MATRIZ DE HALLAZGOS

A JCI 131 electrostatic field mill (FM) was installed at the RUAO in October 2004 (although reliable continuous records did not begin until April 2005). The instrument is commercially available (John Chubb Instrumentation, their website detailing the FM is http://www.jci.co.uk/jci131.html). Field mills have been successfully used for continuous recording of PG at other sites (e.g. Isrealsson and Tammet (2001), Harrison (2006a), De et al., (2006)). The basic design of a FM is a rotating metal electrode beneath an earthed case with electrode-shaped slots at the top, allowing the rotating electrode beneath to be alternately exposed and shielded from the atmospheric PG. This action induces a charge on the electrode, measured as a displacement current with magnitude proportional to the PG above the aperture. Most field mills require the electrode to be earthed at a certain point in their cycle, which is usually done with a metal brush permitted to make and break electrical contact with the electrode (Chubb, 1990). However, the JCI 131 FM is designed to use capacitive coupling between the electrode current and electrometer, thereby removing the requirement of any contact brush (which inevitably degrades with time) and thereby increasing the durability of the instrument. The rotation rate is determined by an optical sensor, so no contact is required for this either. A basic schematic of this FM type can be seen in Figure 4.6.

In addition to the brushless design, the FM is unaffected by mains frequency (50/60Hz) electric fields (useful for urban measurement sites such as the RUAO). There is a large gap between the rotating electrode and outer case to avoid bridging by water droplets,

and drainage holes near the base, making this FM suitable for all weather conditions (see JCI 131 user manual at http://www.jci.co.uk/jci131.html for further details).

Figure 4.6 Basic schematic of the constant capacitance-type field mill (from Chubb, 1990).

Before installation on the RUAO the field mill was tested in the laboratory to ensure the output was linearly proportional to an applied PG (~100-800Vm-1). Additionally, the power supply voltage was changed to determine any voltage effect on FM output. The experimental setup was to clamp the FM vertically and position a horizontal metal plate approximately 3cm above the sensing aperture. The rectangular plate was insulated from the surroundings and connected to the positive terminal of a power supply, thereby allowing a known positive voltage to be applied to the plate, with respect to ground (common to the voltmeter and FM). The plate dimensions were such that a uniform vertical PG would exist between the sensing aperture and plate (i.e. the plate was larger than the sensor aperture, being approximately 25cm by 35cm). The FM was powered by another DC power supply which allowed a variable output voltage. The results of this test are shown in Figure 4.7, for power supplies of 10, 12 and 18V. As illustrated, the FM output was linear with respect to applied PG and appeared unaffected by the input power supply voltage. Further tests revealed a stable FM output with supply voltages down to 7V, well below the minimum expected at the RUAO.

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 0 5 10 15 20 25 30

Applied voltage on plate approx 3cm from FM (V)

F M o u tp u t (V ) 10V 12V 18V Supply

Figure 4.7 Laboratory test of FM to investigate effect of power supply voltage on FM output.

The FM has an auto range capability, allowing automatic switching of sensitivity range settings between 2, 20, 200 and 2000kVm-1 for a full scale output of 2V, with a quoted accuracy of within 1% of operating range sensitivity. Due to the possibility of confusion caused by the auto range setting being activated (monitoring of the actual range set would require a further two research channels, which were not available for this purpose) the range was fixed. This was achieved by supplying 5V to the required range-fixing terminal using power from the RUAO supply (modified to 5V) via a break-out box fixed to the bottom of the support pole. This small box can be seen in part (a) of Figure 4.8 attached to the support pole approximately 30cm from the ground.

