Function And System Design; Input Functionality; Sensor Redundancy; Sensor Drift Monitoring - ABB TTH300 Operating Instructions Manual

4

Function and System Design

Digital transmitters are communication-ready devices with
microprocessor-controlled electronics. They conform to the
requirements of housing IP rating IP20 and are suitable for
integration into DIN A and DIN B sensor heads.
With HART transmitters, an FSK signal is superimposed on the
4 ... 20 mA output signal in accordance with the HART
standard to facilitate bidirectional communication.
With PROFIBUS PA transmitters, communication takes place
in accordance with PROFIBUS - MBP (IEC 61158-2),
PROFIBUS PA profile 3.01.
With FF transmitters, communication takes place in
accordance with FOUNDATION Fieldbus H1 (IEC 61158-2),
ITK Version 5.x.
The transmitters can be configured, polled, and tested using a
DTM or an EDD.
As an option, the transmitter can be fitted with a type A or a
type AS LCD indicator. Type AS is used exclusively for
visualizing current process values. Type A also supports the
option of configuring the transmitter. It is recommended that
you use this combination.
The electrical connection between the LCD display and
transmitter is provided by a 6-pin flat ribbon cable with a plug
connector. The LCD display can only be operated when
connected to transmitters that have an LC display interface.
4.1

Input functionality

4.1.1

Sensor Redundancy

To enhance system availability, the TTH300 has two sensor
inputs.
The second sensor input can be used redundantly for both
resistance thermometers (2 x three-wire circuit or 2 x two-wire
circuit) and thermocouples, or for a mixture of the two.
Sensor redundancy (or sensor backup) always involves
measuring the temperature of the two sensors and calculating
the mean value on the basis of this.
This value is provided at the output of the transmitter. Should
a sensor fail, the temperature measurement for the sensor that
remains in operation is provided at the output of the
transmitter.
A relevant diagnostic message is provided via the EDD or
DTM, or shown on the display. The measured value remains
available and maintenance measures can be taken at the
same time.
12 OI/TTH300-EN Rev. D | TTH300
4.1.2

Sensor drift monitoring

When two sensors are connected, sensor drift monitoring can
be activated via the EDD or DTM.
The sensor drift monitoring can be activated for the following
sensor types:
— 2 x resistance thermometer (RTD), two-wire circuit
— 2 x resistance thermometer (RTD), three-wire circuit
— 2 x resistor (potentiometer), two-wire circuit
— 2 x resistor (potentiometer), three-wire circuit
— 2 x thermocouple
— 2 x voltage
— 1 x resistance thermometer (RTD), two-wire circuit, and
1 x thermocouple
— 1 x resistance thermometer (RTD), three-wire circuit, and
1 x thermocouple
— 1 x resistance thermometer (RTD), four-wire circuit, and
1 x thermocouple
To activate sensor drift monitoring, the transmitter must first
be configured for the sensor types referred to above.
Following this, the maximum permissible sensor deviation
must be configured, e.g., 1 K.
Since sensor response times may differ slightly, it is then
necessary to configure a limit time period during which the
sensor deviation has to constantly exceed the maximum set.
If the transmitter records a larger sensor deviation during the
defined time period, a HART, EDD, and DTM diagnostic
notification - "Maintenance required" - is generated according
to NE 107. At the same time, diagnostic information is shown
on the LCD display.
If drift monitoring is used for the same types of sensor (2 x
Pt100 or 2 x thermocouple), the mean value calculated from
the two sensors is mapped to the transmitter's output signal
as a process variable in redundancy mode.
If a thermocouple is used for Pt100 drift monitoring, the Pt100
sensor (see chapter "Electrical connections" on page 11) must
be connected to channel 1 and the thermocouple to
channel 2.
The measured value from channel 1 (Pt100) is mapped to the
transmitter output as a process variable.
NOTICE
Before configuring the maximum permissible sensor
deviation for drift monitoring, sensor adjustment with respect
to the sensor channel 1 value must be carried out with the
help of the TTH300 DTM.