The handling and general storage of measuring tools and equipment are important for maintaining their accuracy. It includes the feeler gauge, a simple tool that comes commonly in a set consisting of several blades of different thicknesses.
According to the ISO 17025 standards, the feeler gauge equipment in laboratories and industries is to be kept while ensuring the following for maintenance of accuracy of the feeler gauge:
- The feeler gauge must be properly handled, transported, used, and stored
- The feeler gauge must be checked for accuracy before any check
- The feeler gauge must have an accuracy that is acceptable for a particular tolerance limit allowed in a procedure/process
Symptoms of Results From Feeler Gauge that Merits Calibration
The physical condition of a feeler gauge can affect its stability for measurements and reliability. If the physical condition deteriorates, then it results in an inconsistent result of measurements taken using a feeler gauge. The calibration of the feeler gauge must be ensured when the following is observed:
- The accuracy or certainty of the result affects the validity of overall results that are obtained taken from a particular feeler gauge.
- The feeler gauge has kinking, bending, and visible tearing.
- The calibration is required to make the feeler gauge maintain the metrological stability of the results taken with it.
Conditions / Prerequisites for Calibration
Timely and professional calibration of feeler gauges is always preferable for the most accurate readings. As per the ISO 17025 standards, the following are important to ensure calibration of the feeler gauges:
- The industry or laboratory must have an established calibration program that is updated with time.
- The feeler gauge that is calibrated must be labeled or coded to ensure that it is re-calibrated after the expiry of period of validity.
- The feeler gauges that are damaged during working must be clearly labeled as out of use unless their accuracy is proved using various tests.
Brief In-House Calibration of Feeler Gauge
The feeler gauge is usually calibrated in-house with the aid of an accurate and calibrated micrometer. The following steps may be followed for the purpose:
- The feeler gauge is cleaned with some oil and cloth for a good finished surface.
- The micrometer screw gauge is opened and adjusted to the reading that indicates the size of the feeler gauge.
- Then the feeler gauge is tried to insert between the jaws of the micrometer, following may be observed:
- The feeler gauge fits nicely– It shows that the gauge is well calibrated as per the dimensions highlighted on it
- The feeler gauge fits loosely– It shows that the feeler gauge is either damaged or has undergone extensive wear to not be able to be equal to the dimension highlighted on it
- The feeler gauge does not fit– It is surely an indication of damage or some material change to the gauge due to extra pressure it may have suffered or varying conditions of temperature during usage.
- However, the above-mentioned method can only be used for in-house calibration and the professional calibration from certified laboratories is always desired and required.
Where to Get the Feeler Gauge Calibrated
The feeler gauge is not recommended to be calibrated by the companies and therefore requires periodic calibration from set standard providers. As per the ASTM D7091, the parties which can calibrate a feeler gauge are limited to the following:
- The equipment manufacturers and their authorized agents.
- Several laboratories are available which are duly accredited with ISO 17025 standards to ensure calibration of a feeler gauge. These labs issue a calibration certificate upon calibration.
Labs to Choose for Calibration
Certified Reference Materials (CRM) are samples that are established to be used in calibration on an international level. Laboratories have quality control samples that are checked against CRMs for consistent calibration. This helps maintain conformity to the standard of certified reference materials.
A lab’s scope of accreditation enlists the set of activities that the lab is authorized to perform. Therefore, prior to getting a feeler gauge calibrated from a 3rd party lab, it is important to know about their accreditation certificate and whether they are authorized to calibrate a feeler gauge or otherwise.
As per the ISO 17025 standards, the lab has to identify the opportunity for improvement in its operations, policies, audit outcomes, and analysis of data and reports. Actions are accordingly taken to ensure improvements for having a higher and better standard of calibration.
Routine Verification of Calibration Done On A Feeler Gauge
A calibration certificate should not be taken as a guarantee that the feeler gauge may not need any calibration until the expiry of validation of the certificate. Therefore, it is important to select and maintain a sample to re-verify the feeler gauge after a definite interval of time to confirm that it is not erroneous.
For coated feeler gauges, coating thickness standards are there certified options to ensure calibration of these gages. The average of a series of readings taken with the gage should lie within the tolerance limit acceptable for the gage. This helps to uphold the standard of accuracy of feeler gages while in use.
Measurement Standards for Calibration of Feeler gauge
Such standards are maintained to allow for calibration near to the standard set by Certified Reference Materials. The standard of accuracy in calibration drops as we move towards secondary standards. The accepted measurement standard for a feeler gauge are categorized as follows:
- National Metrological Institute’s national standard which is regularly maintained.
- Primary standards which cannot be further calibrated by other measuring standards, but can be compared to other primary standards.
- Secondary standards that can be traced (measurement traceability) to the primary standard
The company itself is bound to follow certain standards in its working to ensure that the feeler gauges that it uses are always showing accurate results. The following can be implemented to ensure accurate testing of feeler gauges in companies:
- The reference standard used in the laboratory and maintained regularly
- Transfer standard to compare standard with other standards
- Working standard that can be used frequently on an increased frequency of operation