Running a mercury arc lamp from a TIG welder (10/06/11)

I have four ancient mercury arc lamps, found in a skip ages ago. They're 500W lamps, made by Ushio, part number USH-500D (http://www.ushio.eu/superhighpressuremercury.html). They date from 1983-1987 and all have about 800 hours used time on them. Out of curiosity, I thought I'd try running one from the TIG welder, using the HF start on the welder to trigger the lamp. Turns out it works pretty well, although the minimum current on my welder (about 20A) is much higher than the maximum current for this lamp (8A).

The lamp is mounted vertically and is hooked up to the welder. The anode goes to the ground terminal of the welder, and the cathode to the "hot" terminal. The cathode is uppermost. Current is measured with a 10mΩ sense resistor, made from a 0.5m length of 1mm copper wire.

After triggering, the current rises to around 23A in a few seconds and the intensity of the light output grows. I've only run it for a maximum of 10s at a time, because I'm operating it over the rated current. Both electrodes glow bright red hot after operation. The lamp emits copious amounts of UV radiation and is very harmful when uncovered.


Lamp in box

Setup

Running. Meter is indicating about 23A current.

Video of operation:

After running it, I noticed a small crack appearing near one of the electrodes. The other lamps have similar and bigger cracks, so they obviously are very near the end of their life! These lamps are still available for around $400 (http://www.soslightbulbs.com/ushio-500d-ush500d-mercury-lamp.aspx).