Eurotherm Chessell 4102C, Руководство пользователя

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Page 37
HA249928
Issue 15 Jly 06
100 MM CHART RECORDER USER GUIDE
Start
100
90
Input signal value
End
Alarm off only when value falls below 90.
Alarm without hysteresis
Alarm with hysteresis
7 GLOSSARY OF TERMS
Alarm A function which is triggered when an input signal or a signal derived from it reaches a cer-
tain value. Once triggered, the alarm can cause a relay output to change state.
Analogue input An input which changes in a smooth (non-stepped) way (e.g. thermocouples, resistance ther-
mometers).
Attenuator A resistive device which reduces the signal voltage by a known ratio (usually 100:1)
Break response The recorder can detect an open circuit at its input terminals and, the instrument’s response
to an open circuit can be defined as ‘None’, ‘Drive high’ or ‘Drive low’. If none’ is selected
the trace is allowed to drift according to what the input wiring is picking up (acting as an
aerial). Drive high (low) causes the trace to be drawn at the extreme right (left) side of the
chart.
Chart cassette A mechanical paper transport system for containing and feeding the chart past the pens or
printhead at a known speed. The cassette includes reservoirs for unused (pay-out tray) and
used (take-up tray) sections of chart.
Cold Junction Compensation Also known by the abbreviation CJC. The voltage generated by a thermocouple (TC) junc-
tion depends on the temperature difference between the actual bonded junction (the hot junc-
tion), and the other (non-bonded) end of the conductors (the cold junction (CJ)). Thus, for
any reading from a TC to be accurate, the temperature of the CJ must be taken into account.
This can be done in three ways: Internal, External or Remote.
Internal. The recorder has integral temperature detectors measuring the temperature near the
terminal blocks (the cold junction for directly connected TCs).
External. For remote TCs, the cold junction can be held at a known temperature. This tem-
perature is entered (in degrees) as a part of the CJC configuration.
Remote. For remote TCs, an auxiliary temperature detector can be used to measure the cold
junction temperature. This detector is then connected to a separate input channel. This input
channel number is entered as a part of the CJC configuration.
Configuration This is used as a verb to mean ‘the process of telling your recorder what you want it to do’,
and as a noun to mean ‘the way in which the recorder has been set up (or configured)’.
Continuous trace This is used to describe recorders which have a single pen associated with each process
variable, and this pen traces the value continuously. See also multipoint recorder.
Digital (discrete) input An input which has only two states (on or off). Examples are switch inputs or voltage pulse
inputs.
Event input A discrete (switch) or digital (voltage level) input.
Hysteresis When an input signal is ‘hovering’ near a setpoint, then an annoying and potentially damag-
ing series of alarms can be generated, instead of just one alarm the cause of which can be
dealt with if necessary. To avoid this, a ‘hysteresis’ value of 1% span is applied, which
effectively puts a dead band round the set point. For example with a span of 0 to 1000 an
absolute high alarm with a set point of 100 will have a hysteresis value of 10. The alarm is
triggered when the input signal value rises above 100, but will not re-trigger until after the
alarm has been ‘cleared’ by the process value falling below 90. An attempt to depict this
example is given in the figure below.
Input channel An input circuit which accepts voltage, current or digital input signals from the user.
Input signal A voltage, current or digital input applied to the recorder input circuits. See also Analogue
input and Digital (discrete) input.