Calibration of laboratory instruments is an essential activity which gives confidence on reliability of your results. Both determinate and indeterminate errors contribute to variations in your results. It is possible to control to some extent the indeterminate errors through Good laboratory practices and training. Determinate errors on the other hand can be overcome through periodic calibration of the instruments used for analysis.
How often calibration is necessary?
Any accredited laboratory needs to maintain a calibration schedule of equipment used by it.National Accreditation Boards for Calibration and Testing laboratories recommend calibration frequencies based on regulatory bodies guidelines. In India NABL has issued such guidelines in the new draft document 103 http://www.nabl-india.org/index.php?option=com_content&view=article&id=148&Itemid=71 which arepresented here for reference purpose.
|Type of Equipment/Instrument/ Calibration Item||Recommended Frequency of Calibration||Intermediate Checks||Remarks|
|BALANCES||Every years or on repair ;||(a) Each weighing do Zero check
(b) One Month- One point check using a calibrated weight close to working capacity
(c) Six months- Repeatability checks at the upper and lower ends of the scale using a calibrated weight
|Reference weights to be used should be standard class F2 or better with established permissible errors.|
|WEIGHTS||Weights have to be calibrated externally by an accredited laboratory, at least once in two years.||Six monthly temperature check around working range||Maintain parameters to an accuracy of within a range as stipulated in methods.|
|Temperature Controlled Enclosures/ Thermostatically Controlled Equipment as Ovens, Incubators, Aging, Vacuum; ; Environmental Conditioning chambers;||Preferably from an accredited calibration laboratory.|
|Temperature Controlled enclosure Autoclave, Temp controlled waterbath, furnances||To be carried out based on usage|
|Thermometers (Liquid in glass)||Calibration from accredited laboratory.||Once a year Check at ice point or at points of use.||Ice point. If outside five times the uncertainty of the calibration,complete recalibration is required.|
|rRTD, PRT, Thermocouple with indicator||Every year, across working range or at points of use from an accredited calibration laboratory.|
| TIMING DEVICES
Stop watches, clock,
|Hygrometer||Calibration by an accredited calibration laboratory|
|Pressure Gauge||Calibration by an accredited calibration laboratory|
|Volumetric labware (Burette, Pipette and Volumetric flask)*||Internal calibration on receipt.||It should be verified that the tolerance is in desired acceptable limit (Class A & Class B glasswares)|
|Initial and recalibration after two years from an accredited calibration laboratory||Once a year against standard soap bubble meter|
|Dry Gas Meter||Initial and recalibration after three years from an accredited calibration laboratory|
|Pitot tube||Initial and recalibration after three years from an accredited calibration laboratory|
|Orifice Plate||Initial calibration from an accredited calibration laboratory|
|Gas sensors||Initial, recalibration after two years||Using standard gases.|
| UV-Visible / IR / FT-IR
Absorbance and wavelength accuracy for the working range
| Atomic Absorption
|Performance check by f Cu CRM as per manufacturer’s instruction.|| When used, Standard solution of
|Generally the performance check is done against the initial value checked at the time of installation.|
|Conductivity Meter||Once a year, Full range calibration with CRM|| Monthly, Reference standard, One point calibration. Adjust cell constant if necessary.
When used, Standard solution
Chromatograph, GC-MS/HPLC / LC-MS
| Quarterly, System
and noise level
|When used, Standard solution / mixture|
|pH Meter||Once a year, Full range calibration with CRM||To be checked with working standard before use.|
|Micropipettes||Once a year internal calibration|
The document under section 5.5 classifies laboratory equipments in following categories:
- General Service instruments which are not directly involved in measurements,e.g,hotplates,stirrers,etc
- Volumetric instruments such as volumetric flasks,burettes, pipettes,etc
- Measuring instruments such as phH meters,thermometers,spectrometers, chromatographs, electrochemical instruments, etc
- Physical standards such as weights, measuring scales, reference thermometers,etc
Calibration of general purpose instruments is not necessary as their contribution to measurements is not critical.
It is not necessary to re-calibrate length scales as the manufacturer guarantees the length against a standard with established traceability. It is only necessary to store such units within controlled conditions of temperature and humidity.
Volumetric glassware comes under different classes, namely, Class A and class B .Class A glassware is supplied with a calibration certificate and class B glassware is calibrated against class A glassware. Volumetric glassware if used within the temperature range 25°C +/- 2°C does not require frequent calibration.
Frequently used analytical instruments also require frequent calibration. The frequency of calibration will depend on usage. It is best to decide the intervals on the basis of in house standard operating procedure guidelines which in turn are based on equipment supplier recommendations.
In order to generate high level of confidence on your results calibration should be carried out even out of prescribed schedule when an instrument has undergone major repairs, is relocated or its capabilities are enhanced through addition of add-on accessories.