Even a small urine stain on the carpet encourages your dog to continuously re-mark that spot. Unfortunately, urine stains aren't always obvious, especially on dark carpets -- and you can't remove what you can't find. Under the ultraviolet light of a black light, dried urine crystals are easy to see.
Vacuum the carpet thoroughly before using the black light. In addition to urine, your black light will illuminate lint, fur and other small particles. Although the differences in color and smell make it hard to confuse a urine stain with lint, removing these items ahead of time allows you to hone in on the urine-stained spots.
Turn off the lights, close the shades and shut the door. Ultraviolet light illuminates urine most effectively when the room is completely dark. Wait until night if the carpeted room has windows. The darker the room, the easier it will be for you to locate those sneaky urine stains.
Put on your safety goggles, available at any hardware store, to protect your eyes from the ultraviolet light. Especially if you anticipate spending more than a few minutes mapping the spots on the carpet, your eyes will need protection. Once you have on your protective eyewear, grab your black light and your masking tape and turn off the lights.
Scan the carpet by holding the black light 1 to 2 feet above the floor. Keep in mind that ultraviolet light will turn food and fluid stains various shades of fluorescent white, blue or green. Urine stains, however, are not bright. Urine crystals are dull in color, and will appear as pea-soup green or brownish-yellow.
Confirm that the spot you found is indeed urine with the old smell test. Urine, of any type or age, has an odor that is unmistakable. Once you've confirmed that the stain is urine, stick a small piece of masking tape on the center of the stain so you know where to concentrate your cleaning efforts. For larger stains, use several pieces of masking tape to mark the outer limits of the stain's perimeter.
- Repeat the black light search a few days after you clean the urine stains. Older and deeper stains often require multiple cleaning sessions before they're completely gone.
- When you saturate the stain with cleanser, start a few inches outside the perimeter, and work your way toward the center. Urine spreads under the carpet padding, which means you must clean a space slightly larger than the stain.
- Don't let your dog on the stained carpet until you've confirmed that you completely removed the stain. Your dog's acute sense of smell will allow him to find and mark even a trace of urine hiding deep in the carpet.
Christina Bednarz Schnell began writing full-time in 2010. Her areas of expertise include child development and behavior, medical conditions and pet health. She holds a Bachelor of Arts in international relations.