KROHNE TT 51 SERIES EN - Manual (Page 6)

3
DESCRIPTION OF THE SUBSYSTEM
6
TT 51 SERIES
www.krohne.com 09/2010 - 4000869801 - AD TT 51 SIL R01 en
Description of the subsystem
3.1 Functional principle
The TT 51 series supports up to two sensor channels with general input circuits that may be
configured for RTD and/or thermocouple temperature sensors.
All safety related calculations are based on these connections.
Functional principle of the TT 51 series is based on the analog to digital and back to analog signal
conditioning. The temperature sensors used are either Resistance Temperature Device(s) (RTD)
or thermocouple(s) (T/C). The RTD has a temperature dependent, non-linear, variable resistance
while the T/C generates a low level, highly non-linear, EMF (voltage) that depends on the
temperature difference between opposite ends of the T/C wire pair. Hence the connection end of
the T/C (cold junction) constitutes a temperature reference or base value that has to be
measured in order to determine the temperature at the critical spot (hot junction). This action is
referred to as cold junction compensation (CJC). One or two sensors of the same or different
types may be connected.
The low level analogue signal from temperature sensors is amplified and filtered before
converting it to a digital signal. The digital signal is less prone to electromagnetic interference.
Digital signal processing like sensor linearization, calculation, temperature drift compensation
etc. is controlled by processors, isolated and converted back to analogue 4...20 mA output signal.
The TT 51 are smart temperature transmitter which improves predicting problems within the
industrial safety instrumented systems – SIS, reducing the manual testing.
The TT 51 is a modular and configurable system with the ability to pre-configure inputs for
measuring sensor(s) and outputs to fault conditions. Configuration of the transmitter is
protected by password.
.book Page 6 Thursday, September 9, 2010 4:19 PM