UFC 3-570-06
JANUARY 31 2003
7-13.3.3
This data is then applied to the following formula:
mV IR Drop
Current =
7-13.3.4 The direction of the current flow is determined from the polarity of the mV IR
drop reading measured. For example using Figure 7-35, a positive IR drop indicates
current flow from right to left, while a negative IR drop would indicate current flow from
left to right.
7-13.4
Multi-Combination Meter. The multi-combination meter has the capability
of measuring current by the "Null Ammeter Method." This meter can safely measure
line currents less than two amps. Do not use this method in areas over two amps of line
current. Stray current variations or other fluctuations in the measurement may make
keeping the meter nulled to obtain an accurate and reliable measurement too difficult.
This method does not require any information on the length of the test span, diameter of
the structure, or the type of metal being tested.
Figure 7-36. Null Ammeter Method, Using the Multi-Combination Meter
M ult i-Co mbi natio n M et er
MC M
MO D EL B 3 -A-2
COA RSE
-
-
+
FI NE
+
TO
LEFT
TERM
POLARI TY
CONTA CT
POLARI TY
ZER O
ZERO
CHE CK CKT
RH EO
VOLTS
IN PUT RES
AM PS
VOLTS
INT BTR Y
AMPS
1.5 V
3V
W ITH
AMPS
Ba tte ry
-
CONTROLS
-
ADJ
+
ON /OF F
+
VOLTS
BI AS
(If R equ ired)
Outsid e Ci rcu it (A )
In side Circuit (m V)
-
+
Pip el ine or ca bl e
7-54
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