In calibration and temperature measurement, the word stability is used very often. Whether we are using a dry block calibrator, liquid bath, furnace, temperature chamber, or any other temperature source, stability plays a very important role in deciding the reliability of the calibration result.
Many people confuse stability with accuracy or uniformity, but these terms are not the same. Stability simply tells us how steady the temperature remains with time at one fixed location after the temperature source has reached its set temperature.
For example, if a dry block is set at 100 °C, it may not remain exactly 100.000 °C every second. It may slightly increase or decrease with time. This variation over a defined period is known as the stability of the temperature source.
What is Stability in a Temperature Source?
Stability is the ability of a temperature source to maintain nearly the same temperature over time at a fixed point.
In simple words, stability answers this question:
“Once the temperature source reaches the set temperature, how much does the temperature change with time?”
If the temperature remains almost constant, the source is considered stable. If the temperature keeps moving up and down significantly, the stability is poor.
For example:
| Time | Temperature Reading |
|---|---|
| 10:00 AM | 100.01 °C |
| 10:05 AM | 100.02 °C |
| 10:10 AM | 99.99 °C |
| 10:15 AM | 100.00 °C |
In this case, the readings are very close to each other. So, the temperature source has good stability.
But if the readings are like 99.70 °C, 100.30 °C, 99.80 °C, and 100.25 °C, then the source is fluctuating too much. This means the stability is poor.
Why Stability is Important in Calibration
During calibration, we compare the reading of an instrument under calibration with a reference standard. For this comparison to be meaningful, the temperature source must remain steady.
Let us take a simple example. Suppose we are calibrating a temperature sensor at 100 °C. If the temperature bath is continuously changing between 99.8 °C and 100.2 °C, then it becomes difficult to decide the actual temperature at which the sensor is being calibrated.
This directly affects the calibration result.
Good stability helps in:
- Getting reliable calibration results
- Reducing measurement uncertainty
- Improving repeatability of readings
- Making better comparison between the standard and the instrument under calibration
- Avoiding unnecessary errors during calibration
In short, a stable temperature source gives more confidence in the calibration result.
How Stability is Observed
Stability is observed by placing a suitable temperature sensor or reference thermometer at a fixed location inside the temperature source. After the source reaches the required set temperature and sufficient stabilization time is allowed, readings are recorded over a defined period.
For example, readings may be recorded every minute for 30 minutes, or as defined in the calibration procedure.
The highest and lowest readings observed during this period are then used to calculate stability.
How Stability is Calculated
The basic formula for stability is:
Stability = Maximum temperature observed – Minimum temperature observed
For example:
- Maximum temperature observed = 100.03 °C
- Minimum temperature observed = 99.98 °C
So,
Stability = 100.03 – 99.98 = 0.05 °C
This means the temperature source changed by 0.05 °C during the observation period at that fixed location.
A smaller stability value generally means a more stable temperature source.
Stability is Not the Same as Accuracy
This is a very important point.
Accuracy tells how close a measured value is to the true value.
Stability tells how much the temperature changes with time.
A temperature source may be stable but not accurate. For example, if a bath is set at 100 °C and it remains steady at 99.80 °C, it is stable because the value is not changing much. But it may not be accurate with respect to the set value.
Similarly, a source may sometimes reach the correct temperature but fluctuate too much. In that case, accuracy may look acceptable at one instant, but stability is poor.
Stability is Not the Same as Uniformity
Stability is also different from uniformity.
Stability is related to temperature change with time at one location.
Uniformity is related to temperature variation from one location to another location inside the working area.
For example, in a temperature bath, one point may be 100.00 °C while another point may be 100.08 °C. This difference is related to uniformity, not stability.
So, the simple difference is:
Stability = variation with time at one point
Uniformity = variation from point to point
Both are important for calibration, but they represent different performance characteristics.
Common Causes of Poor Stability
There are several reasons why a temperature source may show poor stability. Some common causes include:
1. Insufficient stabilization time
If readings are taken too early, the source may still be settling. This may give the impression of poor stability.
2. Controller oscillation
If the temperature controller is not properly tuned, it may overshoot and undershoot around the set point.
3. Ambient temperature variation
Large changes in room temperature may affect the performance of some temperature sources, especially at low or high temperatures.
