Liquid Temperature; Other Liquid Variables - ABB COPA-XM 3000 Series Instruction Manual

Series 10DX3000 Magnetic Flowmeter Instruction Manual
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 - 0.072) x Electrode Dia, in cm
ac
where,
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0.072 is the electrode barrier resistance in megohms; i.e., 36 k x 2/10
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 µS/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.

4.2.1.2 Liquid Temperature

Having established the minimum liquid conductivity requirements for a given application, any liquid
which exhibits equal or higher conductivity may be metered without concern for any system compen-
sating adjustments. However, due regard for the effect of the liquid conductivity versus temperature
should be considered.
Most liquids exhibit a positive temperature coefficient of conductivity. It is possible for certain
marginal liquids to become sufficiently non-conductive at lower temperatures so as to hamper
accurate metering. However, the same liquid at higher or normal environmental temperatures may be
metered with optimum results. The possibility of an adverse temperature conductivity characteristic
should be investigated before attempting to meter such a liquid. Process or ambient temperatures
are also limited by the meter materials specification.
Other normal effects of temperature, such as influence upon liquid viscosity and density, the size of
the metering area, and the flux density of the magnetic field, have negligible or no effect upon
metering accuracy.

4.2.1.3 Other Liquid Variables

Other liquid variables such as viscosity, density and liquid pressure have no direct influence on
metering accuracy. Liquid density has no effect on volumetric flow rate since only the area of the
meter pipe and liquid velocity are required to determine the rate of flow. Viscosity and metering
pressure are restricted to physical limitations alone, such as the leakage pressure of the meter pipe
flange connections.
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