Endress+Hauser Deltabar FMD72 - Function and System Design; Measuring Principle - Electronic Differential Pressure Measurement; Measuring System

Deltabar FMD71, FMD72
Endress+Hauser 7
Function and system design
Measuring principle -
electronic differential
pressure measurement
Devices with ceramic process isolating diaphragm (Ceraphire
®
)
The ceramic sensor is an oil-free sensor, i.e. the process pressure acts directly on the robust ceramic
process isolating diaphragm and causes it to deflect. A pressure-dependent change in capacitance is
measured at the electrodes of the ceramic substrate and the process isolating diaphragm. The
measuring range is determined by the thickness of the ceramic process isolating diaphragm.
Advantages:
• Guaranteed overload resistance up to 40 times the nominal pressure
• The ultrapure 99.9 % ceramic (Ceraphire
®
, see also "www.endress.com/ceraphire") ensures:
– Extremely high chemical durability
– Less relaxation
– High mechanical durability
• Can be used in absolute vacuums up to 150 °C (302 °F)
• Small measuring ranges
1 2 3 4
p
A0020465
1 Air pressure (relative pressure sensors)
2 Ceramic substrate
3 Electrodes
4 Ceramic process isolating diaphragm
Devices with metallic process isolating diaphragm
The process pressure deflects the metal process isolating diaphragm of the sensor and a fill fluid
transfers the pressure to a Wheatstone bridge (semiconductor technology). The pressure-dependent
change in the bridge output voltage is measured and evaluated.
Advantages:
• Can be used for process pressures up to 40 bar (600 psi)
• Fully welded process isolating diaphragm
• Small, flush-mounted process connections
• Significantly reduced thermal effect e.g. compared to diaphragm seal systems with capillaries
p
1
2
3
4
A0016448
1 Silicon measuring element, substrate
2 Wheatstone bridge
3 Channel with fill fluid
4 Metal process isolating diaphragm
Measuring system
The FMD71/FMD72 consists of 2 sensor modules and one transmitter. One sensor module measures
the hydrostatic pressure (high pressure) and the other one the head pressure (low pressure). The
level (electronic differential pressure) is calculated in the transmitter using these two digital values.