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Emerson Process Management GmbH & Co. OHG 5-21
X-STREAM XE
Instruction Manual
HASXEE-IM-HS
04/2010
5
Startup
5.7 Checking the Settings
Tab. 5-1: Analog Output Signals Settings and Operation Modes
Output signal, if
Column A B C D E F G
"OutRange" "FailMode"
Failure
signal level
acc. NE 43
Measured
value is
valid
Measured value is
below lower limit
("Low scale")
Measured value is
above upper limit
("High scale")
An internal
failure
occured
Cable is
severed
0-20 mA Track - 0 ... 20 mA < -19 mA > 21 mA undened 0 mA
4-20 mA Track - 4 ... 20 mA < -19 mA > 21 mA undened 0 mA
0-20 mA LOW - 10% below 0 ... 20 mA -0.2 mA 20.5 mA -2 mA 0 mA
4-20 mA LOW - 10% below 4 ... 20 mA 3.8 mA 20.5 mA 2.4 mA 0 mA
0-20 mA HIGH + 10% above 0 ... 20 mA -0.2 mA 20.5 mA > 22 mA 0 mA
4-20 mA HIGH + 10% above 4 ... 20 mA 3.8 mA 20.5 mA > 21.6 mA 0 mA
If "OutRange" is set to 0-20 mA, a 20
mA signal
is generated, if the measured concentration
is equal to "Max Scale"
. A 0 mA signal is ge-
nerated if the sample gas concentration is 0
(dead zero).
However, a severed cable also results in a
signal value of 0. An external data acquisition
system thus cannot detect such an failure and
accepts a gas concentration of 0.
The commonly used method of detecting a
severed cable is to apply an offset: a concen-
tration corresponding to the lower range value
is assigned an analog signal of 4 mA, enabling
to detect a severed or disconnected cable.
This live zero mode is activated by setting “Out
Range” to 4-20 mA.
Operation modes conforming to
NAMUR 43
(NE 43) recommendations
The operation modes described above do not
generate a signal which enables detection of a
failure in the measurement system. In such ca-
ses the behaviour of the output signal is unde-
ned: either the last value is held, or a random
value is sent. System failures thus cannot be
detected by an external data acquisition system.
NE 43 contains recommendations for setting
analog outputs in order to avoid these situa-
tions. These are implemented by X-STREAM
analyzers as follows:
Setting "FailMode" to HIGH + 10% or LOW -
10% denes specic analog output signals in
case of a failure. Since these values do not
occur under normal operation conditions, a data
acquisition system is enabled to distinguish
between the following situations (table 5-1):
• Valid signal (signal within valid range; co-
lumn C)
• Signal out of range (signal rises or falls
slowly to the limits given in columns D or E
and holds this value until the concentration
returns to a valid level).
• Failure (signal out of range; column F)
• Severed cable (no signal; column G)
Operation Modes acc. NAMUR NE 43