Once the FM was installed at the RUAO at the top of a pole, with aperture pointing vertically upward, the instrument required in situ calibration due to the inevitable distortion of the ambient PG by the instrument and support pole. This distortion will be large (greater than a factor of 2) as can be seen in Figure 4.8b. For this model, both the ground and equipment were assumed to be perfect conductors, and the air a perfect insulator. Although a theoretical equation based on the electric field around an earthed vertical pole is suggested for calibration by the FM user manual, the PW was used as an absolute method of calibration, as the wire was suspended 1m in the air, away from the support poles. The closest end of the PW was situated approximately 2m from the FM pole, and measurement of raw FM output voltage was compared to PG measured by the PW 1m

above the surface (see section 3.2), allowing the FM aperture at 2m elevation to be calibrated to give PG at 1m. From this comparison, the calibration equation was determined to be

1m

387

12

F

=

⋅FM

+

where F1m is the PG at 1m in Vm-1, FMraw is the output of the FM at 2m in V.

Figure 4.8 The FM and support pole installed at the RUAO in part (a) and (b) the modelled distortion of potential and therefore PG around the earthed FM and pole itself. The black lines are lines of equipotential (starting with 20V near the surface and increasing in 20V intervals with height), with the filled colour representing the PG. The ambient PG was 100V/m (i.e. green). It can be seen that the presence of the 2m structure causes an increase in PG of more than a factor of two at the top of the FM, where the PG is sensed. The figure (b) was generated using Maxwell SV electrostatic modelling software, assuming the air has zero conductivity, with the surface and all objects being perfect conductors.

To test for an instrument for temperature dependence, the FM was screened with an earthed metal cover over the sensing aperture so that no electric field would be experienced. The instrument was left like this for five days to measure output voltage at different air temperatures. The results of this experiment are shown in Figure 4.9. From these results it can be seen that a small negative response to increasing temperature is apparent, at -6.2x10-4VK-1. Using equation (4.3) this is equivalent to a PG drift of -0.24Vm-

1

K-1. Assuming a large diurnal temperature range of 30K, this would therefore mean a corresponding variation of 7.2Vm-1, which considering the usual mean fair-weather PG value of approximately 100Vm-1, represents a “worst-case” temperature-induced diurnal PG variability of 7% of the mean. A low mean output voltage (0.009V) during this experiment confirms a negligible electric field experienced by the FM as it was covered by the metal plate.

y = -6.20E-04x + 1.10E-02 R2 _{= 5.18E-01} -0.004 -0.002 0.000 0.002 0.004 0.006 0.008 0.010 0.012 0.014 0.016 -6 -4 -2 0 2 4 6 8 10 12 Air temperature (°C) F M r a w o u tp u t v o lt a g e ( V )

Figure 4.9 Raw output voltage of screened FM versus air temperature as five minute means between 2-6 February, 2007, with linear trend line.

A scatter plot of five minute average field mill-derived PG (using equation (4.3)) and the PG measured using the PW, taken over 16 days, is shown in Figure 4.10. Unlike the FM that is suitable for all-weather recording, the PW is only functional during dry, turbulent conditions when the insulation is not degraded by moisture and there is sufficient air movement adjacent to the wire to allow a continuous supply of ions (Harrison, 2004a) This is typically for a few hours either side of midday, but can be significantly longer during the summer. When both instruments were working it can be seen that a linear relationship exists, in accordance with the derived linear calibration equation (4.3).

0 50 100 150 0 50 100 150 200 Field Mill PG (Vm-1) P a s s iv e W ir e P G ( V m -1 )

Figure 4.10 Comparison between PG recorded by the PW at 1m above the ground and the FM at 2m during fair-weather (from Bennett and Harrison, 2006a), using the manufacturer’s suggested calibration. The line of best fit was then used to calculate the actual FM calibration equation (4.3).

The range was set to allow a PG range of +/- 1947Vm-1 corresponding to a raw output voltage of +/- 5V (RUAO logging range) once the calibration equation (4.3) is applied. This setting, when combined with the calibration equation (4.3) gives a manufacturer’s quoted accuracy of within 7.7Vm-1 (corresponding to 1% of uncalibrated range sensitivity). This quoted accuracy is expected to be a considerable overestimate for routine measurement, based on results such as Figure 4.7 and the observed variability of the FM under both laboratory and operational conditions. This PG range was found to be adequate for routine fair-weather observation, whilst keeping a high resolution of 0.4Vm-1 for the RUAO logging system voltage resolution of ~2.4mV (10V range at 12-bit resolution).