[volt-nuts] Solartron Schlumberger 7150 and 7150 Plus design oddity.

David C. Partridge david.partridge at perdrix.co.uk
Wed Nov 10 09:06:55 UTC 2010

Circuit health warning.

Both these meters have a slight design oddity that mean that the rear sockets should only used in 4 or 5 wire mode for ohms and temperature measurements.

I discovered this when I had connected a PT100 RTD to the meter and switched to measure DC volts.  The meter read about 0.108 volts which related very accurately to the 108 Ohm resistance of the PRTD at a room temperature of about 20 Celsius.

The reason for this is that the Ohms force (or Source) terminals are still connected to the current generator when the meter is set to measure other things than resistance.   If you connect your kelvin clips to a circuit to measure (e.g.) DC Volts, then this can result in "unexpected" readings and could cause damage in some circuits.

David Partridge

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