Central Pneumatic 43760 Specifications download pdf (Page 18)

TT383 – Operation and Maintenance Instruction Manual

2.2

Power Supply

Takes power of the loop-line to power the transmitter circuitry.

Power Isolation

Just like the signals to and from the input section, the power to the input section must be isolated.

Isolation is achieved by converting the DC supply into a high frequency AC supply and galvanically

separating it using a transformer.

Temperature Sensors

Some basic concepts about temperature sensors are presented below.

Thermocouples

Thermocouples are constructed with two wires made from different metals or alloys joined at one

end, called measuring junction or "hot junction". The measuring junction should be placed at the

point of measurement. The other end of the thermocouple is open and connected to the temperature

transmitter. This point is called reference junction or cold junction.

For most applications, the Seebeck effect is sufficient to explain thermocouple behavior as

following:

How the Thermocouple Works (Seebeck Effect)

When there is a temperature difference along a metal wire, a small electric potential, unique to every

alloy, will occur. This phenomenon is called Seebeck effect. When two wires of dissimilar metals are

joined at one end, and left open at the other, a temperature difference between the two ends will

result in a voltage since the potentials generated by the dissimilar materials are different and do not

cancel each other out. Now, two important things must be noted. First: the voltage generated by the

thermocouple is proportional to the difference between the measuring-junction and the cold junction

temperatures.

Therefore the temperature at the reference junction must be added to the temperature derived from

the thermocouple output, in order to find the temperature measured. This is called cold junction

compensation, and is done automatically by the TT383, which has a temperature sensor at the

sensor terminals for this purpose. Secondly, if the thermocouple wires are not used, all the way to

the terminals of the transmitter (e.g., copper wire is used from sensor-head or marshaling box) will

form new junctions with additional Seebeck effects. It will be created and ruin the measurement in

most cases, since the cold-junction compensation will be done at the wrong point.

NOTE

Use thermocouple wires or appropriate extension wires all the way from sensor to transmitter.

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 °C.

Standardized thermocouples that are commercially used, whose tables are stored in the memory of

the TT383, are the following:

 NBS (B, E, J, K, N, R, S & T)

 DIN (L & U)

Resistive Temperature Detectors (RTDs)

Resistance Temperature Detectors, most commonly known as RTD’s, are based on the principle

that the resistance of metal increases as its temperature increases.

Standardized RTDs, whose tables are stored in the memory of the TT383, are the following:

 JIS [1604-81] (Pt50 & Pt100)

 IEC, DIN, JIS [1604-89] (Pt50, Pt100 & Pt500)

 GE (Cu10)

 DIN (Ni120)

For 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 constantly.

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