Transmitters capture the signals from sensors which measure physical parameters such as temperature, path, angle, pressure or force. They perform galvanic isolation and convert the signals into standardized analog output signals: 0 ... 20 mA, 4 ... 20 mA or 0 ...10 V. These standard signals can be transmitted to indicators, recording devices, and/or standardized controllers such as PLCs and can be used for control applications.
2.1 Temperature Measurement with Resistance Thermometers
Temperature is an important control parameter in many industries, with resistance thermometers and thermocouples being the most important sensors.
Resistance thermometers are highly accurate temperature sensors with long-term stability. They are used to measure temperatures by measuring the temperature-dependent electrical resistance.
Resistance thermometers are primarily used to measure low and moderate temperatures, e.g. in the air conditioning, process and food industries.
Pt100, Pt1000 and Ni100 are the most common resistance thermometers. The first part of the identifier indicates the resistance material, the second part the resistance in ohms at 0 °C. Pt100, for example, indicates a resistance thermometer made with platinum and a resistance of 100 Ω at 0 °C.
The ThermoTrans 205/206, A 20210 and P 32100 as well as the PolyTrans P 32000 transmitters allow for connection of common resistance thermometers using 2-, 3-, or 4-wire configurations
(see Fig. 11).
Figure 11: Connection of resistance thermometers
Thermocouples use the effect named after T. J. Seebeck that a junction of two dissimilar metals produces an electric voltage which is influenced by the difference between the temperature at the contact point (reference junction) and the temperature of the point of measurement (see Fig. 12). When the thermoelectric voltage VT is measured using a suitable transmitter and when the temperature at the reference junction is known, the temperature at the point of measurement can be calculated based on the characteristic curve of the thermocouple. This calculation is normally performed in the temperature transmitter.
Figure 12: Connection of resistance thermometers
The temperature at the reference junction can be measured or set externally. In practice, a measurement is either taken "internally" in which case the reference junction is inside the temperature transmitter, or "externally" in which case the reference junction and the corresponding temperature sensor – e.g. a Pt100 – are outside of the transmitter. The voltage generated by the thermoelectric effect is very low, only a few microvolts per Kelvin. As a result, thermocouples are primarily used in high temperature areas – e.g. to take measurements in furnaces, melts and plastics machines – in order to obtain an interference-free measurement signal.
A large number of metal pairings was tested for use in practical thermocouples. Subsequent standardization led to a limited range of standard thermocouples with set material pairings. These can be used for the majority of all industrial temperature measurements. The characteristics including permissible tolerances of thermocouples are defined internationally in the IEC 584-1 standard and in DIN 43710 which is not valid any more and is to be applied only to old plants.
IEC 584-1, EN 60584-1
Type J Iron/Constantan
Typ T Copper/Constantan
Type K Nickel Chromium/Nickel
Typ E Nickel Chromium/Constantan
Typ N Nicrosil/Nisil
Type S Platinum Rhodium/Platinum
Type R Platinum Rhodium/Platinum
Type B Platinum Rhodium/Platinum
DIN 43710 (not valid any more)
Type L Iron/Constantan
Type U (Cu-CuNi)
High-temperature thermocouples like W3Re/W25Re
W5Re/W26Re (tungsten-5%rhenium/tungsten-26%rhenium) have not been included in the aforementioned standards yet. Their characteristics are specified in ASTM E 988-96.
The Knick ThermoTrans 210/211, P 32100 and PolyTrans P 32000 transmitters can process the signals of all common thermocouples.
2.2 Force Measurements with Strain Gauges
Force, weight, torque, mechanical stress and the resulting parameters can be measured using strain gauges. These make use of the effect that a change in length of a conductor due to strain results in a proportional change in its electrical resistance. In a strain gauge transmitter, this resistance change is detected as a measure for the strain, force, etc. and is then converted to a standard signal for display and further processing. In practical industry applications the filling weight of a container, for example, is measured by load cells (force sensors, force transducers) which usually have integrated strain gauge full bridges. The bridge sensitivity is specified as signal level at the bridge output in millivolts related to the supply voltage in volts, i.e. for example 2 mV/V. When the sensitivity is known, e.g. from the calibration certificate of the sensor manufacturer, it is adjusted at the transmitter. If only the nominal sensitivity is known, the actual sensitivity can be determined by calibration with a defined mechanical load. Correspondingly, the zero point can be adjusted at the transmitter by using a tare function. The strain gauge bridge can be connected in a 4-wire configuration (see Fig. 13). A 6-wire configuration is used if the error caused by the current-carrying supply lines must be reduced. Here, the bridge supply voltage is measured through separate lines, virtually currentless and therefore error-free. In practice, also sensors that are equipped with 6 lines are operated with 4-wire transmitters by combining 2 excitation lines each. Alternatively, an external supply may be used to take advantage of the 6-wire connection (see Fig. 13).
Figure 13: Supply of strain gauges
The Knick SensoTrans DMS A 20220 and P 32200 as well as the PolyTrans P 32000 transmitters have been designed for standard strain gauges using a full-bridge configuration. The sensitivity and zero point of calibrated sensors can be adjusted with rotary switches on the device or by using the Paraly SW 111 communication software. A tare function to set the zero point can be activated by a pushbutton on the device or in the software. Correspondingly, the sensor sensitivity can be calibrated at a defined load “at the push of a button“.
2.3 Path and Angle Measurements with Potentiometers
Among other things, potentiometric sensors are used to detect linear or rotary movements. Potentiometric path or angle sensors contain resistance elements made of conductive plastic material or wire-wound resistance elements with movable sliding contacts. Potentiometer transmitters determine the resistance ratio produced by the sliding contact so that its position can be detected and output as standard signal.
Resistance transducers are used for setpoint specification in rail vehicles,on ships, during industrial production, and in many areas of machine and apparatus engineering. Resistive sensors are used for detecting the actual value of actuators, for measuring distances or thickness, and for various position measurement tasks.
The Knick SensoTrans R A 20230 and P 32300 as well as PolyTrans P 32000 transmitters capture potentiometer signals up to 50 kΩ in 3- or 4-wire configurations. The range of the motion to be mapped to the range of the output signal can easily be adjusted with a button on the device or by using the Paraly SW 111 software.