How fluorometers measure a fluorescent tracer such as PTSA in parts per billion, what upsets the reading, and how to keep the optics clean.
Some molecules absorb light at one wavelength and immediately give off light at a longer one. This is fluorescence. A fluorometer shines light at the water and measures the light that comes back at the emission wavelength.
- Excitation. A light source (commonly a UV LED) shines into the water through an optical window, at the wavelength the tracer absorbs.
- Emission. The tracer molecules give off light at a longer wavelength. A filter in front of the detector passes only that wavelength.
- Measurement. The detector’s signal is proportional to the tracer concentration, over the working range. The fluorometer reports it in parts per billion (ppb).
PTSA (1,3,6,8-pyrenetetrasulfonic acid) is a fluorescent dye added to treatment products at a fixed ratio by the manufacturer.
- It is inert. It is not consumed by the treatment chemistry, does not bind to deposits, and stays in the water.
- So the tracer level indicates product dose. If the product contains a known ratio of PTSA, the PTSA reading, multiplied by that ratio, gives the product concentration in the water. For example, if 100 ppb PTSA corresponds to 100 ppm product, the controller can feed product to hold the PTSA at 100 ppb.
- It measures what was fed, not what is active. The reading shows the product is in the water at the right dose. It does not show whether the active ingredients are being used up — for example by scale or corrosion — which needs separate tests.
Because the reading is in ppb, small changes in the optics or the water make a big difference to the result.
From the existing PTSA guidance:
- Bubbles collecting on the optics refract the light and upset the measurement.
- Fouling. Oil, suspended solids, and high calcium, iron, or other minerals that nucleate under UV light all foul the optics.
- Turbidity and color change what the probe sees, so different manufacturers’ instruments can disagree widely on the same water.
Also:
- Background fluorescence. Some organics and other treatment chemicals fluoresce near the same wavelengths and raise the reading. Instruments correct for this in different ways.
- Temperature. Fluorescence falls as temperature rises. Most probes compensate automatically, but large swings still add error.
- Oxidizers. High levels of chlorine or other strong oxidizers can degrade the tracer over time, so the reading reads low compared with the product actually fed.
- Ambient light reaching the optics, for example in a clear flow cell in sunlight.
- Dirty sample line. A sample line with deposits can release or absorb material and delay the reading.
- Mount the probe vertically, with flow going up through it, so upward flow carries bubbles away from the optics.
- Filter out suspended solids where appropriate.
- Keep the optics wet and dark. Use an opaque flow cell or one out of direct sunlight, in a line that does not drain when flow stops.
- Steady flow within the manufacturer’s range. Too little flow lets bubbles and deposits settle; too much can draw in air.
- Sample well-mixed water, well away from the product injection point.
¶ Maintenance
- Expect more cleaning in hard or dirty water. High LSI water may need a more frequent cleaning schedule.
- Clean the optical window with a soft cloth or cotton swab and the cleaner the manufacturer recommends. Mineral scale usually needs a mild acid clean; oil needs a detergent or alcohol wipe. Never scratch the window.
- Calibrate with a zero and a standard. Zero in clean deionized water, then calibrate with a PTSA standard of known ppb, or an in-process point from a calibrated handheld.
- Check against a handheld from the same manufacturer as the probe, not a different make.
- Check after every clean. If the reading jumps after cleaning, the probe was fouled; a large jump means cleaning is overdue.
- Service life. With no consumable electrode, fluorometers commonly last several years. Light sources age over time; many probes report source intensity or a cleanliness value as a diagnostic.
Wear eye protection and gloves when using acid or solvent cleaners, and follow Chemical Handling. Do not look into a UV source.
For step-by-step checks, see Troubleshooting PTSA Inline Probes.
Models documented in this wiki: