not be guaranteed
•2020/5/26
•Endress+Hauser China 周晓
•实际的解决方案
•温度的考虑及全绝缘和半绝缘探头
•At high operating temperatures in the vessels the electronic is protected by a temperature spacer.
•电导率
•Conducting media
•Low conducting media •Non conducting media
JLK •A change of the dielectric constant has no influence on the capacitance because the filled part of the vessel wall apparently
电容培训
2020年5月26日星期二
•电容器基础
•Each electrical charged material is surrounded by an electrical field.
•Between different charged materials •electrical field lines from positive to negative are created.
•Two unequal charged metal plates build a capacitor.
•2020/5/26
•Endress+Hauser China 周晓
•电容器的基础
•The principle of the capacitance level measurement is based on the change in capacitance through the change of the level. •The capacitance of a capacitor depends on 3 factors:
••-••--
••-•••--- ••--
•+•••--+
••+•-+•-
•In order to convert the change •capacitance into an electrical •signal the capacitor is connecte •to an AC voltage.
•Coffee
•4,6
•Slaked lime
•5
•Quick lime
•3,3
•Coal lumps
•4,5
•Coal dust
•2,3
•Feed
•2,3 ... 3
•Flour
•4,5
•Nylon granulates •1,8
•Silica powder •2
•Rice
•3,5
•Sand, dry
• •
•Cs =
dielectric constant: oil = 2 chlorine = 1.8
capacity of the probe
•Conductive media
rod at a distance of 250 mm from vessel: 13 pF/m
•Lmin =
DCmin • Cs
•C
•R
•2020/5/26
•C : capacitance [F] • : 2··f [1/s] •f : measuring frequency [Hz] •XC: capacitive blind resistance
•Endress+Hauser China 周晓
•电容器基础
•Normally, cylindrical tanks are used in the field. •Then, the capacitor is formed from the probe and the tank wall.
•D
•Tank wall
•d
•L
er
•Capacitive rod
•2020/5/26
•Endress+Hauser China 周晓
•测量原理
••-- •••--••-- ••--
••-••-- •- ••-- •-
•••--- ••-- •••-- ••--
••-- ••-- ••-- ••-••--
•The measuring current does not only depend on the dielectric constant, but also on the electrical conductivity of the
•The dielectric constant has to be stable. If it changes, e.g. by humidity the level canno be determined reliable anymore.
moves to the probe.
media.
•2020/5/26
•Endress+Hauser China 周晓
•最小探头长度
•For a limit detection (Nivotester) the minimal change of
the capacitance DCmin should be 5 ... 10 pF.
•Endress+Hauser China 周晓
•实际的解决方案
•Rod and rope probe
•Full- or partially insulated probes
•Probes with ground tube
•2020/5/26
•Endress+Hauser China 周晓
•实际的解决方案
= 2,5 cm
•Endress+Hauser China 周晓
•最小探头长度 ( 非导电介质, 如：油 )
•Limit detection in oil:
•With ground tube:
•Lmin
=
DCmin •Cs•(er-1)
=••55
5 pF pF/m•(2-1)
=
9
cm
•e•L•min
•AC voltage source
•Capacitive rod
•Capacitance changes caused by the level result in a change of the measuring current.
•Measured current
•2020/5/26
•Endress+Hauser China 周晓
•With ground tube:
•Lmin
=
DCmin •Cs•(er-1)
=••55
10 pF pF/m•(2-1)
=
18
cm
•e•L•min
•Without ground tube:
•Lmin
=
DCmin •Cs•(er-1)
=••13
10 pF pF/m•(2-1)
=
77
cm
•2020/5/26
•Without ground tube:
•Lmin
=
DCmin •Cs•(er-1)
=••13
5 pF pF/m•(2-1)
=
38,5
cm
•2020/5/26
•Endress+Hauser China 周晓
•最小探头长度 ( 非导电介质, 如：油 )
•Continuous measurement:
•For cer) the minimal
change of the capacitance DCmin should
•be 10 pF (FEC12).
•Non conductive media er =
•Lmin =
DCmin •Cs•(er-1)
•2020/5/26
•Conductive media
•Fully insulated •Partially / fully
probe
•for aggressive med
insulated probe
•Non conductive •Fully insulated •Partially insulated
•2020/5/26
•Endress+Hauser China 周晓
•电容器基础
•Because of the change in polarity at an alternating voltage, electrical losses will be created. These losses are described by an ohmic resistor XC parallel to the capacitance C.
•3 ... 5
•Washing-powder •1,1 ... 1,3
•液体
•
•Alcohol •Petrol •Chloroform •Desmophen 5100 •Desmodur 44 V •Deuterium •Liquid gas •- Chlorine •- Air -140°C •- Butane •- Propane •Glycerin •Fuel oil •Latex •Petroleum •Water
•L•min
•2020/5/26
•Limit detection in acids:
•Lmin =