Measurement of static electricity in insulating liquids
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UDC 621.317.7.082,77:621.315.615
An insulating liquid becomes electrified when pumped, which results in a fire hazard arising from sparks between the liquid and metal vessels [1-3]. A major problem in researching this electrification is to determine the charge density in a known volume; no instruments are currently manufactured for this purpose, because devices of dynamic type [4, 5] are complicated to make and rather unreliable. Also, there are major difficulties in obviating explosion hazards. It is possible to use a Faraday cage for monitoring charge in a pipeline [6], but this involves opening the pipeline for sampling, so the measurements can be made only spasmodically. Space-charge relaxation has also been employed [7, 8]; the relaxation current I is dependent on the space charge q and on the electrical parameters of the liquid: I = q?/~so,
(1)
where eeo/~ = T is the relaxation time constant, s is the relative dielectric constant of the liquid, so is the dielectric constant, and y is the electrical conductivity. The relaxation current may be measured directly on an electrically insulated part of a pipeline, which is connected to ground via a measuring instrument, or else with electrodes immersed in the liquid; in either case, the relaxation time constant is judged from the measurements of the electrical conductivity. The conductivity is usually measured as far as possible under similar conditions. However, the error of measurement is considerable, while the pumping conditions may vary during the measurement, which causes additional error. The relaxation current can be measured more precisely from the voltage drop U across resistor R, which is inversely proportional to the electrical conductivity of the liquid: R = K~/7; then (i) gives U = IR = Klq/8~o.
The relative changes in s produced by external factors are considerably less than those in determining the space charge for a nonpolar liquid, so i/sso can be determined by experiment or taken from tables and assumed constant. Then the measured charge will be proportional to the voltage: q = KU,
(2)
where K = sgo/Ka is a coefficient independent of the electrical conductivity. Transducers have been developed for industrial use in which R is provided by the leakage resistance between electrodes immersed in the uncharged liquid. Figure i shows such a device schematically, which is meant principally for use in vertical parts of pipelines. This is a section of the pipeline insulated from ground and connected to the measuring instrument; it acts as an electrode surrounding the working volume. This section is itself surrounded by an additional electrode hermetically connected to the pipeline at the input and output of the measuring section. This has channels to allow a neutral specimen of the liquid to enter the space between the electrodes. The output impedance of the device is reduced by providing the inner surface of the additional electrode and the other surface of the measuring section with a set of plates, with the plates in one set alternating with those
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