Mercury Vapour

Mercury Vapour

A mercury vapour lamp is a particular kind of high-intensity discharge (HID) lamp that emits illumination using mercury vapour.
When an electric current is run through a tube holding mercury vapour and other gases, the mercury atoms become excited and emit ultraviolet light.
This is how the lamp operates.The lamp's interior phosphorescent coating is then activated by the ultraviolet light, which causes it to produce visible light.

Low Pressure Mercury Discharge

💢Mercury Has Different excitation level

💢Electrical discharge in Low Pressure Mercury Vapour is mostly(typically 63%) converted into

UV-C radiations -Two resonanceradiations radiations are at 253.7 nm & 185 nm

💢Approximately 3% is converted into visible radiations –predominantly at 405 nm (Violet), 436 nm (Blue), 546 nm(Green) & 578 nm(Yellow). The overall effect of the mixture isa discharge of Pale Blue colour light, but obviously with verysmall efficacy.

💢The remaining Input energy is dissipated as heat radiations.

Low Pressure Mercury Vapour UV Lamp

High Altitude The most common application of mercury discharge is in fluorescent lamps, which produce visible radiation using the fluorescence principle.

To convert UV radiation into visible light, the lamp's interior wall is coated with phosphor.

💥Fluorescent Lamp will be discussed in details in the next module.

💥But Low Pressure Pressure Mercury Mercury Vapour Lamp without without any phosphorphosphorcoating producing UV Rays, are also used in some specific fields.

UV Lamp Application

⋙ UV lamp finds its application as germicidal or Ozone absorption lamps. UVis also essential for Chromatography, Photochemistry, Calibration of UVDetectors, Calibrations of Spectrophotometer, and more.

⋙ UV-C light is germicidal – it deactivates the DNA of bacteria, and otherpathogens and thus destroys their ability to multiply and cause disease. UV-C germicidal lamp has its application in medical sector (pharmaceutical production, laboratories, hospitals and clinics) Water purification, Industryand Food Processing

⋙UV also attracts insects and flies. So UV lamps are also used in electricinsect killer. A light source (emitting UV and blue visible radiations) attractsinsects to an electrical grid, where they get fatal electric shock by touchingtwo wires with a high voltage between them. It is housed in a protectivecage to prevent human and larger animals to touch the high voltage grid.

High Pressure Mercury Vapour Lamp :

Construction

⋟ Normally operated vertically.

⋟ Inner Arc Tube - hard glass i.e quartz in order to withstand high temperature

⋟ It consists of Tungsten electrodes, Mercury and small amount argon gas.

⋟ Outer glass (Elliptical shape) - Soda-Lime silicate silicate glass and for higher rating Borosilicate glass.

⋟ Space between inner & outer glass tube - completely or partially evacuated to reduce convection heat loss.

⋟ For cold start an auxiliary electrode - molybdenum or tungsten positioned close to one of the main electrode, but connected to the opposite supply polarity through a limiting carbon resistor of 10-30 KΩ

HPMV Lamp : Working Principle

⇛ Argon (Filling gas) pressure – 2500-5000Pa, Mercury dose 18 to 36 mgdepending on the lamp rating.

⇛Initially supply is in between main and auxiliary electrode resulting dischargethrough argon – reddish radiation.

⇛ After some time Hg is vaporised, ionisation and discharge of Hg starts.

⇛After complete vaporisation of Hg, a low resistance path is created betweentwo main electrodes electrodes and the arc is shifted shifted to main electrodes electrodes.

⇛ Auxiliary electrode along with the limiting resistance becomes inactive.

⇛ When fully run up the lamp operates at 1 to 2 Atmosphere. For higher pressurefurther spectrum broadening is achieved generating more white light .

⇛ Due to increased pressure comparatively broad spectrum is developed andhence bluish white radiation together with UV, but it is deficient at Red end ofthe spectrum.

⇛ The phosphor coating on the inner wall of the outer envelop increases thelamp efficacy by converting UV into Red Light (600-750nm). Colour renderingproperty is also improved.

HPMV Lamp: Electrical & Optical Characteristics and Application

🧿Operating Voltage – Supply Voltage

🧿Operating Temperature – 600˚C

🧿Power rating – 80 – 1000 W

🧿Takes 4-5 min for full brilliance

🧿Colour – Bluish White

🧿Life – 10000-24000 working hours

🧿CRI (Typical (Typical 250 W) – 16-20 (Clear Bulb) & 40- 50 (Phosphor Coated)

🧿CCT (Typical 250 W) – 6000 K (Clear Bulb) & 3800 K (Phosphor Coated)

🧿Luminous efficacy – 35-65 Lm/W

🧿Application – High ceiling manufacturing bay, Railway Yard, Airport, Shopping Centre, Stage, Sports Lighting etc.

🧿After damage of lamp, disposal of mercury is another problem - health hazardous

🧿Now a days almost obsolete due to poor efficacy and CRI.