Re: Aquatic Plants Digest V2 #57

>Andrew replied:

>Last night I fiddled around a bit and discovered that if I place the probe
>near the ballasts there is little or no interference. I presume that it is
>the current running through the tubes them selves, therefore this would
>continue to happen even if I used magnetic ballasts. My current thoughts on
>rectifying the problem is to section the probe of under the hood from the
>lights by using a wall made from plumbers lead. Does this sound feasible? Is
>it necassary to earth the lead? Will the magnetic field pass through the
>glass lids and into the water, therefore going around the lead barrier? Is
>there some way of making something along the lines of an earth leakage
>circuit to drainthe magnetic field? Sorry for all the questions. I will try
>your current suggestions when I get home tonight. I relly dont want to have
>to spend money on buying a sump just to accomadate the Ph probe.

The ballast probably has a steel cover that limits radiated EMI
(electro-magnetiic inteference) however it has these great antennas
attached to it (bulbs and wires).

Don't use lead, you don't need the toxicity, and it ain't that great as a
magnetic shield. If the inteference is primarly electrostatic, any grounded
metal shield will work, just try a little aluminum foil for a quick test,
however I suspect the inteference has a magnetic component, and if this is
true, you  would get better results with a "ferrous" shield (to bad your
Vegamite does't come in cans (or tins?)). By "ferrous", I mean iron or
metals like iron which have magnetic propeties, mu-metal for example. You
can't really drain a magnetic field, but you can "short it out" or contain
it with a "ferrous" shield (the magnetic field is induced in the shield,
not the circuitry).

And yes, the magnetic field will pass through glass and water. Try putting
a can over the sensor. Also try shortening the distance from the sensor to
your sensor's electronics.

A varnish, copper and iron magnetic ballast will drive the lights at the
frequency of your line voltage, but the plasma discharge in the lights can
still generate high frequency noise. I don't know if the problem will
diminish, I'd try to make the electronic balast work.