Functional Safety Manual
for the Memosens transmitter Liquiline M CM42 SIL
Version:
Page:
2.0
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be done by the operator of the safety chain. So we are here never talking about
accuracy, but always about precision!
To use the safety functions, the device has to be switched to the safe "SIL mode" and
“SIL measurement mode” using the display and the keys/navigator. After switching to
SIL mode the device is able to execute safety functions.
Note!
If not otherwise stated, all comments/remarks/restrictions/etc. of this document refer
to safety functions SAF1 and SAF2. SAF3 and SAF4 are special safety functions, which
are not executed constantly like SAF1 or SAF2.
Note!
In SIL measuring mode the following formulas are used:
The pH value is computed for a measured voltage U at temperature Tk by using:
pH = - (U / S
Tk
) + pH
NP
S
Tk
= slope at temperature Tk
,
zero-point pH
NP
: both from pH adjustment.
The pH value is always automatically temperature compensated (ATC) using the
formulas from chapter 4.7.1.
The system does only allow slopes between 50.0 and 61.0 and zero points between 6.0
and 8.0. All other values will not be accepted. Sensors with slope or zero point outside
these ranges can not be used.
2.1.1
Safety Function 1 (SAF1) – limit value monitoring
Monitoring of the measured value (internally converted from voltage to pH value):
If leaving a user-defined pH-interval I = [M min, M max], an error current is set.
Instead of an interval it is also possible to define an upper or a lower limit only. Then
the other limit is equal to the minimum (4.0 mA) or maximum (20.0 mA) possible
measured value.
The value of I
min
in the figure below is 1.0 pH, the value of I
max
is 14.0 pH (0.0 pH up
to 14.0 pH).
