A complete current measuring system normally consists of a coil and an integrator. These are usually provided separately to allow for flexibility in designing measuring systems.
Individual coils and integrators, even if they are made to the same specification, will differ slightly in their mutual inductances. This is not a problem when a coil and integrator are calibrated together as a unit because the integrator can be adjusted to suit the coil, but in some cases it is convenient to be able to replace a coil without the need to re-calibrate the whole transducer.
Rocoil has developed an interchangeable coil system where each coil has a calibration resistor which is built in to the output cable or the connector. The resistor automatically compensates for the variation in mutual inductance. For this system to work the integrator must be designed appropriately. Effectively this means that the integrator must have specified input resistance.
Coils which have been calibrated for interchangeability will normally have a resistance value marked on them near the connector (e.g. 2500R, 820R). Compatible integrators will be marked with the same resistance value. Interchangeable coils do not necessarily have to be the same length but for each system there is a maximum length of coil that can be used. More information about interchangeable coils.
Coils which are provided for a user to use with their own electronics are normally supplied without calibration resistors. In this case we measure the mutual inductance of each coil and this information is supplied with the coils.
If you are unsure about which integrator is right for you, read more about them here.
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