HA249928
Issue 15 Jly 06
Page 38
100 MM RECORDER USER GUIDE
7. GLOSSARY OF TERMS (Cont.)
Linearisation table
Most transducers produce an output which is not directly proportional to the input. For
example, the voltage output from a thermocouple does not vary linearly with the tempera-
ture it is exposed to. The recorder uses a ‘look-up’ table to find a temperature value for any
mV input from a specified thermocouple type. Similar tables exist for other transducers
such as resistance thermometers.
Measured value
An umbrella term which means: the value of an input channel, derived channel, totaliser,
counter, timer etc. measured in mathematical units as a proportion of the span. See also
Process variable.
Multipoint recorder
This is used to describe recorders which have multiple pen printheads rather than individual
pens to produce the trace on the chart. Each trace is made up of dots, produced by the print-
head as it traverses across the chart at regular intervals. Advantages are that many more
traces can be laid down on the chart, the traces can be annotated for identification and mes-
sages can be printed on the chart. Disadvantages are that fast transients may be missed at
low chart speeds.
Operator interface
A term used to describe the controls (e.g. pushbuttons, keypads) that are used to operate and
configure the unit.
Paper transport system
This includes the chart cassette and the mechanical system, motors etc. needed to move the
chart through the cassette. The paper transport system is often considered to be an integral
part of the writing system.
Pen
A fibre-tipped disposable stylus with an integral ink reservoir. Used to draw (trace) the
value of a single process variable on the chart in continuous trace recorders.
Pen tray
With modular recorder designs, each pen has its own mechanical system (including motor
and feedback device) associated with it to drive it backwards and forwards across the chart.
Pen tray is the general term for such mechanical systems. With some recorders, the pen
drive electronics are integral with the pen tray.
Printhead
This is a device which, together with a disposable multi-colour cartridge, allows multi-point
recorders to mark the chart.
Process variable
An umbrella term which means: the value of an input channel, derived channel, totaliser,
counter, timer etc. measured in engineering units (e.g. Degrees Celsius). See also Measured
value.
Relay output
A set of contacts which changes state as a result of an alarm becoming active. Relays are
energised continuously except when ‘in alarm’, so that if power to the recorder fails they go
into their ‘alarm’ state.
Resistance thermometer
Also known as a resistance temperature detector (RTD), a resistance thermometer is con-
structed of a material whose resistance varies in a known way with the temperature it is ex-
posed to. The resistance variation is non-linear, but for any given type, this non-linearity is
well known and invariable and is compensated for by linearisation tables in the recorder
memory.
Setpoint
Also known as ‘threshold’, this is the point at which an alarm becomes active or inactive.
See also hysteresis.
Shunt
The input circuit of each recorder channel measures voltage signals. If current signals are
connected to the recorder, a low value resistor must be placed across the inputs, to convert
the current signal to Volts, according to Ohms law (Volts = Amps x Ohms). Thus, a 0 to
20 mA (0.02 Amps) signal applied across a 250
Ω
resistor produces a voltage range of
0 to (0.02 x 250) Volts = 5 Volts.
Such resistors are called ‘Shunt resistors’ or ‘Shunts’ for short, and are usually of very close
tolerance.
Span
Span has two common meanings: the right-most grid of the chart, or the value given by
(maximum value - minimum value). The two meanings are identical where the minimum
value is zero.