4. Poor circulation or stirring
In liquid baths, poor stirring can cause unstable temperature conditions.
5. Frequent opening of chamber door or bath cover
Disturbance during measurement can affect stability.
6. Improper sensor placement
If the sensor is not placed properly or is moved during measurement, readings may fluctuate.
7. Load effect
Adding too many instruments or heavy probes may disturb the thermal condition of the source.
How to Improve Stability
Good stability depends on proper use, maintenance, and environmental control. The following practices can help improve stability during calibration:
Allow sufficient time for the source to reach and stabilize at the set temperature. Do not start recording readings immediately after the display reaches the target value.
Use proper immersion depth for the probe or sensor. Poor immersion can cause heat loss and unstable readings.
Avoid unnecessary movement of the sensor during measurement. The sensor should remain fixed at one location.
Keep the lid, cover, or chamber door closed as much as possible during stabilization and measurement.
For liquid baths, ensure proper stirring or circulation.
Avoid placing the temperature source near air conditioners, doors, windows, direct sunlight, or heat-generating equipment.
Do not overload the temperature source with too many instruments at the same time.
Maintain the equipment as per the manufacturer’s recommendation and check controller performance periodically.
Practical Example
Suppose a calibration laboratory is using a dry block calibrator at 100 °C. After reaching the set value, the laboratory waits for proper stabilization. Then readings are recorded for 30 minutes using a reference thermometer inserted at a fixed position.
The readings vary between 99.97 °C and 100.02 °C.
So, the stability is:
100.02 – 99.97 = 0.05 °C
This value can be used in uncertainty estimation as per the laboratory’s procedure.
However, the laboratory must also check other factors such as uniformity, reference standard uncertainty, resolution, repeatability, and calibration method before finalizing the measurement uncertainty.
Role of Stability in Measurement Uncertainty
Stability is one of the important contributors to measurement uncertainty in temperature calibration. If the temperature source is not stable, the calibration result becomes less reliable.
For example, if a bath fluctuates by 0.20 °C during calibration, this variation must be considered. A higher stability value may increase the overall uncertainty of measurement.
Therefore, laboratories should evaluate and document the stability of temperature sources used for calibration. This is especially important for laboratories working under ISO/IEC 17025 requirements, where measurement uncertainty and equipment performance must be properly controlled.
Stability in Different Temperature Sources
Dry Block Calibrator
In dry blocks, stability depends on heater control, block design, sensor placement, ambient conditions, and loading. Dry blocks are convenient and portable, but proper insertion depth and suitable inserts are very important.
Liquid Bath
Liquid baths generally provide good stability when proper stirring is available. The liquid medium helps in distributing heat evenly, but bath fluid condition and circulation must be maintained.
Temperature Chamber
In chambers, stability depends on air circulation, chamber loading, door opening, controller performance, and sensor location. Stability may be different at different parts of the chamber.
Furnace
At high temperatures, stability can be affected by heater cycling, insulation, controller tuning, and ambient conditions. Sufficient soaking time is very important before taking readings.
Best Practices for Recording Stability
To record stability properly, the following points should be considered:
Use a calibrated reference thermometer or suitable sensor.
Place the sensor at the required fixed location.
Allow sufficient stabilization time.
Record readings for a defined time period.
Do not disturb the sensor during observation.
Record maximum and minimum values.
Calculate stability using the difference between maximum and minimum temperature.
Mention the observation period and location clearly in the record.
A stability value without time duration and location is incomplete. For example, instead of writing only “Stability = 0.05 °C”, it is better to write:
Stability = 0.05 °C over 30 minutes at the center location.
This gives a clearer technical meaning.
Conclusion
Stability is a simple but very important concept in temperature calibration. It tells us how steady a temperature source remains with time at a fixed point. A stable temperature source helps produce reliable, repeatable, and technically valid calibration results.
It is important to remember that stability is different from accuracy and uniformity. Stability is related to time-based variation, while uniformity is related to location-based variation.
For good calibration practice, the stability of dry blocks, liquid baths, chambers, and furnaces should be checked, recorded, and considered in uncertainty estimation. A well-stabilized temperature source not only improves measurement confidence but also supports better compliance with quality and calibration requirements.
