Basic function principles
Fig.130: EM37xx ModulesThe pressure measuring modules EM3701, EM3702 and EM3712 record direct differential and relative pressures of non-aggressive gases.
As in electronic signal acquisition the pressure measurement takes place via a terminal module. The pressure measuring modules convert the measured pressure into an electrical signal and make it available to the higher-level control system with a resolution of 16 bits.
With the aid of user calibration, the measuring module can be adjusted to the ambient conditions.
A temperature compensation facility is integrated on the chip and increases the stability of the measurement result.
The status LEDs indicate proper function or errors such as range exceedance.
Pressure measuring modules measure the pressure relative to a reference value. A distinction is made between three physical measurement methods:
- Absolute pressure measurement: The pressure is measured relative to an absolute vacuum. Absolute pressure measurement is used in barometers, for example.
- Relative pressure measurement: The pressure is measured relative to a defined reference pressure. The ambient pressure is used as a reference pressure.
The average atmospheric pressure at sea level is 1013.25 hPaabsolute = 0 hParelative (at 0 °C and 45° latitude). The ambient air pressure depends on the altitude, the temperature and the latitude of the measuring site.
Values of lower ambient air pressure are referred to as negative pressure and have a negative sign. Values of greater ambient pressure are referred to as overpressure and have a positive sign.
P(relative) = P(absolute) -P(ambient); see diagram: Diagram showing relative and absolute pressure
The relative pressure measurement is applied e.g. for filling level measurements in aerated tanks. - Differential pressure measurement: The difference between any two pressures is measured (note limit values). Differential pressure measurement is used for monitoring filters, for example.
Measuring method | Reference value | Value range | Example |
|---|---|---|---|
Relative pressure measurement | Ambient air pressure | EM3702: EM3712: | Filling level measurement of aerated tanks, pneumatic systems |
Differential pressure measurement | Pdifference = P1 – P2 ; | -100 hPadifferential ≤ Pdifference ≤ 100 hPadifferential | Monitoring of filters |
Fig.131: Diagram showing relative and absolute pressureEM3701 - single-channel differential pressure measuring module
Fig.132: EM3701 single-channel differential pressure measurement EM3701The EM3701 terminal module can measure pressure differences between 0 and 100 hPa (0 and 100 mbar) between two hose connections.
EM3702 - two-channel relative pressure measuring module (0 hPa to 7500 hPa)
Fig.133: EM3702 two-channel relative pressure measurementThe EM3702 terminal module can measure pressure values between 0 and 7500 hPa (0 and 7.5 bar) at each hose connection. The pressure is measured relative to the ambient air pressure.
EM3712 - Dual channel relative pressure measuring module (-1000 hPa to +1000 hPa)
Fig.134: EM3712 dual channel relative pressure measurementThe EM3712 terminal module can measure pressure values between -1000 and +1000 hPa (-1 and +1 bar) at each hose connection. The pressure is measured relative to the ambient air pressure.
Installation and connection technique
The pressure measuring terminal samples the pressure values directly. Additional measuring instruments are unnecessary. This avoids connection systems and saves available space compared with the use of conventional measuring instruments. The installation of the pressure measuring terminals is simple and fast and can be carried out without any additional mounting tools. The measuring hoses are connected directly to the quick couplings of the pressure measuring terminal. Standard commercial plastic hoses can be used as measuring hoses. In terms of connections and installation space the pressure measuring terminals are designed like a "normal" 24 mm Bus Terminal and can be installed with minimum space requirement directly in the Bus Terminal system.