Calibration and Calibration curve

Calibration is the process to check the accuracy of instruments used for analysis. It is performed to find errors and to configure the instruments for better results. At the end of the calibration of instruments, results are drawn in the form of a calibration curve. It is the comparison of relative concentrations of sample to the known concentration of the standard sample.

A calibration curve is the performance of an instrument. It is also known as the standard curve. Every instrument needs calibration after specific periods. Calibration is a comparison of obtained results to the standard results. Calibration is necessary to improve the performance of that instrument. It is also necessary to minimize different types of errors.

What is calibration

Calibration is an analytical procedure for determining instrumental accuracy. It is performed to check whether our instrument is properly working or not. It provides information about accuracy in results. It helps to find out different types of errors that are affecting our results. After performing calibration we minimize these errors and improve our results.

If we are performing an analysis in the lab. We are using different apparatus and instruments. We are using analytical balances, beakers, flasks, pipettes, and different analytical devices i.e, HPLC, UV visible, IR, AAS, etc. So every instrument that is used in the laboratory must be calibrated for accurate results.

The general procedure of calibration

The calibration process consists of the following steps

  • Check the instrument properly
  • Plan the experiments
  • Select the reference standard accordingly.
  • Prepare standard of required concentrations
  • Perform experiment (3-5 readings)
  • Make measurements
  • Draw calibration curve
  • Evaluate the results obtained
  • Estimate the uncertainty associated with the values obtained for test samples.
  • Find out the errors
  • Make adjustments if required
  • At the end attached the certificate of calibration

Types of calibration

There are different types of calibration depending upon the method of analysis and the types of instruments used during analysis.

Electronic calibration

Electronic calibration involves the calibration of any instruments that measure any electrical parameters. It is done to check the performance of instruments that measure the different electrical parameters like

  • Voltage
  • Current
  • Resistance
  • Inductance
  • Capacitance
  • Time
  • Frequency
  • conductance
  • electrical power

Electronic calibration involves the use of precise devices. These devices are much more precise than instruments to be calibrated. By comparing the results with these devices we evaluate the performance of electrical instruments and make the necessary adjustments to improve our results accordingly.

The instrument used for measuring these parameters are

  • Voltmeters
  • Ammeters
  • Wattmeters
  • AC / DC shunts
  • Multimeters
  • Transformers

Mechanical calibration

Mechanical calibration involves the calibration of simple tools that are used to measure changes in the dimension of objects. Mechanical calibration factors are

  • mass
  • Force
  • Dimension
  • Angle
  • Volume
  • Flatness
  • torque
  • Density
  • Viscosity
  • Speed
  • vibration

The tools that are used for measuring these factors are calibrated in controlled conditions. There are the different instruments used for mechanical calibration i.e

  • Accelerometers
  • Scales
  • Balances
  • Load Cells
  • Screw Gauges
  • Micrometers
  • Verniers calipers
  • Height Gauges
  • Torque Wrenches
  • Screwdrivers
  • Weight & Mass Sets

Thermal calibration

Thermal calibration is related to temperature measuring devices. Temperature is one of the most important factors in the different analytical processes. Some of the chemical reaction takes place in a controlled environment. They required a very narrow range of temperature. So the devices used for measuring temperature must be calibrated for better results. Different devices are used for measuring temperature i.e,

  • Mercury Thermometers
  • Thermisters
  • Platinium resistance thermometers
  • Thermocouples
  • Thermal cameras

These devices are used in different places like stability chambers, ovens, furnaces, water baths, column ovens, incubators, freezers, etc. so these must be calibrated periodically to get accurate results.

Pressure calibration

Pressure calibration involves the calibration of pressure measurement devices. Just like temperature pressure is also one of the important parameters of a different process.

Many industrial processes rely on the measurement of pressure. We measure the pressure of different gases and the pressure of the hydraulic system. Different devices are used for the measurement of pressure i.e

  • Barometers (most commonly used)
  • Digital Pressure Gauges
  • Digital Indicators
  • Transducers
  • Transmitters
  • Analogue Pressure Gauges
  • Test Gauges

These devices must be calibrated periodically. This will help to improve our results.

Calibration of solution

Calibration of the solution is also known as standardization. We are using solutions of different concentrations in different chemical processes. We prepare solutions accordingly but due to different reasons (moisture, impurities, etc), some solutions are not of desired concentrations. Then we standardized this solution to find out the actual concentrations of solution. The standardization is done through titration. Calculations of final results are made according to standardize+ concentration. This will helps to find actual results.

Optical calibration

The term optical is related to light. Optical calibration involves the calibration of instruments that are used to measure optical properties. These instruments are like spectrophotometers, detectors, polarimeters, refractometers. We are analyzing different substances by using these instruments. So these must be calibrated to get accuracy in results.

Optical calibration is done by using different types of lamps and irradiance sources of known power density.

