How to estimate chlorine sample-line delay
Estimate how long water takes to reach an online chlorine analyser. Use line volume and sample flow, then check the estimate on site.
To estimate chlorine sample-line delay, divide the water volume between the take-off and sensor by the actual sample flow. For example, a 0.60 litre line at 0.30 litres per minute gives an estimated transport delay of two minutes. The analyser’s response and logging interval are separate.
Measure the water path
Use the internal diameter of the tubing, the length of each section and the measured flow in the branch that feeds the sensor. Use litres for volume and litres per minute for flow so the answer is in minutes. Include other wetted volumes upstream of the measuring point where you know them, such as a filter bowl.
Worked example: a 12 metre sample line
A straight 12 metre tube with an 8 mm internal diameter holds approximately 0.60 litres. That is the cross-sectional area of the bore multiplied by the length. At 0.30 litres per minute, the water takes about two minutes to travel through the tube. At 0.15 litres per minute it takes about four minutes. These are illustrative figures, not Halogen operating specifications.
Use the right time for the comparison
A sample collected at the process take-off at 10:00 would reach the end of that example line at about 10:02 if flow stayed steady. Use a suitable sample point near the analyser when possible. If you sample at the take-off, record the time and account for transport before comparing the result against the online trend.
Why the measured delay may differ
Filters, fittings, branches and mixing in the cell can change the observed lag. Flow can vary as a line fouls or the sample supply changes. Check the flow under representative conditions and compare timing during an approved site check. PMA’s MP5 field-trial account shows why verifying the sample supply belongs in this investigation.
Record delay separately from instrument response
Log the take-off location, internal diameter, line length, flow and observed delay. Separately record the instrument response, display update and SCADA logging settings. For a replacement analyser, carry those records into the retrofit survey and recheck them when the sample route changes.
