1.1
Output options
•
1, 3 or 6 extra relays (first optional relay can be set for simple on/off alarm/control or PI
control using pulse width or frequency control)
•
Isolated analog retransmission (single or dual analog outputs) configurable for 4–20mA, 0–
1V or 0–10V. Analog retransmission can be set for linear operation or logarithmic operation
using 1 to 5 decades. The first analog output is configurable for retransmission or PI control
•
Isolated RS485 or RS232 serial communications (ASCII)
•
Isolated Digital output - binary or BCD up to 16 bit, NPN or PNP output types available
•
Optional outputs are available in certain combinations e.g. Extra relay plus RS232
1.2
Conductivity measurement general information
The instrument measures conductivity by placing an AC voltage across the two conductive surfaces
(electrodes) of the conductivity cell and measuring the resulting AC current passing through the
solution. For a given cell the AC current will increase as the conductivity of the solution increases.
Any external voltage‘s present in the solution where the cell is located may cause inaccuracy and
possibly instability in the reading. Typical sources of external voltages are level sensors and badly
earthed electrical equipment such as pumps which have contact with the solution. The amount of
AC current produced by the cell depends on the conductivity of the solution, the area of the cell
electrodes and the distance between the electrodes. Any deposits which coat the cell will reduce
the surface area available and therefore cause inaccurate readings. If cells are likely to become
coated in use they will either have to be regularly cleaned or a non contact (inductive) type cell
used. The use of non contact cells is not covered in this manual.
If resistivity, ppm or percent are selected for viewing then the instrument simply measures con-
ductivity using a conventional conductivity cell and converts this reading into the required display
units. For ppm readings the conversion factor must be manually entered.
Cell K factor
- The instrument can only supply a given current range through the solution
being measured therefore a cell designed for use with pure water will not be suitable for use in
measuring very high conductivity since the instrument will not be capable of providing sufficient
current for stable measurement at both extremes. This instrument requires that the resistance
of the solution be 80Ω or higher for accurate measurement. To overcome this problem cells with
different sensitivity levels are manufactured and this sensitivity level is known as the K factor. A
cell with a higher K factor will use less current in a given solution than a cell with a lower K factor.
See the table at the beginning of this chapter for typical measuring ranges for common K factor
cells. The correct K factor cell should be chosen to suit the range required for measurement.
Temperature compensation
- Since conductivity changes with temperature the conductivity
value displayed is referenced to a given temperature, usually 25
o
C. This means that the value
being seen on the display is not necessarily the actual conductivity of the solution at that time
but is the conductivity value which would be seen if the solution temperature was 25
o
C. If 25
o
C is
not the required reference temperature i.e. if it is required to view what the conductivity reading
would be at a different temperature then the required temperature value can be set at the
SOL
"C
function.
This instrument allows for either manual or automatic temperature compensation. If no tempera-
ture sensor is used with the cell then the known temperature of the solution can be entered at the
dEF "C
function. If a temperature sensor is used the the reading obtained from this sensor can
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PM4COMAN-2.2-1