Flow calibration

Flow calibration involves the calibration of devices that are used for the measurement of flow rate. Flow rate is the rate at which liquid moves through pipelines, vessels, at a given time. There are different devices used for the measurement of flow rate i.e,

  • Thermal Mass Flowmeters
  • Laminar flowmeters
  • Gas and Air Rotameters
  • Turbine meters

These devices are used to measure the linear, nonlinear, mass, or volumetric flow of liquids and gasses. These devices must be calibrated because many chemical processes are dependent on flow rate. For example, all HPLC analyses are carried out at a specific flow rate.

Medical devices calibration

Medical devices calibration involves the calibration of devices used in medical processes. Major medical instruments are

  • X-rays machines
  • Ventilator
  • Infants Incubator
  • BP Apparatus (Sphygmomanometer),
  • Dialysis Machine,
  • ECG Unit,
  • Hematology Analyzer,
  • Pulse Oximeter,
  • Microscopes
  • MRI machines
  • CT scanners

These instruments are used for diagnostic purposes. Doctors prescribed medicine according to the results of these instruments. So these must be calibrated for accurate results.

Apparatus calibration

Different apparatus are used in labs for chemical processes. These are beakers, flask, pipettes, cylinders, etc of different volumes. All of these apparatus must be calibrated because are making dilution of different concentrations by these apparatus. If these are not calibrated then our solutions are not of required concentrations. We will not get accurate results. So all of these must be calibrated for better results.

Why calibration is so important?

Calibration is necessary to

  • Find out the different types of errors
  • Find out the uncertainties in results
  • Check the performance of instruments
  • Make the adjustment or correction of these errors
  • To enhance accuracy in results
  • To maintain the quality standards

What is a calibration curve?

A calibration curve is a plot of instrumental response to the analytical signals changes with the concentrations of analytes. We prepare a standard solution of known concentration. And check the machine response by changing the concentration according to the standard solution. The concentrations of the standards must lie within the working range of the technique used.

A calibration curve is used to find out the concentration of analyte in a sample. It will help to find out different types of errors. A calibration curve is one of the important parameters of calibration. It tells us whether the instrument used for analysis is properly working or not.

Calibration curve formula

y = mx + c

where,

  • Y b = y axis
  • m =slope
  • c = intercept

This equation is also known as the straight line equation. Results of calibrations are plotted in the form of the calibration curve. Calculation of results is made by using this straight line equation. For changing concentration it must be a straight line graph.

Calibration table for different laboratory instruments

Calibration standard for commonly used lab instruments
Instrument nameStandard used (reference standard)
HPLCCaffeine
FTIRPolystyrene films
UV-visiblePotassium dichromate
PolarimeterSucrose solution
RefractometerWater
ViscometerSilicone
Melting point apparatusCaffeine, Acetanalide, Diclofenac sodium, etc.
Dissolution apparatusPrednisone and salicylic acid tablets
pH meterBuffers of pH 4, 7, 10.
Atomic Absorption spectroscopyDifferent gases (Ne)

Concept bergs

Define calibration.

In general, calibration is the determination of instrumental accuracy. It is the comparison of obtained results to standard values.

What is the purpose of calibration?

The main purpose of calibration is to get more precise results by sorting out the different types of uncertainties and errors and making adjustments. Calibration is done to check the performance of instruments used for any purpose.

What is a calibrator?

Calibrators are the reference standards that are used for the calibration of different instruments. These are used to adjust the instrumental accuracy.

What is meant by the calibration curve?

A calibration curve is the plot of instrumental response to the changing concentration of analytes.

Why calibration is so important?

Calibration is important to

  • Find out the different types of errors
  • Find out the uncertainties in results
  • Check the performance of instruments
  • Make the adjustment or correction of these errors
  • To enhance accuracy in results
  • To maintain the quality standards

What are the different types of calibration?

Types of calibrations

  • Electronic calibration
  • Mechanical calibration
  • Thermal calibration
  • Apparatus calibration
  • Pressure calibration
  • Calibration of solution
  • Optical calibration
  • Flow calibration
  • Medical devices calibration

How many points should a calibration curve?

A calibration curve should have at least three points. But in general practice, a calibration of five points is used.

How are measuring instruments calibrated?

Measuring instruments are calibrated accordingly. Each of the measuring instruments has its own reference standard for calibration. These reference standards are called calibrators.

How to calibrate a Digital weight?

Digital weights are calibrated according to their measuring capacities. It involves the lower limits to the upper limits of digital weights. These are calibrated by putting different standard weights. These weights are put on the digital weights and balance is set to calibration modes.

When an instrument required calibration?

An instrument required calibration

  • Newly installed instruments required calibration.
  • After repairing or modifications
  • After a specific time period
  • After shifting from one place to another
  • Before and after critical analysis
  • When results are not proper

How can we calculate results from the calibration curve?

Results are calculated by using a straight line equation

y = mx + c

Where,

y = y axis

m =slope

c = intercept

Reference books

  • Vogel’s Textbook of Quantitative Chemical Analysis sixth edition by J Mendham, RC Denney, JD Barnes, M Thomas.

Reference links

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