When you're ready to take noise measurements, it's strongly recommended that you perform a field calibration using an Acoustic Calibrator. You should calibrate your sound meter before you start to measure, and then once you've finished you should perform a reference check against the calibrator and make note of any drift that has occured. You can find out more about performing Field Calibrators by reading this post and watching a short video.
LAB23, the latest guidances from the United Kingdom Accrediation Service (UKAS), states the recommended calibration intervals for different types of measuring equipment, including:
For a professional calibration and repair service with a fast turnaround, contact Castle Calibration or visit www.castlecalibration.com and search for your equipment.