How to decide calibration frequency of laboratory instruments?

Calibration of laboratory instrumentsis 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 103http://www.nabl-india.org/index.php?option=com_content&view=article&id=148&Itemid=71which 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) 6个月——重复性检查上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 enclosureAutoclave, 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,	Every Year
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)
Flow Rota meters	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 Spectrophotometer / Colorimeter	Quarterly-Photometric Absorbance and wavelength accuracy for the working range		Using CRM
Atomic Absorption Spectrophotometer	Performance check by f Cu CRM as per manufacturer’s instruction.	When used, Standard solution of specific element	通常对th性能检查完成e 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
Gas Chromatograph, GC-MS/HPLC / LC-MS	Quarterly, System performance including: Resolution, sensitivity, repeatability, retention time 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.