An Absolute Charge Sensor for Fluid Electrification Measurements

A new instrument, the Absolute Charge Sensor (ACS), has been developed that measures the net charge density entrdned in a fluid flow, independent of the fluid’s electrical properties, velocity, and any electrification processes within the instrument. Through a shielded sampling probe, the ACS periodically transfers a small volume of charged fluid to an electrically isolated Faraday cage with no fluid exiting. The charge density is calculated from the known filling flow rate and either the current or voltage, respectively measured by low or high impedance electrometers connected to the electrically isolated Faraday cage. The ACS overcomes the ambiguity in current or voltage measurements of probes placed into the fluid flow where it is impossible to separate contributions from impacting charge in the flow from charge separated at the probe interface by the very charge generation and transport processes that are under study. The instrument is typically operated to measure charge densities as low as a few pC/m3 using a sampling flow rate of 0.18 cm3 with a sampling period on the order of 1 min. A Couette charger apparatus that simulates flow electrification processes in transformers and fuel pumps has been used to test the ACS using transformer oil or gasoline. Results show good correlation between the ACS and continuous flow measurements. In preparation for field tests of electrification charging of transformer components, laboratory tests were also performed in a pump driven flow loop. ACS measurements of charge density were made at the inlet and outlet of an oil drum electrically grounded through an electrometer. Charge density measurements in flows having Reynolds numbers as high as 3000 were in good agreement with the charge conservation law, i.e. the difference in convection currents into and out of the grounded oil drum equal the measured current through the electrometer. 0018-9367/01/0400-181$1.00