Hach ORBISPHERE 366 Series, Basic User Manual

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3.6.2 Analog output signal
The analog output signal, representing the measured gas concentration, is available from the
instrument in the ranges from 0 to 20 mA, or in the range from 4 to 20 mA. In addition, the model
3662EX has a second analog output of 0 to 2 Volts. Depending on the instrument model, separate
output connections are available for the "safe" area and for the "hazardous" area.
Scale these outputs between two custom concentration limits. If the lower of these limits is
represented by AL, and the upper one by AH, the relation between the output current I (or voltage V)
and the gas concentration C is as follows:
0 – 20 mA C = AL + (AH - AL) * I/20
4 – 20 mA C = AL + (AH - AL) * (I - 4)/16
0 – 2 Volts C = AL + (AH - AL) * V/2
The analog output saturates at 20 mA (or 2 Volts), or is blocked at 0/4 mA (or 0 Volts) under various
conditions, as summarized below:
Analog output Explanation
0/4 mA (or 0 Volts) Thermal cutoff or below analog output low limit (AL)
0/4 mA (or 0 Volts) < signal < 20 mA Normal operating condition
20 mA (or 2 Volts) Over range, above analog output high limit (AH), or sensor disconnected
3.6.3 Alarm relay output (3660EX only)
There are three separate alarm relay connections on the instrument's output card (NO and NC), for
each alarm limit relay (High and Low), and system (SYST) alarm. The following table lists the
possible alarm conditions.
Conditions Low alarm
NC to Com*
High alarm
NC to Com*
Low alarm
NO to Com*
High alarm
NO to Com*
Alarm syst
NC to Com*
Alarm syst
NO to Com*
Instrument power
switch off closed closed open open closed open
No alarm:
Low Limit < measured
gas < High Limit open open closed closed open closed
Low alarm:
measured gas < Low
Limit closed open open closed open closed
High alarm:
measured gas > High
Limit open closed closed open open closed
System test closed open
Sensor not connected closed open open closed closed open
Thermal cut off closed open
* Low NC, High NC, Low NO, High NO, and Com identify the alarm relay connections on the instrument's output
card.
Note: The system alarm is turned on when the following situations are met:
Instrument off - Alarm high - Alarm low - Sensor out - Thermal cut off
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