Thermocouple when senses gives "mv" readings to the
transmitter. RTD gives "ohm" readings to the transmitter.
For checking put the multimeter in resistance mode and if
the the sensor is throcouple the meter will indicate
almost"0" resitance whereas the RTD would indicate
around "119 ohms" in normal temperature like 30deg.C
Thermocouple is basically couple of two dissimilar matel
wire which generate certain emf on puting the couple end in
higher temperature source.As emf is generated then it can
measure in terms of mV.Phisically we can check the T/c by
checking it two wires, they will be of different
RTD is resistance temperature detector works on the basis
of resistance changes when temperature changes. Physically
they can be identified by wire exposed to make
connection.In normally everyone uses three wire RTD, in
which 2 wires are of same insulation colour and resistance
between them will be nearly 2-3 Ohms. The resistance b/w
third wire and any of those wire will be 119.5 Ohms at room
temperature.RTD never generate emf hence we can't measure
any mV b/w any of wire of RTD.
rtd can be checked by measuring the resistance accross the
terminals. At room temp rtd has got a resistance around
110ohm when neasured across its terminal.
t/c can be checked by measuring mv at the terminals
connected. the corresponding mv is corelated with mv v/s
temperature chart to find the temp.
more answers are here. Answer No 7 is currect.
but one thing is missed about the color coding of thermo
couple. mostly in industrial section we are using K, E, J,
T , types we can understand the these types from the color
code of TC. you will get further details on
the output of thermocouple is mV so check the mV in diff
temperatures and the output of RTD is resistance so measure
the resistance at normal temp it should be greater or equal
to that ohm specific of RTD type .For Example if it is PT100
so it will shows minimum 100 ohm if RTD is in working condition.
Diff bet T/C and RTD
both are used to measure temperature but RTD is more
accurate than T/C
RTD can be used upto 800 deg but T/C can be used upto 1750 deg
thermocouple is active devise and works on seeback effect
prociplal which is manufature by using two dissmillar metal
because of this diffrence in property of diffternt alloy
when join togeather at one end it will give milli volts
when u measure across other end of the terminal and it can
use for the measuring higher range temperatue till max 1300
RTD is passive devise and positive temperature coefficent
its resistance increse as temperature increses and it made
up of platinum and works on wheatstone bridge principal and
its temprature measuring range is from -270 to max 700 and
acuracy is good and it will act variable arm of wheat stone
We do have a problem in the company that is one PT connected to PlC which is mean 1 out of 1 in PLC system we do have the PAHH is 55 mbar and PALL 8 mbar with a timer of 15 sec for the PAHH the problem started when PAHH goes more than the set point which is 55 it reached up to 71 but due to timer doesn't trip . We force it and we checked everything nothing there . We replaced the transmitter problem still there the reading goes more than the 55 bar the range for transmitte is 0- 60 mbar . So we did loop check , aculaizing valve , wiring all are okay . What the possible causes for this ???