Inductive Voltage Divider (IVD) calibrations at NPL are traceable to the UK national standards of Alternating Voltage Ratio. These standards are based on IVDs, designed and built at NPL, which have very stable and repeatable characteristics. The standard IVDs are themselves calibrated, approximately every five years, using a build-up technique (sometimes called a boot-strap technique) utilising stable transformers and isolated windings.
NPL provides UKAS-accredited calibration of IVDs at specified discrete frequencies within the range 40 Hz to 120 kHz. We maintain two separate measurement systems which are optimised for particular frequency bands.
Note: For measurements below ~ 400 Hz it may be necessary to go off-frequency by several Hz in order to minimise interference from mains harmonics.
The NPL operates a calibration service for Strain Gauge Bridge Indicators (SGBIs) which is traceable to the national standards for A.C. Voltage Ratio. SGBIs are force measuring instruments and the method of calibration used is where the resistive strain gauge transducer is replaced by two calibrated Inductive Voltage Dividers. Two types of measurements are available. Absolute measurements energise the IVDs directly from the test instrument, and maintain traceability to national standards. Linearity check measurements use a stable centre-tapped 100:1 step-down transformer between the SGBI and the IVDs to providing more resolution, but do not have traceability to national standards.
The techniques above apply to SGBIs that are manufactured by HBM (Hottinger Baldwin Meßtechnik) of the types DK38, DMP39 and DMP40.
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