YOKOGAWA SC200 User Manual

IM 12D7B2-E-H
4-5. Sensor selection and diagnostics
4-5-1. General remarks
The EXA SC200 continuous sensor diagnos-
tics can lead to the indication of faults due
to the fouling of the connected sensor. The
fault will be indicated with a signal field on
the display and (if activated) by a 22 mA sig-
nal. The selection of the correct conductivity
cell with the suitable cell constant is explai-
ned in the next section.
4-5-2. Selecting a conductivity cell
First select the range limits for the EXA in-
strument.
For conductivity values below 1 µS/cm the
only choice is a cell with cellconstant 0.01
cm
-1
. From 1 µs/cm up to 10 mS/cm a cell-
with cellconstant 0.1 cm
-1
can be used. A
cellconstant of 1 cm
-1
is used between 10
µS/cm up to 100 mS/cm. The cell with cell-
constant 10 cm
-1
is used from 100 µS/cm to
1 S/cm. All these values refer to the actual
measured values at the operating tempera-
ture.
Above 20 mS/cm with C=1 cm
-1
and 200
mS/cm with C=10 cm
-1
4-electrode cells
are advised.
Yokogawa delivers cells from a choice of
chemically resistant materials like stainless
steel, epoxy resin, PTFE and PVDF.
The cell determines the principle used to
measure the conductivity: there is a distinc-
tion between 2-electrodes and 4-electrodes
cells.
The selection of 2 or 4 electrode measuring
principle is from the service level §7-9.
4-5-3. Selecting a different cell-con-
stant
The EXA SC200 can be programmed to ac-
cept any cell with a cell-constant between
0.01 and 50/cm. The adjustment of the cell-
constant must be executed from the service
level §7-7.
4-5-4. Integral temperature sensor
The factory setting for the temperature sen-
sor is a Pt-1000 element. To get a different
temperature setting the service level setting
at § 7-4 is used.
A separate temperature sensor can be con-
nected, if no integral temperature element is
present in the conductivity cell.
Choose a Ni-100, NTC or Pt-1000 sensor
to get the correct temperature reading.
4-5-5. Sensor diagnostics and related-
error messages
The check for polarisation of the electrodes
is only working correctly when the cell-con-
stant has been chosen to match the measu-
ring range. The check is automatically
switched off when the 4-electrode measu-
ring principle is selected. The check can
manually be switched on or off: refer to
§7-14. The factory setting for the polarisa-
tion check is off.
18

YOKOGAWA SC200 Specifications

General

Brand	YOKOGAWA
Model	SC200
Category	Transmitter
Language	English