CRITERIOS DE EVALUACIÓN Y ESTÁNDARES DE APRENDIZAJE EVALUABLES PRIMER CICLO ESO EVALUABLES PRIMER CICLO ESO

With resistance thermometers, the electrical resistance varies with temperature. The output signal is obtained by passing a constant measurement current through the resistance and measuring the voltage drop across it. This voltage drop is given by Ohm’s law:

Formula 23:

The smallest possible measurement current should be selected (see also Chapter 4.7.3), to avoid heating of the sensor. A measurement current of 1mA can be assumed to have no significant ef- fect. This current produces a voltage drop of 0.1V with a Pt 100 resistance at 0°C. The signal volt- age must now be transmitted via the connecting cable to the indication or evaluation point with as small an error as possible. There are four different connection methods here:

4.4.1

2-wire arrangement

Fig. 27: 2-wire circuit

The connection between the thermometer and the evaluation electronics is made with a 2-core ca- ble. Like any other electrical conductor, this also has a resistance, which is connected in series with the resistance thermometer. As a result, the two resistances add together, so that the system indicates a higher temperature. With longer distances, the cable resistance can amount to several ohms, and may introduce significant errors into the measured value.

Example:

Conductor cross-section: 0.5 mm2,

Conductor material: copper,

Resistivity: 0.17Ω mm2 per m,

Cable length: 100m, conductor length is twice the cable length (loop), e.g. 200m. Formula 24:

With a Pt 100, 6.8ohms corresponds to a temperature increase of 1.7°C. To avoid this error, the lead resistance is compensated electrically: the input circuit of this type of instrument always as- sumes a lead resistance of 10 ohms. When the resistance thermometer is connected, a compen- sating resistance is inserted in one of the measuring leads and the sensor is initially replaced by a 100.00ohms resistor. The compensating resistance is then altered until the instrument indicates 0°C. The compensating resistance combined with the lead resistance is then 10ohms. Because of this comparatively expensive compensating operation, and the fact that the effect of temperature on the measuring lead is not detected, the 2-wire circuit is being used less and less.

V

=

R I⋅

R

0.17 Ω mm

m

2 100 m⋅

0.5 mm

---

=

0.68 Ω

⋅

=

The following table lists the resistance of a 10m long copper connecting cable (outgoing and return leads).

Table 18: Resistance of a 10m long copper connecting cable relative to the conductor cross-section

4.4.2

3-wire arrangement

Fig. 28: 3-wire circuit

To minimize the effects of the lead resistances and their fluctuations with temperature, a 3-wire cir- cuit is normally used instead of the 2-wire circuit described above, whereby an additional lead is run to one terminal of the resistance thermometer. As a result, two measuring circuits are formed, one of which is used as a reference. With the 3-wire circuit, it is possible to compensate for both the value of lead resistance and its variation with temperature. However. an essential requirement is that all three conductors have identical properties, and are exposed to the same temperature. Because these requirements are usually satisfied to an adequate level of accuracy, the 3-wire cir- cuit is the most widely used method today. No adjustable lead compensation is required.

4.4.3

4-wire arrangement

Fig. 29: 4-wire circuit

The 4-wire circuit is the optimal connection method for resistance thermometers. The measure- ment result is not affected by either the lead resistances or their variation with temperature. No lead compensation is required.

The measuring current is fed to the thermometer via the supply leads. The voltage drop at the re- sistance sensor is picked up via the measuring leads. If the input impedance of the connected electronics is many times higher than the lead resistance, the lead resistance can be neglected. The voltage drop determined in this way is thus independent of the properties of the connecting leads.

It should be noted that with both the 3-wire and 4-wire circuits, the circuit is not always run all the way to the sensing element. The connection from the sensor to the terminal head in the fitting, re-

Conductor cross-section (mm2)

0.14 0.22 0.5 0.75 1.5

ferred to as the internal connection, is often run as a 2-wire circuit. As a result, the same problems are encountered with this connection - although to a much lesser degree - as those described for the 2-wire circuit. The total resistance, given by the sum of the resistance of the internal connection and the resistance sensor, is then designated as the thermometer resistance, in accordance with DIN 16 160.

4.4.4

2-wire transmitter

Fig. 30: 2-wire transmitter

2-wire transmitters are used to get round the problems described for the 2-wire arrangement and, at the same time, avoid the need for multicore cables. The transmitter converts the sensor signal to a normalized 4 — 20mA current signal, directly proportional to temperature. The transmitter is also supplied via the two connections, and the quiescent current here is 4 mA. Because of the elevated zero, this arrangement is also referred to as “live” zero. Another advantage of the 2-wire transmitter is that, since the signal is amplified, the circuit is much less sensitive to interference. There are two arrangements for positioning the transmitter. Because the circuit carrying the unamplified signal must be kept as short as possible, to reduce the susceptibility of the signal to interference, the transmitter can be mounted directly in the terminal head of the thermometer. Although this is the optimal solution, it may not always be possible because of design constraints, or the fact that in some cases the transmitter might be difficult to access in the event of a fault. In such cases, a DIN- rail mounting transmitter is used, installed in the control cabinet. However, the advantage of im- proved access here is achieved at a cost, as the circuit carrying the unamplified signal must cover a longer distance.