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Emerson Process Management GmbH & Co. OHG 6-9
Instruction Manual
HASAxE-IM-HS
05/2006
6 Initial Startup
6-2 Checking the Instrument's Setup
Normally it is expected that a gas concentration
of "0" generates an similiar analog output of
0 mA. Such an analog output operation mode
is selected by setting the "Signal range"
parameter to 0-20 mA.
But: A signal cable break results in a "0" signal,
too, and an external data acquisition is not
capable of detecting such a failure and accepts
a gas concentration signal of "0".
To avoid this the commonly used methode is
driving the analog outputs in LIVE-ZERO mode:
A concentration corresponding to the lower
measuring range limit (e.g. "0") is assigned to
an analog signal of 4 mA. Thus a failure like
cable break is clearly detectable by a signal of
0 mA.
This LIVE-ZERO mode is selected when
setting the parameter "Signal range" to
4-20 mA, which is the factory default setting.
Operation Modes corresponding to
NAMUR Recommendation 43 (NE 43)
Both modes described above do not provide
a signal capable to detect a failure within the
measuring system. Here the output signal
behaviour is undefined: either it keeps the last
value or is set to an arbitrary value: Measuring
system failures are not detectable by an exter-
nal data acquisition system.
NE43 gives recommendations how to setup
analog outputs to avoid above situation and the
X-STREAM series analyzers consider NE 43:
Setting "Signal range" to values other than
0-20 mA or 4-20 mA defines specific signal
levels for analog outputs in case of measuring
system failures. During normal operation these
values are not output, so an data acquisition
system is capable to distinguish between
• cable break ("0" signal),
• failure (signal outside accepted range, but
differing from "0")
• valid measuring value (signal within
accepted range)
In case there is no failure but only a measuring
range overrun or underrun the output signal will
increase / decrease until it is equal to the limit
given in table 5-1 below and then keeps this
value until the measuring signal is back within
the measuring range.
Special feature for single channel
instruments!
These special NAMUR compatible operating
modes adjust the channel 2 analog output to a
value below the accepted range but above the
failure limit. This ensures that a data acqusition
system recognizes the signal as a non valid
measuring signal and does not give an alarm
signal due to a missing measuring channel.
Table 6-1 at page 6-9 shows a comparison of
all available modes.