitor

Two known-good probes read incorrectly on the same input.

Investigate:

Reference Resistor

The thermistor is often compared against a precision resistor.

If that resistor drifts:

Every probe connected to that channel may read incorrectly.

ADC Error

Correct probe resistance.

Correct divider voltage.

Displayed temperature wrong.

Now the conversion or processing path becomes more likely.

Calibration

Temperature systems may use calibration values to correct:

Offset Error

Reference:

37.0°C.

Device:

39.0°C.

At:

30°C reference,

device:

32°C.

A consistent +2°C error suggests something different from a nonlinear error.

Scale Error

Reference:

20°C → device 20°C

Reference:

40°C → device 45°C

Now the error increases across the range.

That suggests:

Calibration Cannot Fix Wrong Probe Type

If the device is expecting one thermistor curve and you install another:

Do not calibrate around it.

Use the correct probe.

Calibration Cannot Fix Intermittent Cable

If the reading jumps when the cable moves:

Repair or replace the probe.

Temperature Simulator

Some patient simulators can electrically simulate specific temperature probe values.

Instead of producing actual heat, they present the monitor with the expected electrical resistance or signal.

What a Temperature Simulator Proves

If the monitor reads:

37°C

with a simulator:

You have tested much of the monitor's input path.

You have not tested the physical patient probe.

Probe vs Monitor Isolation

Simulator:

Correct.

Actual probe:

Wrong.

Think probe.

Simulator:

Wrong.

Known-good probe also wrong.

Think monitor input.

Resistance Substitution

In some systems, a known precision resistor can simulate a specific temperature.

Only do this when manufacturer documentation provides the correct values and method.

Environmental Temperature Sensors

Medical devices also use thermistors internally.

Examples include:

Battery Thermistor

Battery packs may contain a temperature sensor.

The charger uses it to decide whether charging is safe.

Battery Too Hot Message

Actual battery:

Normal.

Temperature line open.

Device reports:

Battery temperature fault.

The battery cells may not be overheating at all.

Fan Control

A device may increase fan speed as internal temperature rises.

The thermistor becomes part of a control loop.

Incubator Temperature Control

An incubator may measure:

and use those values to control heater output.

Closed-Loop Temperature Control

Think:

Desired temperature

↓

Heater output

↓

Actual temperature

↓

Temperature sensor

↓

Controller adjusts heater

Sensor Error Can Become a Control Error

Suppose actual temperature:

36°C.

Sensor incorrectly reports:

34°C.

Controller may increase heat.

Now a measurement error can create a real temperature problem.

Redundant Temperature Sensors

Safety-critical equipment may use more than one sensor.

If they disagree:

The device may alarm or shut down heat.

Overtemperature Sensor

Some devices use a separate safety sensor independent of the primary control sensor.

That is intentional redundancy.

Refrigerator Example

Primary sensor:

Controls compressor.

Independent sensor:

Triggers high-temperature alarm.

Failure in one may not affect the other.

Temperature Probe Cleaning

Probe contamination can affect:

Use approved cleaning methods.

Fluid Intrusion

Moisture in a connector can create:

Connector Contamination

A small amount of conductive residue may shift resistance enough to affect a precision thermistor circuit.

Probe Damage

Inspect for:

Physical Damage Can Affect Thermal Response

A probe tip may remain electrically functional but become thermally isolated by damaged material.

It may read correctly eventually but respond very slowly.

Response-Time Failure

Reference temperature:

Eventually correct.

But takes several minutes longer than expected.

That may still be clinically unacceptable depending on application.

Ambient Air Sensor

Incubators and warmers may use air sensors positioned in specific airflow.

Move the sensor:

Reading changes.

Placement Matters in PM

If the service procedure specifies reference probe location:

Follow it.

Temperature gradients can be significant.

Reference Temperature

For accuracy testing, you need a trustworthy reference.

Possible tools include:

depending on device and procedure.

Ice Water Is Not Automatically a Precision Calibration Bath

Improvised temperature references can introduce error.

Use the specified verification method.

Temperature Bath

A controlled bath provides a stable known temperature around the probe.

Stirring Matters

Without fluid circulation:

Different parts of the bath may be at different temperatures.

Probe Depth Matters

If one probe tip sits deeper than another:

They may not experience identical temperatures.

Stabilization Time

Always allow both:

to stabilize.

Real-World Example: Patient Probe Reads High

Patient simulator:

Monitor temperature input accurate.

Known-good probe:

Accurate.

Original probe:

Reads approximately 3°C high.

Failure follows probe.

Real-World Example: All Probes Wrong

Three known-good probes read:

2°C low

on same monitor.

Simulator also reads low.

Monitor input calibration/reference circuit becomes more likely.

Real-World Example: Intermittent Temperature

Reading jumps:

36.8 22 36.9 22

when cable moves near connector.

Intermittent open conductor reproduced.

Real-World Example: Battery Overtemperature

Device refuses charging.

Battery physically cool.

Service data shows impossible temperature.

Battery temperature-sensing circuit is faulty.

Real-World Example: Incubator Overheats

Displayed air temperature:

35°C.

Independent thermometer:

39°C.

Heater remains active.

Air temperature sensor is underreading.

Measurement fault created a control problem.

Common Mistakes

Assuming Displayed Temperature Is Actual Temperature

Verify with an independent reference.

Recalibrating Before Checking the Probe

Probe failure is common.

Ignoring Thermal Stabilization

Temperature needs time.

Using the Wrong Probe Type

Connector fit does not guarantee compatibility.

Ignoring Added Connector Resistance

Precision temperature circuits can be sensitive.

Treating a Sensor Error as Only a Display Problem

In a control system, a wrong sensor can cause wrong heater or cooling behavior.

A Useful Troubleshooting Framework

For a temperature problem, ask:

What temperature is actually being measured?

Then:

Is the correct probe installed?

Then:

What does an independent reference measure?

Then:

Does a known-good probe correct the problem?

Then:

Does a simulator or known resistance produce the expected reading?

Then:

Is the problem in the probe, wiring, input circuit, or calibration?

Another Useful Question

Ask:

Is the sensor wrong, or is the temperature truly wrong?

That distinction is extremely important.

What Did You Actually Prove?

If a thermistor measures the expected resistance at one temperature:

You proved:

It matches the expected electrical value at that test point.

You did not prove:

If a monitor reads correctly with a temperature simulator:

You proved:

The monitor input and conversion path function correctly at the simulated test point.

You have not proven:

The physical patient probe is good.

Final Thoughts for Biomeds

Temperature measurement is really an electrical conversion problem.

Think:

Temperature → Sensor Resistance → Voltage → ADC → Display.

Then, if the value controls something:

Display/Controller → Heater or Cooling System.

When temperature looks wrong, do not immediately adjust calibration.

First ask:

Is the actual temperature wrong?

Is the probe correct?

Is the resistance correct?

Is the monitor interpreting it correctly?

Once you separate the physical temperature from the electrical measurement, troubleshooting becomes much easier.

And as always:

What did you actually prove?

— Jake

Important Note

Temperature sensor types, thermistor curves, probe compatibility, calibration methods, acceptable tolerances, response-time requirements, and service procedures vary by medical-device manufacturer and model. Follow current manufacturer documentation and use calibrated reference equipment or approved simulators when verifying temperature performance.

Related Biomed Basics

Back to Biomed Basics