4.2 Operating Characteristics
4.2.1 Liquid Variables
4.2.1.1 Liquid Conductivity
The magnetic flowmeter requires a liquid conductivity of 5 microsiemens per centimeter or higher for
operation. This minimum liquid conductivity requirement is not affected by the length of the signal
interconnection cable when remote mounting of the signal converter is required, as long as the
factory-supplied interconnection cable (with driven shields) is utilized. The nominal maximum trans-
mission distance is limited to 30 meters (100 feet), however longer distance can be accommodated
(contact factory for details).
The conductivity of a given liquid,
σ
, may be determined experimentally under a filled meter
condition, as follows:
1) Remove the Converter housing cover. Disconnect the electrode signal interconnection leads
from terminals "1" and "2" of the signal converter. (These leads should be identified so that they will
be properly reconnected.)
2) Measure the resistance between signal leads "1" and "2" with an ac ohmmeter.
CAUTION
Do not use a DC ohmmeter for this measurement as polarization
effects will produce completely erroneous data.
The conductivity of the process liquid (in microsiemens/cm) may be determined from the electrode ac
resistance measurement (in megohms) by substitution of values in the following equation.
σ
=
1
(R
ac
- 0.072) x Electrode Dia, in cm
where,
0.072 is the electrode barrier resistance in megohms; i.e., 36 k x 2/10
6
Electrode diameters vary with meter size, however, the values given in Table 4-1 will serve as typical.
For example, assuming the measured ac electrode resistance (full pipe and zero flow) is 192,000
ohms and electrode diameter is 7.92 mm (0.792 cm), then
σ
=
1 = 10.52
µ
S/cm
(0.192 - 0.072) x 0.792
This is above the threshold for specified measurement accuracy for the particular liquid, meter size
and signal converter combination. Liquid conductivities at the operating temperature may also be
determined from standard reference works for many pure liquids. Company Field Engineers are
equipped to determine the conductivities of special liquids at the user’s site as an engineering
service.
10D1475 MINI-MAG MAGNETIC FLOWMETER INSTRUCTION MANUAL
FUNCTIONAL DESCRIPTION
4-3
