Nace CP 1, Руководство по эксплуатации

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Field Measurements 5:12
CP 1 – Cathodic Protection Tester Course Manual
©
NACE International, 2000
02/01/05
If in the above example we wished to estimate the voltage drop for a known
current, we would again apply Ohm’s Law. To estimate the voltage drop
for a current of 5 amps, simply solve Ohm’s Law for voltage.
E = I x R = 5 A x 0.01
Ω
= 0.05 V =50 mV
An easier method for calculating current is to determine the amps/millivolt
rating of the shunt. There are two values associated with the shunt, one in
amps and the other in millivolts. Except for wire-type shunts, most shunts
will have these values stamped on them. You can determine the
amps/millivolt rating by dividing the amp value by the millivolt value. This
calculation tells you how many amps are flowing per millivolt measured
across the shunt. After you measure the millivolt drop across the shunt,
multiply the millivolt drop by the amps/millivolt rating. This will give you
the current flow through the shunt in amps.
For example,
Given:
Shunt data: 15 A/50 mV
Measured voltage drop: 28 mV
Determine shunt rating:
Rating = 15A/50 mV = 0.3 A/mV
Calculate current:
Current = 0.3 A/mV x 28 mV = 8.4 A
For shunts that you test regularly, writing the amps/millivolt rating on the
shunt will speed up your work considerably when taking data. You need
only to multiply the rating by the millivolt drop to obtain the current.
Direction of Current Flow
You not only need to know the amount of current flowing through the
shunt, but you also need to determine the direction of flow. This is done in
the same manner as with voltmeters. Remember that when current enters
the meter on the positive terminal, you will get a positive indication on a
digital meter, and on an analog meter the needle will swing to the right.
Remember also that the voltmeter is connected in parallel with the shunt.
Consequently, the direction of current flow through the meter will be the
same as that through the shunt.