pH

The Sutro monitor measures the pH of the water sample with a phenol red pH indicator. Phenol red changes color depending on the pH of the water it is mixed into from 6.8 to 8.2 pH. If the pH is above 8.2 or below 6.8, phenol red will remain the same color, causing 6.8 and 8.2 to be the limits of this indicator.

The Sutro monitor measures the pH to within ±0.2 or less, similar to many residential test kits. The monitor also reduces the potential error caused by:

• inaccurate volumes of water vs reagent
• improper lighting when reading tests
• inadequate reagent mixing
• reagent contamination
• long term reagent stability
• incorrect test timing

How does it compare?

Phenol red is used in many test kits for colorimetric determination of pH. Generally, these test kits segment the 6.8-8.2 pH range into 0.2-0.4 pH increments.  The accuracy of these test kits depend on the reagents, test tubes, and test process by the user. In the best case scenario, these should be accurate to within 1 color comparator block depending on how the range is segmented this will be usually be 0.2 to 0.4.

Test strips use similar methods to determine the pH of the water. Procedure and reagent stability are equally important in these test kits and have similar accuracy to within 1 color comparator block depending on how the range is segmented, usually 0.2-0.4 pH increments.

Pool stores commonly use liquid reagents or automated test devices when a water sample is brought in for a test. If the pool store is using an automated test device there is still usually a test cartridge or test tube that is required to be filled and cleaned by the technician, so some of the sources of error can still remain. Water samples are also not 100% stable so when they are transported meaning the pH and free chlorine may not be exactly the same as what is in the pool after transport.

Free Chlorine

The Sutro monitor measures the free chlorine in the water utilizing a DPD oxidation reaction. DPD reacts with the free chlorine in the water coloring the sample dependent on the amount of free chlorine in the sample. The monitor reads the concentration from 0 to 5 ppm Cl2. Above 5 ppm Cl2 the response is no longer linear so the monitor will report that the free chlorine is above 5 ppm Cl2 at that point.

In cases where the free chlorine is higher than 10 ppm, the color developed by the DPD reagent can start bleaching out. This will cause a lower chlorine reading than the actual concentration.

The Sutro monitor measures free chlorine within ±0.2 ppm Cl2 while reducing the same types of errors that can cause inaccuracy with manual readings such as:

• inaccurate volumes of water vs reagent
• improper lighting when reading tests
• inadequate reagent mixing
• reagent contamination
• long term reagent stability
• incorrect test timing

Residential test kits are normally accurate within ±0.2 ppm Cl2 when using the FAS-DPD method and can measure higher than 5 ppm Cl2. Basic DPD drop test kits measure within ±0.5 ppm Cl2 and up to 5 ppm Cl2 and test strips can measure to within ±1 color comparator block. If the water sample is brought to a pool store some of the chlorine may be consumed on the trip or during sample collection. So the free chlorine may be lower at the store than in the pool itself.

Bromine

The Sutro monitor uses the same method to measure bromine as it does chlorine in the water. The measurement scale is adjusted for bromine instead of chlorine, similar to test kits that have a chlorine and bromine scale on their color comparators. The Sutro monitor measures the active bromine in the water within ±0.5 ppm Br2.