On a visit to a printing plant in Tainan, the owner pointed at two lamp tubes that looked much alike and asked me: "These two look about the same — how come one's for disinfection and the other for baking ink?" They may look alike, but their "insides" — that is, the pressure of the mercury vapor inside the tube — differ by several orders of magnitude, and the output spectra diverge just as sharply.
Low-Pressure Mercury Lamps: A Clean, Sharp Germicidal Peak
The mercury vapor pressure inside a low-pressure mercury lamp is very low, so once it strikes, almost all the energy concentrates into two resonance lines: a main peak at about 254 nm, plus a secondary peak at about 185 nm. 254 nm is precisely the golden wavelength for disinfection, strongly absorbed by DNA and RNA; 185 nm carries even more energy and can turn oxygen molecules in the air into ozone. This "concentrated energy" is what makes the low-pressure mercury lamp the most efficient UV-C source around — the literature notes that on average about one-third of the input electrical power can be converted into UV-C output, which is why drinking water, wastewater, air and surface disinfection almost uniformly rely on low-pressure mercury lamps.
Medium-Pressure Mercury Lamps: The Broad-Spectrum, High-Firepower Curing Workhorse
The mercury vapor pressure inside a medium-pressure mercury lamp is a great deal higher and the discharge temperature is high too, so the output is no longer two clean lines but a "broad spectrum" that spans UV-C, UV-B and UV-A all the way out to visible light. Its power density (watts per unit area) is far higher than a low-pressure lamp's, letting it cure thick coatings and dark inks all the way through in the instant a conveyor whisks past at high speed — at the cost of throwing off a great deal of heat and infrared, so cooling and shielding have to be built in to match.
An easy way to remember it: for disinfection and pure energy, go low-pressure; for curing and broad-spectrum firepower to drive through the formulation, go medium-pressure. Of course, in recent years UV-LEDs have been steadily squeezing medium-pressure mercury lamps out on the curing side — but that's another trend worth a discussion of its own.
There's no one-size-fits-all answer to light-source selection; it comes down to your water quality, line speed and the absorption characteristics of your formulation. From its base in Kaohsiung, TingHao Technology serves businesses across southern Taiwan close at hand, and if you need an on-site assessment of which mercury lamp to use, call 07-363-0802 or add us on LINE for a chat.