When taking hand readings from the pool side, the CPO® manual states that you should take the sample 18 inches below the surface of the water. Why? This brief article talks about how pools circulate and how that relates to hand readings.
When taking hand readings from a pool, it should be away from a return as this is where chemicals enter the pool, away from “dead zones” such as stairs, corners, water features, etc. as this is where circulation is the worst.
The very top of the pool is not going to have the same exact chemistry as the bottom of the pool. 18 inches down will give a more balanced reading.
One thing not often talked about is how the water going through the pipes often does not test the same as the water you take from the poolside.
Ideally, you want the pool to have a “balanced mixing” where the chemicals enter the pool from the returns, and circulate roughly evenly between the top and the bottom, which looks like this:
In practice though, it’s often not balanced. The gutters or skimmers could be pulling more than they are designed to, upsetting the delicate balance. In most cases, the skimmers/gutters are pulling 75% of the water from the pool and the main drain about 25% but this varies by pool. In many cases, the gutters/skimmers are pulling so much more than the main drains that the bottom of the pool doesn’t circulate as well it should, leading to a “top balanced” pool, where the top side has a higher hand reading than the bottom of the pool.
Top balanced pools will read higher than hand readings taken from the chemistry controller in the mechanical room.
Lastly, and less common, is a “bottom balanced” pool, where the main drains are pulling so much that the top side of the pool consistently has a higher hand reading.
Due to the potential variance between the pool readings and the readings from the pump room, anytime hand readings are being compared to the chemistry controller for the purposes of calibration or just ensuring that the chemistry controller is reading accurately, the hand reading should also be taken from the sample port at the chemistry controller.
By following the 18-inch rule (Most people do “elbow deep” which is fine) and the other rules regarding taking hand readings, you can ensure consistent water quality.
If you suspect your pool is not circulating as it should, consider checking the valving out. It’s possible that the valves that control the amount of water coming from the gutters/skimmers/main drains is not correct, which will result in some skewed readings.