8
TRM138
3.2 Block Diagram Elements
3.2.1 Input Sensing Devices
3.2.1.1
Input sensing devices (transducers) convert physical properties of the facility to
electrical signals transmitted to the instrument for their further processing. Data on sensing
devices used with instruments of various modifications are presented in Table 2.
3.2.1.2
Resistive temperature transducers (RTT) are used to monitor ambient temperature at
the transducer’s location. RTT’s principle of operation is based on the conductor’s ability to change
electrical resistance when the ambient temperature is changed.
The sensing transducer shall be connected to the instrument by a threewire circuit (see
Appendix
2, Figure A2.7,
a)
.
If the RTT connection by a twowire circuit is required,
Appendix
4
shall be followed.
3.2.1.3
Thermoelectric temperature transducers (thermocouples) (TC) are also used to
monitor temperature. A TC represents a thermoelectric circuit arranged by two heterogeneous
metallic conductors with two junctions.
Junction point of heterogeneous conductors is termed
as a
“working”
junction of thermocouple
and their ends are
tails
or
“cold
” junctions. The “working” junction of the thermocouple is placed
in a temperature monitoring location while the tails are connected to the instrument’s input pins
(see
Appendix
2, Figure A.2.7,
b)
.
ATTENTION!
Only thermocouples with insulated and unearthed working junctions can
beused with the instrument since negative sides of their tails are merged at the TRM138 entry
point.
3.2.1.4
Active converters with analog output signal are used to monitor various physical
properties (pressure, temperature, flow rate, level, relative humidity, etc.). The exit signal of such
sensing transducers is a direct current voltage varying on the linear law or a current.
The connection diagram of the sensing transducer with a current terminal is presented in
Appendix
2, Figure A.2.7,
c
.
ATTENTION!
“Minus” terminals of active sensing transducers in the instrument are merged.
3.2.1.5
An instrument of any modification can work with various types of sensing transducers
from those presented in Table 2 for the specified modification. When connected to the instrument,
the sensing transducers are assigned with serial numbers of the entries to which they are connected
(entry 1 corresponds to sensing transducer d1, entry 2 – sensing transducer d2, and so on). Type
of each sensing transducer is set by the user in parameter
int (PL1)
when the instrument is
prepared for operation (see Section 6).
Note.
When the instrument’s programmed parameter is referred to, programming level number of this para
meter is indicated in brackets. A full list of programmed parameters is presented in
Appendix 3
.
3.2.2 Input Parameters Measuring
3.2.2.1
The instrument measures the facility’s input parameters (temperature, pressure, etc.)
by consecutive interrogation of operational sensing transducers and conversion of signals received
from them into digital values. When being processed the signals are filtered from noise and corrected.
3.2.2.2 Interrogation of Sensing Transducers
3.2.2.2.1 Sensing transducers are interrogated in a closed cycle by means of
AC1
automatic
commutation device controlled by the microprocessor under the program set by the user. This
program includes the list of serial numbers of all operational sensing transducers (interrogation
list) as well as interrogation priority for each of them.
3.2.2.2.2 Any sensing transducer included in the interrogation list is switched on automatically
after its NSC type is set in
int (PL1)
parameter. If
int (PL1)
parameter is set to
oFF
(“switched
off”) the sensing transducer is excluded from the interrogation list.
The queue and frequency of interrogation of each sensing transducer are defined by
the interrogation priority which is set as a dimensionless numerical value (from 1 to 8)
in
Prt (PL1)
parameter individually for each sensing transducer. The maximum numerical
value corresponds to the highest interrogation priority.
Instrument Design and Operation
Section 3