PT 100 SENSORS - WORKING PRINCIPLE

 

Most of the temperature measurements on board the vessels carried out by using Pt100 sensitive element.

What is really a PT-100 sensor ?

Pt100-Pt (Platinum) are resistive elements which change own impedance depends from applied temperature.

Why Platinum? due to the fact that  is more stable compared with other materials.

What 100 is standing for? It is 100 Ohms resistance at 0 0 C.

If we have on board Pt200 or Pt1000 it means 200 Ohms or 1000 Ohms at 0 0 C .

Wrong temperature indication from sensor on monitoring computer, how to deal with?

1st step : You have to measure Pt100 resistance and compare with Calibration_table.pdf.

For example: Pt100 has measured resistance is 120 Ohms and at the table you find closest value 119.4 Ohms corresponding to 50 0 C.

2nd step: Compare indicated temperature with your finding as per calibration table and design where a failure is.

Pt100 measuring sensors could be with 2,3 and 4 wires connections.

Why is it so?

Two wires connection. This connection easy for technician, but measuring error arising for long distance cable, for example resistance of 2 copper wires with 10 meter distance will be 1.4 Ohms and that error must be added for measuring circuit. In case we have Pt100 own resistance 103.9 Ohms at 10 0 C plus 1.4Ohms error on computer we will observe approx. 13.5 0 C.

 image 1

R1=R2=ρ*l/S, where ρ- resistivity of copper (in our case) copper wire 0.0175 Ohms*mm2/m

                                       l- Length of wire, meter

                                       S- Wire’s cross-section, mm2

In order to eliminate error created by long distance connection 3-wires and 4-wires connections using, where additional wires reduce existing error due to parallel wire connections.                

 4-wires connection

 image 2

What is really measured Pt100 for

2-wires connection is Pt100+(R1+R2)

3-wires connection is Pt100+ (R1-R2)

4-wires connection is Pt100 unaffected by lead resistance.