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Re: Metal halide article
Mark Fisher asked about comments on the article on metal halide bulbs
and ballasts in Aquarium Frontiers.
I'm not an expert on metal halide lighting, but my initial comments are
as follows:
1. The author assumes that the power consumed is the product of the
voltage and current. This may not be the case with reactive loads where
the power factor must be considered. He should have used a wattmeter.
You pay for true power which takes the power factor (power factor is the
cosine of the phase angle between the voltage and current) into account,
rather than the apparent power. Whether or not this caused a
significant error in this case is unknown to me.
2. He concludes that the electronic ballast/bulb combination is better
because it runs cooler and consumes less electricity. Seems to me that
it should since it's driving the bulb to a significantly lower PAR
level.
3. The VA consumed by the electronic ballast and bulb (192 VA) is less
than the 400 watt rating of the bulb. This should have been a red flag
for him. This may be due to underdriving the bulb (as evidenced by the
lower PAR), or possibly because his current meter was unable to
accurately measure the current consumption of the ballast. He should
have used a true rms meter with sufficiently high freqency response to
ensure accurate measurements. Most low cost meters are not of the true
rms variety and are accurate only for 60 Hz sinusoidal measurements.
4. He adds the temperatures of the bulbs and ballasts to arrive at a
measure of heat generated. I think this is misleading, especially if
the ballasts were not the same size. For equivalent PAR outputs I
don't see how an electronic ballast can make the bulb any cooler.
FWIW, I hear that the Digital Oceans ballasts are no longer for sale due
to poor reliability. Ice cap ballasts are supposed to be better, but I
think Frank Reiter and others may not agree.
Don