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Low resolution high precision ohmmeter

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article image The BS407 with the Kelvin Clip lead.

THURLBY Thandar’s BS407 precision ohmmeter, available from Emona Instruments , offers resistance ranges from 2mΩ up to 20kΩ with an accuracy of 0.1%. The resolution on the lowest range is 1mW.

Most digital multimeters offer a best resolution which is at least one thousand times poorer than this.

A very high resolution multimeter may attempt to resolve fairly low resistances, but it does so by resolving extremely low voltages (typically one nano-volt per micro-ohm), making it subject to severe errors from thermal emf effects and noise.

In contrast, the BS407 generates sufficient test voltage across the unknown resistance to combat noise and reduce thermal emf effects to manageable levels. Current reversal can then be used to eliminate any remaining thermal offset.

The BS407 is a fully portable instrument which operates from internal NiMh rechargeable batteries. The battery charger is built into the instrument and charging can take place during operation.

The BS407 uses a four terminal measurent system via high quality Kelvin Clip lead. For speed and convenience front panel switches are provided for current diversion (allowing in-situ zero adjustment) and current reversal (for identifying thermal emf errors).

An LED indicator warns of open circuit leads thus preventing spurious results from being recorded.

A front panel operated 'clamp' switch is available to limit the maximum voltage across the unknown to 20mV. This is a requirement for the measurement of switch contact resistances to international standards.

The BS407 measures 220 x 82 x 230 mm, and weighs 1.3kg.

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