SMAR TT400WH, Manual

Page: 23 / 46

Operation
2.3
The relation between the measuring junction temperature and the generated millivoltage is tabulated in
thermocouple calibration tables for standardized thermocouple types, the reference temperature being 0
o C.
Standardized thermocouples which are commercially used, whose tables are stored in the memory of
the TT400 Wireless HART TM , are the following:

NBS (B, E, J, K, N, R, S, T)

DIN (L, U)
THERMORESISTANCES (RTDs)
Resistance Temperature Detectors, most known as RTDs, are based on the principle that the resistance
of a metal increases as its temperature increases.
Standardized RTDs, whose tables are stored in the memory of the TT400 Wireless HART TM , are the
following:

JIS [1604-81] (Pt50 & Pt100)

IEC, DIN, JIS [1604-89] (Pt50, Pt100, Pt500, Pt1000)

GE (Cu 10)

DIN (Ni 120)
For a correct measurement of RTD temperature, it is necessary to eliminate the effect of the resistance
of the wires connecting the sensor to the measuring circuit. In some industrial applications, these wires
may be hundreds of meters long. This is particularly important at locations where the ambient
temperature changes a lot.
A 2-wire connection may cause measuring errors. It will depend on the length of connections wires and
on the temperature to which they are exposed (see Figure 2.2).
In a 2-wire connection, the voltage V2 is proportional to the RTD resistance plus the resistance of the
wires.
V2 = [RTD + 2
x
R]
x
I
Figure 2.2 – Two-Wire Connection
To avoid the resistance effect of the connection wires, it is recommended to use a 3-wire connection
(see Figure 2.3) or a 4-wire connection (see Figure 2.4).
In a 3-wire connection, terminal 3 is a high impedance input. Thus, no current flows through that wire
and no voltage drop is caused. The voltage V2-V1 is independent of the wire resistances since they will
be canceled out and is directly proportional to the RTD resistance alone.