Operation; Overview; Operating Notes - Honeywell 7866 Operation And Maintenance Manual

6.1

Overview

This section describes operating guidelines and procedures for the 7866 Digital Analyzer.
6.2

Operating Notes

All three range readings are displayed in the upper display of the 7866 digital controller/indicator. To
select the measured value of a particular range, the operator selects the desired range (1, 2, or 3) by
pressing the LOWER DISPLAY key on the instrument's face. The lower display indicates the selected
range. Each range engages a different scaling equation to calculate the correct percentage of gas. If
preferred, the lower display can indicate the gas being measured. Pressing SETPOINT SELECT toggles
the lower display between the range and the gas being measured. Pressing LOWER DISPLAY selects the
desired gas (CO2inAIR, H2in O2, H2in AIR).
Caution should be used since the range can be inadvertently changed. The normal mode for operating is
RANGE 3 or H2in AIR. In this mode or range, the controller is monitoring the hydrogen content versus
air. The alarms are active in this mode only.
6.3

Operation

The digital display will indicate measured value to the limit of the design range. If the signal from the
sensor is outside the range, the display value will flash a range message. In extreme cases, a recalibration
may be required. However, normal variations due to changing gas flow rates and environmental factors
may cause signal drift.
The alarm relays are normally energized when no alarm condition exists; therefore, the contacts will go to
their alarm position upon loss of line voltage to the controller. Alarm limits can be set by pressing the SET
UP key to enter the Set Up mode. The Alarms Set Up group can then be entered to adjust the setpoints of
Alarm 1 and Alarm 2. Alarm circuits are inoperative when Range 1 and Range 2 are selected. As soon as
the controller is switched to Range 3, the alarms are activated.
Two alarms can be available, each one with its own separate relay contact. When power is OFF, the relays
are de-energized and their normally open contacts are non-conducting (open). When the alarms are
operating, but there is no violation of the limits, the relays are energized and the relays' normally open
contacts are conducting (closed). When an alarm limit is violated the specific relay will de-energize and
create an open circuit. The open circuit can be used to activate an external device.
Alarm 1 is a LOW Alarm type. Alarm 1 will trip and remain on whenever the measured percentage goes
below the entered value. Alarm 1 typically is used as a first warning and to signal the need for make-up
hydrogen gas. A typical setting may range from 96.0 % to 98.0 %. The setting of Alarm 1's limit is the
responsibility of the user to determine. Consult your safety engineer or insurance company for proper
operating parameters.
Alarm 2 is also a LOW Alarm type. This alarm is typically used to indicate an increasingly dangerous loss
of hydrogen from the system. As air replaces hydrogen in the system, the probability of combustion
increases. Alarm 2 is typically used to activate an emergency shutdown circuit and/or to activate an
annunciating horn or siren. In this situation, the process may need to be purged to achieve a safe gas
mixture. The specific alarm limit and proper procedures to handle a low hydrogen situation is the
responsibility of the user to determine. Consult your safety engineer or insurance company for proper
operating parameters.
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6. Operation
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Operation
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