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Wednesday, March 21, 2012

Reading a Schlumberger Sangamo Meter

My production meter is a Schlumberger Sangamo with the following markings below the rotating disk:

CL200 - accurate to 200 AMPS (but it is a 100 AMP meter)
240V - normal household single phase current
3W - 3 wire connection (Phase A, Neutral, Phase B)
Type J5S - fits into a socket with plugs for 3-wire single phase or 3-wire delta
30TA - calibrated at 30 AMPS (30 Test Amps)as specified in ANSI C12.X standards
7.2kh - a single rotation of the disk is 7.2 watt hours and 1000 rotations will be 7.2kWh

From this last factor we can see that 138.8888889 rotations equals 1kWh (1000 / 7.2).

Elaboration:  Since power is force applied for a period of time then the single rotation of the disk is equivalent to a force of 7.2 Watts applied for one hour.  Whatever time it takes will be removed from the power equation (1 rotation = 7.2Wh and 138.8888889 rotations = 1kWh).  So the calibration is equating all amounts of power to linear variations of rotational speed (within zero to 200AMPS) then the force unit (W) is replaced with power (Wh).  Time is irrelevant if the calibration is effective. And for every 138.8888889 rotations we see the digits of the meter advance one kWh. 

Magnetic damping is the method of calibration and it is similar to magnetic damping in beam balances used for reagent measurements in chemistry labs. The placement of permanent magnets on both sides of a non-ferrous plate, usually an aluminum plate on the beam, causes eddy currents as the plate moves through the magnetic flux. These eddy currents exert opposing magnetic forces which slow or dampen the movement of the beam. On the electrical meter the rotating disk is non-ferrous and appears to be aluminum.  The damping magnets are adjustable with a screw in a hole with F (faster) and S (slower) directional markings.  Placing the magnets closer to the disk will increase the magnetic damping or drag on the disk.

Now as to the title of this post -- you can read the kWh numbers directly like an odometer because these meters no longer use alternating rotation dials and multipliers or vernier scales.  The reading today was 626 at sundown.  This is 135kWh for the day.  The smart meter showed 18kWh total inbound off-peak during the night, 0kWh of on-peak, 2.27kW demand (set on Tuesday when pies were baking).  The outbound totals were 38kWh off-peak and 75 kWh on-peak, with a new outbound demand of 17.36kW.

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