Company X runs a large manufacturing facility and possesses a range of acoustical measurement instruments. Once a year, they send all the equipment to XYZ Lab for testing. Company X is currently unsure whether the tests carried out are appropriate.

The following points illustrate how Company X might develop a more appropriate calibration strategy, by following the outline process defined in the previous pages.

Measurement Objectives

To begin, Company X should identify what measurement activities they carry out, and any relevant characteristics of these measurements.

Why are measurements being taken? What is the end purpose?

Company X identified the following measurement activities:

• A Environmental noise surveys, where measurements are taken according to BS 4142 and evaluated against the Environmental Protection Act
• B Investigative measurements to identify noise transmission paths and likely sources of noise
• C Risk assessments to assess employee noise doses around a new piece of equipment
• D Monitoring of employee noise dose

How often are measurements taken?

Company X then considered how often each measurement activity takes place:

	Activity	Regularity
A	Environmental noise survey	Once a year
B	Investigative measurements	Once every 3 months
C	Employee Exposure Risk assessments	Once every 3 months
D	Dose monitoring	Frequently / all the time

What measurements are actually taken?

Company X considered what measured quantities were recorded during each measurement activity:

	Activity	Measure
A	Environmental noise survey	Sound pressure Frequency content Time history of sound pressure
B	Investigative measurements	Sound pressure Frequency content
C	Employee Exposure Risk assessments	Sound pressure
D	Dose monitoring	Time-averaged sound pressure

 

Risk Assessment

Next, Company X considered the risks associated with each measurement activity – that is, the risks to the business that would be incurred if inaccurate or invalid measurements were made.

Risks were split into Technical and Commercial risk, and scored from 0 to 5. A "combined risk level" was then defined by adding the levels of technical and commercial risk, and a level of importance was then attached to each activity.

	Activity	Technical Risk (0-5)	Commercial Risk (0-5)	Combined Risk Level (Technical + Commercial Risk)	Importance
A	Environmental noise survey	5	5	10	Very High
B	Investigative measurements	3	1	4	Medium
C	Employee Exposure Risk assessments	3	5	8	High
D	Dose monitoring	1	2	3	Low

Measurement Traceability

Based upon the risk levels given above, and a consideration of the relative importance of each activity, the following levels of traceability were judged to be relevant for each activity.

	Activity	Importance	Level of Traceability	Additional Considerations
A	Environmental noise survey	Very High	Level 1	It is possible that the results of the environmental noise survey could be used in a court of law; therefore, the highest possible confidence is required in the measurements.
B	Investigative measurements	Medium	Level 2	Investigative measurements, identifying noise sources and transmission paths, are of lower importance.
C	Employee Exposure Risk assessments	High	Level 1	In the event of any health and safety-related legal claims, it would be very important to be able to show that a risk assessment of employee noise exposure had been carried out, measured to a high level of confidence.
D	Dose monitoring	Low	Level 3	As formal employee noise exposure risk assessments are carried out (Activity C) then routine dose monitoring is of lower importance and is primarily useful to trace any trends in noise exposures.

Measurement Instruments

In order to determine an overall calibration strategy, Company X should then consider which instruments are used in each measurement activity. The table below also summarises the traceability required for each activity.

	A	B	C	D
Activity	Environmental noise survey	Investigative measurements	Employee Exposure Risk Assessments	Dose monitoring
Traceability required	Level 1	Level 2	Level 1	Level 3
Microphone S/N 01
Microphone S/N 02
Microphone S/N 03
Microphone S/N 04
Cables
Sound Level Meter A
Sound Level Meter B
Personal Dosemeter
PC with analysis software
Portable calibrator

Calibration Options

From the above table, Company X then was able to define the required level of calibration for each instrument, as follows:

Instrument	Required Level
Microphones	Level 1 OR Level 2, depending upon activity
Sound Level Meter	Level 1 OR Level 2, depending upon activity
Personal Dosemeter	Level 3
Portable Calibrator	Level 1 OR Level 2, depending upon activity
Cables	Cannot be calibrated, but should be continuity checked and inspected for damage
PC with analysis software	Cannot be calibrated, but output should be checked using a portable calibrator, as part of checks made on the measurement chain

An Appropriate Calibration Strategy for Company X

Company X then took all the above information and developed an appropriate calibration strategy as follows:

Firstly, Company X consulted with its calibration laboratory, Lab XYZ

• To ensure that it was UKAS accredited and able to provide a Level 1 traceable calibration
• To discuss the nature of the measurements that Company X was making, to ensure that the calibration procedures would, for example, check the frequency response of the instruments where necessary.

Company X then considered its inventory of instruments, and which were used for each measurement activity.

• Microphones require a Level 1 or a Level 2 type of calibration, depending upon the activity they are used for. Company X decided to reserve two microphones for higher importance measurement activities, to be calibrated to Level 1 traceability at Lab XYZ. All other microphones were to be calibrated to Level 2 traceability.
• Similarly, Company X reserved one sound level meter for higher importance measurement activities, to be calibrated to Level 1 traceability at Lab XYZ. The other meter was to be calibrated to Level 2 traceability.
• Company X only has one portable calibrator, and therefore decided that this should be calibrated to Level 1 traceability, ensuring that it can be used to check instruments used for all activities.

Company X has in-house laboratory facilities, and recruited an experienced engineer able to carry out calibrations and maintain appropriate reference equipment.

• Level 2 traceable calibrations of microphones and sound level meters used on lower-importance activities can therefore be carried out in-house.

Company X consulted the guidance in the UKAS document Lab 23 to determine the maximum calibration interval for each instrument. It was decided that the maximum calibration intervals could be used, as long as the following risk-mitigation activities were carried out

• All instruments to be stored in appropriate, clean, temperature and humidity controlled conditions, and not subjected to impacts, vibration or rough treatment.
• For every measurement, a portable calibrator should be used to check the readings given by the measurement chain. Note that the guidance given in Lab 23 indicates that a portable calibrator should be itself calibrated every year.
• Monthly cross-checks to be made between instruments, and a database to be maintained to monitor instrument sensitivities and drift. Any instruments that appear to be giving unstable readings should be recalibrated at more regular intervals.

Finally, Company X noted that Personal Dosemeters should be checked, and a certificate of conformance issued (Level 3 calibration), every year. However, it was discovered that the cost of sending each dosemeter away for conformance checking was greater that the cost of a new dosemeter. Therefore, new dosemeters are to be purchased at regular intervals, as each dosemeter comes with a manufacturer's certificate of conformance, valid for one year.

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