Operating Characteristics; Liquid Variables; Liquid Conductivity; Liquid Temperature - ABB CK-MAG 10D1477 Instruction Manual

10D1477 MAGNETIC FLOWMETER INSTRUCTION MANUAL

4.2 Operating Characteristics

4.2.1 Liquid Variables

4.2.1.1 Liquid Conductivity

With certain qualifications (see text which follows), the CK-MAG magnetic flowmeter can measure
liquids whose conductivity is as low as 0.05 uS/cm. 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 supplied shielded interconnection cable is utilized. The nominal
maximum transmission distance is 30 meters (100 feet).
A known property of low conductivity liquids is that they generate noise currents within the metering
tube section. The magnitude of the noise currents is dependent on velocity and viscosity of the
metered liquid. This noise is known as triboelectric noise and can swamp the signal measurement
system of the converter in extreme cases. A given low conductivity liquid may be measured with
satisfactory results if the following relationship is true:
σ υ
x
> 0.1
2
V
where:
σ
= liquid conductivity in uS/cm
υ
= kinematic viscosity in cSt (centistokes)
V = liquid velocity in ft/sec
As an example, consider an application to measure liquid flowing at 10 ft/sec whose conductivity is
0.2 uS/cm and whose viscosity is 60 cSt. Using the previous equation:
0.2 x 60 = 0.12
10 x 10
This is a border line case, but will result in an accurate measurement with an acceptable level of
process induced noise.
Liquid conductivities at the operating temperature may also be determined from standard reference
works for many pure liquids. 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.
Since most liquids exhibit a positive temperature coefficient of conductivity, it is possible for certain
marginal liquids to become sufficiently nonconductive 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. Liquid or ambient temperatures are
also limited by the meter materials specification.
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