If you are new to hydroponic gardening, you might be feeling a little overwhelmed by all of the different kinds of grow lights that are available. Each light can be best suited for specific purposes, such as helping plants grow at different stages of maturity. Ones that you might have heard about are high pressure sodium grow lights. This type of grow light produces light when sodium becomes excited, and provides a good light by which to grow plants.
HPS grow lights are also gas-discharge lights. This means that your high-pressure sodium lights give off light by sending an electrical current through a plasma, or an ionized gas. HPS lights typically use the noble gas Xenon to get things started in terms of producing light because it doesn't get in the way of the chemical reactions occurring in the arc tube.
A combination of mercury and sodium lies in an arc tube made out of a product called Alumina. Alumina is made out of aluminum oxide, which allows light to pass through it diffusely. This material is used for the tube because it withstands the substantial chemical activity caused by sodium arc at high pressure. Electrodes are attached to either end of the arc tube, and a/c voltage is used to power the device. The voltage flows to the electrodes, which excites the sodium, causing it to produce light. If the power switch is flipped to the on position, pure uncontrolled voltage would cause the bulb to explode. In order to stop this from happening, high pressure sodium lights are always used in conjunction with a ballast. A ballast is used to regulate the amount of current flowing through the arc. The stabilizing effect of the ballast on the current allows HPS lamps to operate in a steady rather than erratic manner.
HPS lights can last more than 20,000 hours. When they are ready to burn out, they start cycling. The lamp can be started at a low voltage and they heat up while operating. That heat increases the gas pressure in the arc tube, and it takes greater amounts of voltage to maintain the electric arc. As the bulb gets old, it takes more and more voltage to compensate for the high pressure of the gas, and eventually that amount of voltage will exceed what the ballast can supply safely, so the ballast shuts down the arc. This cools down the arc tube, which reduces the pressure of the gas inside the tube, and once it reaches a safe temperature the bulb will come back on again because the ballast will reactivate when the temperature is correct. So the grow light will go on and off several times. When this starts happening, you know that it will soon be time to replace those HPS lamps.
