What is temperature calibration?

• Temperature calibration is a process of determining and correcting the accuracy error of the instrument used in a system that measures temperature.
• Generally, calibration of an instrument is a process of checking and verifying the accuracy of a measuring instrument by comparing it with a standard.

What is a temperature calibrator?

A temperature calibrator is a specific instrument for determining the performance of a temperature sensor within specified limits of uncertainty or accuracy.

What are the types of thermometer calibration?

Standard types for calibrating a thermometer:

1. Boiling point method
2. Freezing point method.

Why is temperature sensor calibration necessary?

Calibration of temperature sensors such as RTD, Thermocouple, or Thermometer is necessary to obtain accurate temperature information of process in the process industry.

What are the different types of temperature calibration in industrial temperature measurement?

There are three different types of calibration in industrial temperature measurement:

1. Temperature Sensor Calibration:

• Sensor calibration is a part of a confusing or ambiguous term because temperature sensors do not have any kind of adjustable output. It means if the sensor reading is not proper it can’t be adjusted.
• Calibration of the temperature sensors is about the verification of the number of offsets from true reading and documenting that offset.
• When the sensor is fixed in any application, proper tuning of that instrumentation is required to read the sensor’s offset output to get an accurate measurement.
• Thermocouples, RTD (PT100s), and thermistors are the most common types of sensors used to measure temperature in process industries and in power plants.

2. Instrument Calibration:

• Instrument calibration is known as traditional calibration.
• An instrument calibrator is a device that produces output specified to trigger sensor output.
• But these sensors also drift over time.
• These are similar to devices such as sensors or instruments which are used to measure process temperature.
• The offset of the instrument is easily seen and can be easily adjusted to meet its published accuracy.
• PIE 510B RTD (PT100) Simulator/Source plus Ohms is an example of an instrument calibrator.

3. System Calibration:

• System calibration is a relatively straight forward process
• System calibration certifies that the whole system produces output with high accuracy.
• A sensor calibrator is used to obtain a defined temperature from the system.
• The combination of both sensor calibration and instrument calibration is known as system calibration.
• Calibration of the sensor and instrument is done separately or the system as a whole and the offset is adjusted accordingly.
• This method requires that the sensor and instrument be always used together.
• Connect the instrument to the sensor that it will be reading from in your application.
• Let us consider an example of testing a sensor and instrument.

Assume that:

1. Sensor has a drift of +1 degree from the specified output.
2. Instrument has a drift of +1 degree.

Both these A & B produces an overall system drift of +2 degree.

Explain various types of Temperature Sensor Calibrators:

The temperature sensor calibrators produce accurate temperatures so that everyone can easily compare sensor readings.

Common types of temperature sensor calibrators are shown below.

A. Block Calibrators:

Block calibrator consists of an accurate heating control system, to heat a metal block to a specific temperature as required by the metal block such as inserting a hole to insert a temperature sensor probe.

Benefits of Block Calibrators:

• Portable and Clean
• Less weight
• Accuracy is high
• Easy to use
• Fast heating and immediate cooling.
• Optional cooling system for calibration of below ambient temperature.

B. Fluidized temperature Bath Calibrators:

The fluidized bed is used to develop highly stable heat references in these fluidized temperature bath calibrators.

This fluidized bed is developed by passing hot air through a bed of a granular solid such as oxides of aluminum.

This granular bed acts more like a liquid to provide excellent thermal contact but it has the ability to attain high temperatures than true liquids. 

Other than calibration, these fluidized bath calibrators have a wide range of applications that includes thermal cleaning of small extrusion tooling, polymer rheometer, reactor heating, and general heat treatment of devices and materials.

Benefits of fluidized temperature bath calibrators:

• Large volume with a high heating capacity
• Can calibrate many sensors at a time
• Wide temperature range
• compared to salt baths these calibrators are more safety,
• stability is more
• uniform than ovens

C. Blackbody Calibrators:

• The black body calibrators work by heating only the surface to a specified temperature.
• These blackbody calibrators are mainly used for calibrating infrared temperature sensors.

How to calibrate a temperature probe?

1. Place the calibrating probe RTD along with the reference probe close together in the temperature source or bath.

2. Connect the sensor leads to note down the measured reading of the probe. This is the readout that is paired with the probe.

3. For every test point or every set point, the temperature of the reference probe and calibrating probe are measured and recorded.

4. As part of the calibration at a higher level the resistance of the probe is measured and new adjustment parameters must be calculated.

5.  Check the error of the device under test or calibrating probe RTD, if it is intolerant or not.

6. Calculate new adjustment parameters using the calibration results for calibrating probes such as RTD or PRT if necessary.

What are common methods for calibrating temperature sensors?

Three common methods for calibrating temperature sensor.

A. Calibrating the electronics with a simulator:

• Calibrating the electronics of temperature measurement and control system
• In the case of a thermocouple, the simulator gets a temperature in Degrees as input and simulates the output in terms of voltage.
• In the case of RTD, the simulator gets a temperature in Degrees as input and simulates the output in terms of resistance.
• The output for both simulators is based on accepted national tables.

Advantages:

• Operation is fast.
• Instant electrical settings
• Doesn’t require stabilization times.
• Calibration equipment is portable compared to equipment used in other methods.
• This meter can be used with interchangeable probes.

Disadvantages:

• Requires separate calibration procedure for each probe 
• Untraceable measurements due to un-calibrated probe.

B. Calibration of both electronics and the temperature sensor in a dry well:

• In this method, the temperature of the dry well is set to the desired test temperature.
• The thermometer, meter, and probe are combined and placed in the dry well. The thermometer reading is now compared with the dry well reading built-in thermometer.

Advantages:

• Both probe and meter are calibrated easily.
• Uses an actual temperature scale.
• System setup is simple with a single instrument called a dry well.

Disadvantages:

• The accuracy of the dry well is limited.
• Changing temperatures in the dry well requires more time.
• An additional error arises if the probe doesn’t reach the bottom of the well. It means the length of the probe must be equal to the depth of the well.
• Probes are not interchangeable after calibration.

C. Calibration of both electronics and the sensor in a dry well using a reference thermometer:

Here, a dry well is used as a stable heat source and the thermometer reading is compared against a more accurate thermometer placed in an adjacent hole in the well.

Advantages:

• Accuracy is good.
• Calibration result is traceable.
• More versatility in matching units under test sizes.

Disadvantages:

• More expensive than other methods.
• Complex setup because two instruments are used.
• Because two instruments are used, the setup is slightly more complex.
• To change temperatures the dry well requires more time.
• Probes are not interchangeable after calibration

What is the equipment used for temperature calibration?

Dry-block calibrator & calibration bath are the equipment used for temperature calibration

Which method is used for the calibration of thermometers?

The ice-point method is the most widely-used method to calibrate a thermometer.

Hold the calibration nut securely with a wrench or other tool and rotate the head of the thermometer until it reads 32˚F (0˚C). Thermometers should be calibrated regularly to make sure the readings are correct.