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Sampling Sample point - tap sampling

Sampling Sample point - tap sampling Why is water not allowed to have sat in the pipe-work or tap for any period of time when sampling? Click here for the answer. 12. Sampling Sample point - tap sampling Why is water not allowed to have sat in the pipe work or tap for any period of time?

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Sampling Sample point - tap sampling

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  1. Sampling Sample point - tap sampling Why is water not allowed to have sat in the pipe-work or tap for any period of time when sampling? Clickhere for the answer. 12

  2. Sampling • Sample point - tap sampling • Why is water not allowed to have sat in the pipe • work or tap for any period of time? • Must reflect true condition of the water being tested. • Need to test water in main pipe and not the off-take pipe. 12

  3. Sampling process • The following steps will describe the sampling process. • Step 1: • Run the tap on full flow for 2 minutes then • reduced to a slow steady flow. • Note: • When the off-take point is a long distance from the main pipe, more time is needed to flush the pipe – 5 minutes or more may be needed. 13

  4. Sampling process • Step 2: • Sample container and cap rinsed with the sample three (3) times. • Step 3: • Fill the sample cell and adjust volume to the • line. 14

  5. Sampling process Step 4: Cap immediately. Why is the cell capped immediately in step 4? Clickhere for the answer. 15

  6. Sampling process Step 4: Cap immediately. Why is the cell capped immediately in step 4? To prevent loss of chlorine from the sample. 15

  7. Sampling When taking a sample, why is it important to run the tap for at least 2 minutes? Clickherefor answer. 16

  8. Sampling When taking a sample, why is it important to run the tap for at least 2 minutes? To ensure that a fresh sample from the main pipe is tested (and not the water that has been sitting in the off-take pipe and tap) 16

  9. Sampling When taking a sample, describe how to correctly prepare the sample cells? Clickherefor answer. 17

  10. Sampling Before taking a samples, are the sample cells prepared? The cells and caps are rinsed three (3) times with the sample. Also - check the cells for dirt and staining – clean them if necessary. Replace the dirty cells if they cannot be cleaned. 17

  11. Measuring free & total chlorine - residual • DPD method - N,N diethyl-p-phenylenediamine. • Forms reddish pink dye - colour proportional to chlorine concentration (the more chlorine present, the darker the pink colour becomes). • Chlorine concentration is measured using a HACH pocket colorimeter • or other HACH models: DR 800 & 2000 series. • Free chlorine – ready to measure within 1 minute. • Total chlorine – ready to measure between 3 - 6 minutes. 18

  12. Chlorine reading • Free chlorine = Hypochlorous acid (HOCl ) • Total chlorine = HOCl + Combined chlorine • Combined chlorine is the sum of all chloramines • Can Free chlorine be greater than Total chlorine? • Clickherefor the answer. 19

  13. FREE CHLORINE COMBINED CHLORINE 20% 80% TOTAL CHLORINE Chlorine reading Can Free chlorine be greater than Total chlorine? Free chlorine shouldn’t be greater than Total but it can happen due to instrumental inaccuracies under certain conditions. 19

  14. Sample cell types – Your HACH chlorine colorimeter will take specific size cells. Learn which ones you need to use in both Low and High ranges. 20

  15. Sample cell types 21

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