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3
Operation of Transmitter
Measuring Mode and Special Modes
3.1
Normal operation for a transmitter means measuring mode during which the con-
centrations of measured gas are monitored and transferred to the control unit. Fur-
thermore the transmitter is able enter additional so-called special modes due to
user operation or faults. Under these conditions the measuring functionality is no
longer given or not to the full extent.
Please consider that a connected control unit has to interpret the transmitter sig-
nals correctly to recognize the special modes and to arrange for suitable safety
measures. This is ensured when using ExTox-Control Units.
All special modes are non-latching.
Description of Transmitter Modes
3.2
Measuring Operation
3.2.1
In measuring mode the transmitter issues the measured concentration via the 4-
20 mA Interface. 4 mA corresponds to the zero point, 20 mA to the end value of
measuring range of the transmitter. The calibration curve is linear.
The range of output currents covers 2 to 22 mA for measuring mode
Under-Scale of Measuring Range
3.2.2
The output current is between 4 and 2 mA, corresponding to the concentration
range 0 to -12.5 % of end value of measuring range. Still smaller measured values
are indicated on the lower limit 2 mA.
Slight (reversible) deviations cannot be excluded due to climatic and other influ-
ences at the place of installation. If the deviations are permanently higher than al-
lowed, calibration and adjustment are necessary. Fixation of allowed deviations can
depend on the range of application and measured gas or it can be determined by
regulations. Please contact ExTox where necessary. In general deviations should
not exceed 5 % of end value of measuring range (3.2 mA).
Over-Scale of Measuring Range
3.2.3
The output current is between 20 and 20.4 / 22 mA, corresponding to the concen-
tration range 100 to 102.5 / 112.5 % of end value of measuring range. Still higher
measured values are indicated on the upper limit 22 mA.
The curve is at first linear up to 20.4 mA and goes then directly to 22 mA.
The measured value has over-run the end value of measuring range and so left the
specification range. Please pay attention to the fact that defects or permanent
changes of the measuring signal due to overload cannot be excluded for some sen-
sor elements. In general we recommend checking the sensor element after such
overloads occurred.
On some measuring principles, for example catalytic combustion, the measuring
signal could drop back into the measuring range in case of very high concentrations
above the upper limit of measuring range. To avoid misinterpretation the down-
stream Control Unit should display the over-run of measuring range as latching. To
facilitate this the transmitter types ...-WT issue a value of 22 mA for ca. 30 s in case
of an over-scale of measuring range, even if the sensor already indicates a value
within the measuring range.
Page 8 of 20
ExTox Transmitter ExSens(-I) and Sens(-I)
Instruction Manual
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