Accuracy, Precision and Pool Chemical Testing

pool testing kits

Are your pool testing kits accurate and precise? 

Pool managers and lifeguards everywhere ask, “How accurate is that test kit?” Often times the question they’re actually asking is, “How precise is that test kit?” Accuracy and precision are two related concepts that are often used interchangeably, but have two distinct meanings.

Accuracy is defined as the degree of closeness of a measured or calculated quantity to its actual, true value. Precision is defined as the degree to which further measurements or calculations show the same or similar results. Precision also accounts for the number of significant digits to which the value can be measured. For example, an accurate dart thrower will hit the bullseye; a precise dart thrower will hit the same place on the dart board repeatedly, but not necessarily the bullseye. Consider water analysis results for pool water: What results are accurate versus precise? Suppose a pool has a pH of 7.45.

> BAD accuracy and BAD precision: The water is tested 10 times and the pH results are: 7.50, 7.32, 7.60, 7.39, 7.56, 7.47, 7.37, 7.52, 7.30, 7. 40. The results do not match the actual pH value (lack of accuracy), nor do they have a tendency toward a particular value (lack of precision).

> GOOD accuracy and BAD precision: The water is tested 10 times and pH results are: 7.41, 7.42, 7.46, 7.47, 7.44, 7.45, 7.43, 7.42, 7.48, 7.45. All results are close to the actual pH (7.45), however the distribution of results still does not have a tendency toward one value.

> BAD accuracy and GOOD precision: The water is tested 10 times and pH results are: 7.56, 7.53, 7.55, 7.56, 7.54, 7.56, 7.55, 7.57, 7.56, 7.55. This set of results shows a tendency toward one value (7.56) which demonstrates high precision, but every measurement is off from the actual pH value (7.45).

> GOOD accuracy and GOOD precision: The water is tested 10 times and pH results are: 7.45, 7.44, 7.44, 7.45, 7.45, 7.46, 7.43, 7.41, 7.45, 7.46. These results show repeated values (high precision) very near the actual pH value (high accuracy).

Water analysis results are used to prescribe treatment protocols. Operators will only be able to make a decision as good as the water analysis results. Imagine a pool with the following water quality characteristics: pH 7.4, temperature 84°F, alkalinity 90ppm, calcium hardness 300ppm and TDS 1,500ppm. The calculated Langelier Saturation Index (LSI) is 0 and the water is perfectly balanced. However, suppose your test kit is not accurate and the same water sample gets the following results: pH 7.5, temperature 84°F, alkalinity 110ppm, calcium hardness 350ppm and TDS 1,500ppm. While the water is still balanced, the calculated LSI of 0.3 indicates the water is more scale forming than in actuality.

Inaccurate or imprecise test kits can lead to unsafe water conditions, damage components of the pool system and unnecessary use of chemicals. Results that are both accurate and precise will help pool operators maintain healthy, balanced water for both swimmers and pool equipment. Investing in a high-quality, accurate water analysis instrument for monitoring pool water chemistry is imperative.

Erin Post is the regional manager of Americas at Palintest USA and Bob Banker the is application specialist for aquatics/pools and spas at Palintest USA. For more information, email 

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