RTDs are widely used for accurate temperature measurement in calibration laboratories, pharmaceutical industries, process plants, HVAC systems, food industries and research applications. They are preferred because of their good accuracy, stability and repeatability. However, like every measuring sensor, an RTD also has some practical limitations. One important effect that should be understood in precision temperature measurement is the self-heating effect.
In simple terms, self-heating in an RTD means the RTD sensor becomes slightly warm because of the measuring current passing through it. This small temperature rise can make the RTD indicate a temperature slightly higher than the actual temperature of the medium.
This effect may look very small, but in accurate calibration and precision temperature measurement, even a small error of 0.05°C or 0.10°C can be important.
What is Self-Heating in RTD?
An RTD works by measuring resistance. To measure this resistance, the temperature indicator, transmitter, bridge, data logger or calibration instrument sends a small electrical current through the RTD element.
When current flows through any resistance, electrical power is generated in the form of heat. The same thing happens inside an RTD sensing element.
This heat is produced inside the RTD itself. Because of this, the RTD element may become slightly warmer than the surrounding air, liquid, dry block or process medium.
So, the RTD may not show the exact temperature of the medium. It may show the temperature of its own slightly heated sensing element.
In simple language:
RTD self-heating = temperature rise of the RTD caused by its own measuring current
Why Does Self-Heating Happen?
Self-heating happens due to the basic electrical heating effect. When current passes through a resistance, heat is generated.
The power generated inside the RTD is given by:
Power = I²R
Where:
I = measuring current
R = RTD resistance
This means that if the measuring current increases, the generated power increases very quickly because current is squared.
For example, if current is doubled, power becomes four times higher. This is why excitation current must be selected carefully in accurate RTD measurement.
The basic process is:
Measuring current flows → electrical power is generated → RTD element warms up → resistance increases → displayed temperature becomes slightly high
Simple Physical Understanding
Imagine an RTD placed inside a liquid bath at exactly 100.00°C. The measuring instrument passes current through the RTD to measure its resistance.
Because of this current, the RTD sensing element generates a small amount of heat. If this heat is not removed quickly by the surrounding liquid, the RTD element may become slightly warmer than the liquid.
The true bath temperature may be:
100.00°C
But the RTD element may become:
100.10°C
In this case, the RTD may indicate a temperature slightly higher than the actual bath temperature. This difference is called self-heating error.
Formula for RTD Self-Heating
The temperature rise due to self-heating depends on the power generated inside the RTD and how easily that heat is removed by the surrounding medium.
The simple formula is:
ΔT = P / D
Since:
P = I²R
Therefore:
ΔT = I²R / D
Where:
ΔT = temperature rise due to self-heating
P = electrical power generated in the RTD
I = measuring current
R = RTD resistance
D = dissipation constant
The dissipation constant tells us how easily heat can escape from the RTD to the surrounding medium. A higher dissipation constant means heat is removed more easily, so self-heating is lower. A lower dissipation constant means poor heat removal, so self-heating becomes higher.
Worked Example of RTD Self-Heating
Let us take a simple example of a PT100 RTD.
At 0°C:
RTD resistance = 100 Ω
Suppose the measuring current is:
I = 2 mA = 0.002 A
Power generated inside the RTD:
P = I²R
P = (0.002)² × 100
P = 0.0004 W
P = 0.4 mW
Now suppose the dissipation constant is:
D = 4 mW/°C
Then self-heating rise will be:
ΔT = P / D
ΔT = 0.4 / 4
ΔT = 0.10°C
This means the RTD may read approximately 0.10°C higher than the actual temperature.
So, the self-heating error is:
+0.10°C
For high-accuracy measurement, this error should not be ignored.
What Affects Self-Heating in RTD?
Self-heating is not the same in all conditions. It depends on several factors.
1. Measuring Current
Higher measuring current produces more heat. Since power is proportional to current squared, even a small increase in current can significantly increase self-heating.
2. RTD Resistance
Higher resistance produces more power for the same current. This is one reason why measurement current must be carefully selected, especially in precision systems.
3. Sensor Size and Construction
Small sensing elements have less surface area and less mass. They may heat up more quickly compared to larger or well-protected RTD elements.
4. Medium Around the Sensor
Self-heating is usually higher in still air because air does not remove heat efficiently. It is generally lower in stirred liquid because liquid removes heat more effectively.
5. Flow or Stirring
Good flow around the RTD helps remove generated heat. Poor airflow or poor liquid circulation increases self-heating.
6. Installation and Immersion Depth
Poor immersion, poor contact, shallow insertion or incorrect mounting can reduce heat transfer and increase self-heating error.
7. Dissipation Constant
The dissipation constant is a direct indicator of heat removal capability. A higher dissipation constant means better heat dissipation and lower self-heating.
Where is Self-Heating More Severe?
Self-heating becomes more noticeable where heat transfer from the RTD to the surrounding medium is poor.
RTD in Still Air
Self-heating is usually highest in still air because air is a poor conductor of heat. If there is no airflow, heat generated inside the RTD element is not removed quickly.
