Regular (daily) calibration checks establish a record of the operation of your instrumentation, not just the sensor, but the entire measurement chain. This is important as cabling and other parts of the measurement chain may wear out, and a recent calibration check for comparison can help rule out a faulty sensor. Calibration also helps to establish the level of uncertainty associated with the measurements. It gives a historical account of the sensor’s performance, and the level of accuracy with which that sensor can perform.
Equipment should be calibrated starting at the first use, and then every time before each use. This will enable confidence in the measurements when some part of the data is brought into question. A post calibration can be wise when taking measurements for long durations of time. This will help determine the drift of the equipment over time, temperature, humidity, and ambient pressure. Recall that all microphones have coefficients related to environmental parameters, and that any major change can cause a temporary small deviation in sensitivity. A temperature change of 5°C can cause a sensitivity change of up to 0.15 dB according to the limits set down by the microphone standards.
When to Perform a Calibration Check
The microphone should be calibrated once the measurement chain has been completely assembled (sensor, cable, signal conditioner, and data acquisition or microphone as part of a sound level meter which is a common core) and once the sensor has been acclimated to the environment in which it will be measuring. It is important to allow the sensor to become acclimated to the environment before calibration; otherwise the operational sensitivity will not be the same as the pretest calibration sensitivity. It is also important to allow the calibrator to run for a minimum of 30-40 seconds to assure a stable signal is being applied to the microphone. During calibration it is important to not touch the microphone, as this can alter the temperature of the microphone by the application of body heat.
How Often Should Accredited Calibration Be Performed?
The duration between calibrations of instrumentation is a balance between risk and cost. A shorter period between calibrations by an accredited laboratory will lower the risk of questionable measurements. Annual calibration is highly recommended for all sensors. Equipment used on a daily basis should have a shorter calibration cycle than equipment used, for example, once a month.
However, several things should be taken into consideration. The cost of necessary correction measures if the instrument has not been reliable over a long period of time. The required uncertainty in measurements may demand calibration on a more regular basis. Regular accredited calibrations will allow trend data to be obtained from calibration records and inform of any tendencies to wear and drift. As components age and equipment undergoes changes in temperature or mechanical stress, performance gradually degrades. This is called ‘drift’. When this happens, test results become unreliable and both design and production quality suffer. While drift cannot be eliminated, it can be detected and contained through the process of calibration.
The hazard exposure of the equipment is very important as well. Extreme climatic conditions, vibration, ionizing radiation, and (electrical and mechanical) shock can have an adverse effect on microphone performance. Microphone storage and transportation should be performed with absolute care (preferably with the microphone stored in its original case); allowing microphones to roll around in a toolbox is ill advised.
The degree to which the personnel are trained can also affect microphone performance. An improperly trained technician may mishandle the microphone during calibration checks, drop the sensor, or unknowingly damage the microphone diaphragm through poor handling. In all of these cases it is important to have a calibration performed by an accredited calibration house to determine any problems that might arise from the continued usage of the instrument.
Lastly, if a legal requirement exists, or if any other requires the need to follow the changes in the instrument over time, tracking from the initial calibration will be required. Beyond that, a calibration history from day one should be kept with a re-calibration once a year. Then a comparison of the results over the years may be tracked. To miss a yearly calibration can put an entire two years’ worth of work at jeopardy if some unknown drift occurred.