Inaccurate water tests – or did I do something wrong after all?

In the aquarium scene we often hear people talking about “accurate” and “inaccurate” tests. But mostly they mean a rougher or finer scaling of the output values. If a test is inaccurate this is mostly because of an application error when dosing the reagents and the amounts of water, or because of the way the test was used. Unintendedly inaccurate readings also often cause errors which result in this statement.

The test works accurately as long as a water test has not expired its best-before date and the application instructions are followed. But remember: once a water test has been opened, it starts to age. That’s why tests should be used 9 – 12 months after the initial opening at the latest. After this period some deviation in the measurement accuracy can occur. The best-before date is only valid for the unopened product.

Often minor problems can have a major impact. In case of problems our customer service will gladly help you. In the following we have listed and described the most frequent application errors for the water test types “drop test” and “test strips”. If you avoid these errors your next measurement will be even more precise.

Typical errors when using the drop tests

  • Contaminated glass cuvettes: Residues from the last measurement or of tap water can strongly affect the measurement. The glass cuvettes need to be thoroughly cleaned and dried after each water test. Rinse the glass before every test with the water to be tested.
  • Partial droplets of reagents: Everyone knows what it is like to have drops filled with air bubbles. Hold the reagent bottle vertically over the glass cuvette and carefully press the bottle. That’s how you avoid air pockets, which reduce the amount of reagent for the test reaction, thus falsifying the result. In practice many testers discard the first drop since this is often filled with air.
  • What spoon end? Heaped or level? Some water tests require the addition of powder. This always means level, NOT heaped spoons. For some tests you use the small, less noticiable, end of the spoon. If too much powder is added for either reason, the result will be inaccurate.
  • Reading errors caused by not using the comparator block: The ambient light has a big influence on the reading accuracy of the test results. To reduce light from entering laterally and enable you to read the results as precisely as possible, the enclosed comparator block needs to be used. The blank test compensates for the intrinsic colour of the water which can mislead your reading. Ensure there is sufficient daylight and avoid strong artificial lighting. Compare the measuring result of a test without a comparator block in a dark room and in the open air. You will be astonished.
  • Service life of test: If the test has passed its best-before date or it has been open longer than 9 – 12 months this can lead to a deviation in test results. If you receive colours which can’t be found in the colour chart this is the most frequent cause. You will find the best-before date on the back of each test case and on the packaging of the individual tests. Please also make a note of the date of the first opening. If none of these factors apply, one of the intended reagents was probably wrongly dosed or mixed up.
  • Waiting times: almost every test has allowed waiting times which should suffice until the reading of the end result. Some tests also require waiting times between the adding of individual reagents. Should these waiting times not be upheld, the measured values will be incorrect.
  • The water quantities are important: always use the enclosed syringe to dose the correct water quantity. Dosing into the vessels by eye will lead to incorrect results since the round form of the container means the surface tension of the water optically bends the water and deceives our eyes.
  • Colour weakness: This primarily affects men and they are often not aware of it. Colour weakness but also red-green colour blindness can be differently pronounced and make reading of some tests difficult. Undetected, this could influence the interpretation of the results. If you are unsure a second person should be consulted as a check.

Typical errors when using test strips

  • Removing excessive water: After dipping the strip into the water there is a lot of excessive water in the individual test fields – sometimes even small water domes. To remove the water the test strip needs to be laterally tapped on a liquid absorbent underlay. Please do NOT shake the strip in the air because through this the measuring fields might react with each other and the water will not be sufficiently removed. This is the most frequent cause of measuring errors and reading problems. If two fields react with each other, these are not evenly changing colours. Position: Always hold the smartphone vertically over the ColorCard. If you hold it at an angle of 45 ° over the ColorCard this will result in distortions.
  • Age of the test strips: The test strip reacts with the contact to water. The more often the package has been opened the more humidity enters into the small tube. If after a measurement the tube has not been closed, the test strips inside may react to the humidity and not deliver correct measuring values anymore.
  • Light and reflections: Sufficient daylight is required for readings. Excessive water on the test strip or direct light from a lamp onto a reflecting surface can lead to lighting-based reflections and an incorrect perception of the colour. Use kitchen paper as matt, non-reflective underlay and carry out the reading in bright light, similar to daylight.
  • Uphold the waiting time: To receive correct measuring results you need to uphold a waiting time of 60 seconds. Since the test strips continue to react steadily the reading of the measuring values, following the exact prescribed time, is the most important factor for a correct measurement.
  • Colour weakness: This primarily affects men and they are often not aware of it. Colour weakness but also red-green colour blindness can be differently pronounced and make reading of some tests difficult. Undetected, this could influence the interpretation of the results. If you are unsure a second person should be consulted as a check.

Comparing values from different water tests

Every test has a fixed scaling range which enables the assignment of measurement data. The steps in a drop test here are different to the JBL ProScan or JBL EasyTest 6in1 . They don’t always have the same scaling and this can result in non-congruent measuring points leading to different measuring results than in your other tests. It is thus important to note that similar measuring results are acceptable: they don’t have to be completely identical. If, however, the values deviate substantially from each other, a faulty measurement has probably been caused by one of the above mentioned reasons.

© 22.10.2016
Matthias Wiesensee
Matthias Wiesensee
M.Sc. Wirtschaftsinformatik
JBL GmbH & Co. KG

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