Electronic Control Fundamentals - Honeywell AUTOMATIC CONTROL SI Edition Engineering Manual

SENSORS/TRANSMITTERS
DIGITAL SENSOR
SPST/SPDT
TEMPERATURE SENSOR
POSITIVE RTD,
OHMS
THERMISTOR, OR
mV
THERMOCOUPLE
RELATIVE
HUMIDITY TRANSMITTER
PRESSURE
TRANSMITTER
DIFFERENTIAL
PRESSURE TRANSMITTER
ENTHALPY TRANSMITTER
TEMPERATURE,
HUMIDITY
— Some RTD element resistances are as low as 100 ohms.
In these cases, the resistance of the lead wires connecting
the RTD to the controller may add significantly to the
total resistance of the connected RTD, and can create an
error in the measurement of the temperature. Figure 3
shows a sensor and controller in relation to wire lead
lengths. In this figure, a sensor 8 meters from the
controller requires 16 meters of wire. If a solid copper wire
with a dc resistance of 20.96 ohms per km is used, the
16 meters of wire has a total dc resistance of 0.335 ohms. If
the sensor is a 100-ohm platinum sensor with a temperature
coefficient of 1.24 ohms per kelvin, the 16 meters of wire
will introduce an error of 0.27 degrees C. If the sensor is a
3000-ohm platinum sensor with a temperature coefficient
of 8.6 ohms per kelvin, the 16 meters of wire will introduce
an error of 0.039 degrees C.
Significant errors can be removed by adjusting a
calibration setting on the controller, or, if the controller
is designed for it, a third wire can be run to the sensor
and connected to a special compensating circuit designed
to remove the lead length effect on the measurement. In
early electronic controllers, this three-wire circuit was
connected to a Wheatstone Bridge configured for lead
wire compensation. In digital controllers, lead wire
compensation on low resistance sensors may be handled
by software offset.
ENGINEERING MANUAL OF AUTOMATIC CONTROL
INDICATING ACCESSORY
DEVICES DEVICES
LED PANEL
ANALOG
GAUGE
DIGITAL
DISPLAY
123.4
REMOTE
SETPOINT
ADJUSTMENT
MICRO–
OHMS
PROCESSOR-
BASED
MANUAL
INTERFACE
CONTROL
SPDT
POWER
OVERRIDE
SUPPLY
TRANSFORMER
24V
12V
Fig. 2. Typical Electronic Control System Components.

ELECTRONIC CONTROL FUNDAMENTALS

INDICATING
CONTROLLERS
DEVICES
TEMPERATURE
LED PANEL
CONTROLLER
ANALOG
GAUGE
DIGITAL
RELATIVE DISPLAY
HUMIDITY
123.4
CONTROLLER
ENTHALPY PROCESSOR–
CONTROLLER BASED OUTPUT
ADJUSTMENT
UNIVERSAL
CONTROLLER
ELECTRONIC
CONTROLLER
Fig. 3. Lead Wire Length.
— The usable temperature range for a given RTD sensor
may be limited by nonlinearity at very high or low
temperatures.
— RTD elements that provide large resistance changes
per degree of temperature reduce the sensitivity and
complexity of any electronic input circuit. (Linearity may
be a concern, however.)
A sensor constructed using a BALCO wire; is a commonly
used RTD sensor. BALCO is an annealed resistance alloy with
a nominal composition of 70 percent nickel and 30 percent
iron. A BALCO 500-ohm resistance element provides a
relatively linear resistance variation from –40 to 120 C. The
sensor is a low-mass device and responds quickly to changes
in temperature.
123
INTERFACING OUTPUT FINAL CONTROL
DEVICES DEVICES
CONTACTOR
OR RELAY
TWO-POSITION
ACTUATOR
MODULATING
ACTUATOR
MICRO–
E/P
TRANSDUCER
INTERFACE
REMOTE
SETPOINT
SEQUENCER
OR STEP
CONTROLLER
OVERRIDE
SENSOR LEAD WIRES
SENSOR
8 m
SENSOR TO CONTROLLER
M15130
ELEMENTS
FAN/PUMP
DAMPER
VALVES
C3097