RTD in Stirred Liquid
Self-heating is usually much lower in stirred liquid. The liquid absorbs and carries away heat from the RTD element more effectively.
RTD in Dry Block or Thermowell
Self-heating depends on contact quality. If the RTD has good thermal contact with the block or thermowell, heat transfer improves. If the fit is loose or contact is poor, self-heating may increase.
In simple words:
Poor heat transfer = more self-heating
Good heat transfer = less self-heating
Why is Self-Heating Important?
Self-heating is important because it directly affects measurement accuracy.
In routine industrial measurement, the effect may be small enough to ignore in many cases. But in calibration laboratories, validation work, precision monitoring and reference measurements, it can become an important error source.
Self-heating is important because it:
- affects RTD measurement accuracy
- contributes to calibration uncertainty
- becomes more noticeable in precision temperature measurement
- can create positive bias in temperature readings
- depends strongly on excitation current and heat transfer condition
- may affect comparison between reference sensor and instrument under calibration
For example, during calibration, if the reference RTD and the test RTD have different self-heating characteristics, the comparison result may be affected.
Self-Heating and Calibration Uncertainty
In temperature calibration, uncertainty evaluation includes all important factors that can influence the measurement result. Self-heating is one of those factors.
If the RTD is used as a reference standard, self-heating should be understood and controlled. If it is significant, it should be included in the uncertainty budget.
The effect may be evaluated by testing the RTD at different excitation currents. If the indicated temperature changes when current changes, self-heating is present.
For high-accuracy temperature calibration, self-heating should not be assumed to be zero unless it is technically justified.
How to Reduce Self-Heating in RTD Measurement
Self-heating can be reduced by using proper measurement practices.
1. Use Low Excitation Current
Lower current produces less electrical power and reduces self-heating. However, the current should not be so low that the measurement becomes noisy or unstable.
2. Follow Manufacturer Recommendation
RTD manufacturers and instrument manufacturers usually specify suitable measurement current. This recommendation should be followed.
3. Ensure Good Thermal Contact
The RTD should be properly inserted and should have good contact with the medium, dry block, thermowell or calibration source.
4. Use Proper Immersion Depth
Insufficient immersion can cause stem conduction error and poor heat transfer. Proper immersion improves measurement reliability.
5. Use Stirred or Flowing Medium Where Possible
Stirred liquid baths and flowing media remove heat more efficiently and reduce self-heating.
6. Avoid High Current in Precision Measurement
High excitation current should be avoided, especially when using RTDs for reference measurement or low-uncertainty calibration.
7. Consider Self-Heating in Uncertainty
If self-heating is significant, include it in the uncertainty evaluation.
Low Current vs High Current
A low measuring current generates less heat and causes a smaller self-heating error. A high measuring current generates more heat and can produce a larger error.
For example:
Low current = lower power = smaller self-heating error
High current = higher power = larger self-heating error
This is why precision instruments usually use carefully controlled low excitation current for RTD measurement.
Practical Example in Calibration
Suppose a PT100 sensor is being calibrated in a dry block at 100°C. The reference thermometer and the sensor under calibration are placed in the block.
If the RTD is excited with a high current and the dry block contact is poor, the RTD may heat itself slightly. The reading may appear higher than the actual block temperature.
This can lead to wrong correction values or increased uncertainty.
To avoid this, the calibration setup should use suitable excitation current, proper sensor insertion, correct block size, good thermal contact and enough stabilization time.
Common Misunderstandings About RTD Self-Heating
Misunderstanding 1: RTD Only Measures Temperature, It Does Not Produce Heat
An RTD requires current for resistance measurement. Because of this current, some heat is always generated.
Misunderstanding 2: Self-Heating is Always Negligible
In many industrial applications it may be small, but in precision calibration, self-heating can be important.
Misunderstanding 3: Higher Current Always Gives Better Measurement
Higher current may improve signal strength, but it also increases self-heating. A balance is required.
Misunderstanding 4: Self-Heating is Same in Air and Liquid
Self-heating depends strongly on the surrounding medium. It is generally higher in still air and lower in stirred liquid.
Good Practices for RTD Measurement
For reliable RTD measurement, use the correct RTD type, suitable wiring method and proper measurement current. Ensure proper immersion depth and stable thermal conditions. Avoid loose fitting in dry blocks or thermowells. Use a stirred liquid bath where high accuracy is required. Allow enough time for stabilization before recording readings.
During calibration, review whether self-heating is significant for the required accuracy. If required, evaluate it experimentally and include it in the uncertainty budget.
Conclusion
The self-heating effect in an RTD is a small but important source of error in temperature measurement. It occurs because the measuring current flowing through the RTD generates heat inside the sensing element.
This heat can make the RTD element slightly warmer than the surrounding medium, causing the displayed temperature to become slightly higher than the true temperature.
The effect depends mainly on measuring current, RTD resistance, sensor construction, surrounding medium, flow condition, immersion, contact quality and dissipation constant.
For routine applications, self-heating may be very small. But for precision temperature measurement and calibration, it should be carefully considered.
The best practical approach is:
Use the lowest practical excitation current, ensure good heat transfer, and include self-heating in uncertainty evaluation where required.
