![Milltronics Mercap MCP01 D series Instruction Manual Download Page 26](http://html1.mh-extra.com/html/milltronics/mercap-mcp01-d-series/mercap-mcp01-d-series_instruction-manual_1796996026.webp)
Page 24
MERCAP – INSTRUCTION MANUAL
7ML19981CM01.1
MST9500 Transmitter
Operating Principles
The instrument's MST9500 transmitter measures the capacitance of a sensor/electrode
relative to the reference electrode (often the tank wall) and transforms it to a 4-20 mA signal.
Applications include level measurement, level detection, flow measurement, and flow
detection.
The measurement capacitance is usually obtained by using an insulated probe inserted in the
tank, forming one electrode of the capacitor. The wall of the tank forms the other electrode of
the capacitor.
A stilling well is used when the silo or tank is not conductive, or when the shape of the tank
cannot guarantee linear measurement. The stilling well is a (grounded) metal tube with vent
openings, which fits around the electrode. The stilling well diameter is somewhat larger than
the diameter of the electrode, depending on the application.
A significant advantage of the MST9500 is the Active Shielding feature. It prevents any
capacitance that may occur in the connection cable, process connection, and non-active
parts of the probe from interfering with the measurement. As a result, the capacitance
registered by the MST9500 consists only of the measuring capacitor, and a more stable and
more reliable measurement is provided.
Due to intrinsic safety requirements, the entire MST9500 transmitter is potted in epoxy resin
that also protects the electronics against mechanical vibration and moisture influences. The
maximum measuring range of the MST9500 is 3300 pF (1pF
≅
10
-12
F).
The electrode is connected by means of a mini-coax cable. The screw connection is intended
for grounding the tank or stilling well.
Note
: This ground must be connected to the tank and/or stilling well.
Conventional
Capacitance
Measurement
Measuring-Circuit
C1 =
Cap. connection point
C2 =
Cap. connection cable
C3 =
Cap. Process connection,
includes inactive part
Ca =
Initial capacitance (air)
Cm =
Cap. Increasement (product)
MST9500 with
Active Shield
Measuring- Circuit