Testing MH bulbs w/ ohmmeter?

[danielevan]

Hey all --

My MH system doesn't fire up; I suspect that the ballast is killing bulbs.

I wanted to test the bulbs; since I don't have another ballast handy, I'm wondering if there's another test route.

Apparently, incandescent bulbs can be tested with a garden-variety ohmmeter; they'll offer a bit of resistance.

Can MH bulbs be tested similarly? I bought a cheap multimeter at radio shack (http://www.radioshack.com/product.asp?catalog%5Fname=CTLG&category%5Fname=CTLG%5F011%5F008%5F002%5F000&product%5Fid=22%2D218), but it doesn't measure any conductivity across any MH bulb I measured -- including some new ones at the LFS.

The meter's specs say "resistance: 100 KOhm"; does one need to put more voltage across the bulb than this meter offers in order to get a reading?

[Chris@IceCap]

MH bulbs cannot be tested with an Ohm meter! Incandescent and fluorescent bulbs have a filament that can be measured, when the filament breaks these lamps no longer light. MH bulbs have two electrodes that an electrical arc flows through. Mh bulbs tend to turn on and off as they begin to fail. Typically MH ballasts do not harm lamps if they fail.

Chris
IceCap Inc.

[PaulErik]

You can not measure the resistance to see if a MH lamp works. A MH lamp is an arc light source. It has two electrodes up to a few inches a part. One way is to use a high voltage low current source and apply it to the contacts. This allows the arc to strike between the main electrodes.

The best way to test a MH lamp is to test it on a known working ballast.

Hope that helps

[danielevan]

Good to know -- thanks.

Chris, my setup is en route via UPS to you folks (150W electronic ballast, pendant, bulb). Maybe let me know what you discover?

Thanks!

[Chris@IceCap]

I'll have our service dept. notify me when it arrives.

[danielevan]

My bulbed stopped firing one day (6 months after the whole setup was new), and I got another bulb.

After changing out the bulb, the second bulb made a "pop" noise when I fired up the ballast, and also wouldn't light. I don't know if the second bulb got fried. Nor do I know if the first bulb was fried...

Thread in the icecap forum: http://archive.reefcentral.com/forums/showthread.php?s=&threadid=640135

[The Grim Reefer]

How would you go about checking the ballast?

[PaulErik]

Depends on the ballast. If the ballast has an ignitor or is electronic you should test it with a working lamp. A standard multi meter can’t handle the starting pulse voltage.

If it’s a magnetic probe start you can use a multi meter and measure the open circuit voltage or test it with a standard mercury vapor lamp of the same wattage.

[Chris@IceCap]

Grim,

We have a ballast analyzer here at IceCap. We can measure lamp voltage and current to see if there is an internal ballast failure. Or, we could just hook it up to one of our 150W setups.

Chris

[The Grim Reefer]

Couldn't be easy could it! I have a old Beckman 310T that is pretty hearty but I wold rather not turn it into a pile of molten plastic.

I guess I am back to my original idea of hooking the wires to the neighbor's cat to do a screetch test:D