RTD transmitter

[mado 0]

what is the need for transmitter 4-20 mA for RTd if I have analoge input module?

[b_carlton 61]

The RTD must be driven by only a very low level current to prevent the heating effect of that current itself masking the ambient temperature reading. To get a useable analog signal even over the full range the resultant voltage drop must be amplified by a great extent. By the time you have arranged for a low level stable driving current and low level amplification you have nearly duplicated the hardware of a transmitter. Google 'RTD circuits' and view the various methods to develop a useable higher level linear signal.

[mado 0]

I didn't understand any of these words , tell me what is the relation of this to my question?

[b_carlton 61]

I was trying to explain that, using the RTD by itself to modify the current or voltage to an analog input card will produce a very unusable signal. It would only vary by a slight amount through its entire range, be very non-linear and inaccurate. I was describing the circuits needed to overcome these problems. A transmitter is necessary for any accuracy.

[mado 0]

thank you , now I got it

[ger1 1]

1. Resistance Temterature Detector (RTD) – Pt100 An RTD is a device used from sensing the temperature of a process. These devices work on the principle that the electrical resistance of metals or semiconductors changes with temperature. The most commonly used metals are platinum, nickel, or nickel alloy. The resistance changes in a linear manner with temperature, although the actual change in resistance per degree is small. For example, a Pt100 has a resistance of 100? at 0°C and 138.4 ? at 100°C . This implies a 1°C increase in process temp will cause the resistance of the RTD to increase by 0.384?. There are also PT1000 sensors that have a resistance of 1000 ? at 0 °C. The RTD is then connected to the 4-20 transmitter where is converted to a 4-20mA current signal. 2. 4-20mA Transmitter Signals from plant sensors are many and varied, ranging from a few millivolts to a hundred volts. Signals can be DC volts, AC volts or a resistance, such as that from a Pt100. The 4-20mA transmitter is a standard way of transmitting information between sensors and the measurement and control equipment. Figure 1: 4-20mA current loop diagram. Source: http://www.iqinstruments.com/iqshop/technical/signals.html As can be seen from fig.1, the transmitter is a current sinking circuit, which means that it will draw a current from an external power supply. This is usually a 4-20mA signal powered from 24V DC supply which is often an incorporated into the measuring instrument to which the transmitter is connected. The amount of current flowing through the loop is determined by the resistance of the Pt100. At 0°C, the resistance of the Pt100 will be 100?, this will allow 4mA to flow in the loop. Similarly, at 100°C, the resistance of the Pt100 will be 138.4?, this will allow 20mA to flow in the loop. The 4-20mA signal is fed to a the Analogue to Digital converter where it is converted into it’s digital equivalent, which can be processed by the PLC program. The 4mA ‘zero’ signal has a dual purpose. The 4mA offset provides the current that the transmitter/transducer needs to operate. It also protects against transducer or cable damage. If the transducer fails, or the signal cable is open or short circuited, the 4mA current will be lost and can be used to give a ‘transducer fail’ alarm. A current signal is used as this is less affected by noise than a signal represented by a voltage and a current signal can travel over long distances.