Which To Test First: TDS vs Resin vs Rinse On WFP Spotting Sequencing Decision Tree

Why this matters

When a water-fed-pole clean dries spotty, three suspects compete: the water purity (TDS), the resin or membrane that produces that purity, and the rinse technique that delivers it to the glass. A technician who guesses in the wrong order wastes a resin change on a rinse problem, or re-rinses for an hour against exhausted resin. The fix is not knowing which suspect is guilty in the abstract; it is testing them in the order that yields a diagnosis fastest with the least teardown. This tree sequences the checks so the cheapest, most decisive reading comes first and each result either closes the case or hands you the next test.

Symptom presentation

The complaint is uniform fine spotting after WFP drying, usually visible once the glass dries in direct light. Note whether spotting is uniform across all panes (points to water or resin) or concentrated at pane edges and bottoms (points to rinse and run-off). Uniform spotting on the first pane of the day with a fresh fill is a different signal than spotting that appears only after several hours of production, which suggests progressive resin exhaustion.

Quick checks

Three readings, taken in this order, because each is faster and more decisive than the next teardown step:

• Brush-head TDS first. It is a 30-second meter reading at the point of use and it splits the entire problem. Spot-free drying requires 000 to 010 ppm at the brush. This is the most decisive single number and costs nothing to take.
• Resin or membrane state second, only if TDS is high. Reading the inlet TDS and comparing to the brush-head TDS tells you the purification stage's rejection performance without opening a vessel.
• Rinse technique third, only if TDS is good. If the water is pure but glass still spots, the cause is downstream of purity and you isolate it by re-cleaning one pane with a controlled rinse.

Isolation tree

Step 1: Read brush-head TDS. If it is above 010 ppm, the water reaching the glass is not pure enough to dry spot-free. Go to Step 2. If it is 000 to 010 ppm, the water is fine; skip resin entirely and go to Step 3.

Step 2 (TDS high): Read source/inlet TDS. For a DI-only system, any brush-head reading materially above source-after-RO means the resin is exhausted or channeling; regenerate or replace. For an RO/DI system, compute membrane rejection: rejection percent equals (inlet TDS minus permeate TDS) divided by inlet TDS, times 100. A healthy thin-film composite membrane rejects 90 to 98 percent. If rejection has dropped below roughly 90 percent, the membrane is fouled or the DI is masking a dying membrane until it suddenly fails. Replace the spent stage, re-fill, and re-read at the brush. A confirmed 000 to 010 ppm at the brush closes a water/resin cause.

Step 3 (TDS good, still spotting): The problem is rinse technique or frame contamination, not water. Re-clean one isolated pane: agitate the glass fully, then rinse from the top sill downward with a full-pane flush, lift the brush off the glass while water is still flowing, and do not re-touch. If that pane dries spot-free, the original spotting was rinse technique (insufficient flush volume, rinsing too fast, or letting dirty frame water sheet over clean glass). If even the controlled pane spots at good TDS, suspect contaminated hose or tank (algae, mineral scale in a hose left full), or pre-existing etch on that glass.

Confirming diagnosis

• Water cause confirmed when brush-head TDS is above 010 ppm and drops to 000 to 010 ppm after the purification stage is serviced, and re-rinsed panes then dry clear.
• Resin vs membrane split confirmed by the rejection calculation: low rejection at the membrane is a membrane fault; good rejection but high brush TDS is a DI fault.
• Rinse cause confirmed when one controlled-rinse pane dries spot-free while adjacent normally-rinsed panes spotted, at identical good TDS.
• Equipment-contamination cause confirmed by flushing the hose to waste, reading TDS at the far end, and finding it elevated versus the cart outlet.

Why this order, not another

The order is fastest-and-most-decisive first. The brush-head TDS reading is a 30-second meter check that splits the entire problem space in one number, so it goes first regardless of what you suspect. Resin and membrane diagnosis is second because it only matters if TDS is already proven high, and it needs nothing more than a second meter reading at the inlet plus a rejection calculation, no vessel teardown yet. Rinse technique is third because isolating it requires actually re-cleaning a pane and waiting for it to dry, the slowest test, and it is only worth doing once you have ruled water out. A common field mistake is to start by swapping the resin cartridge on any spotting complaint; if the real cause was a rinse problem at already-good TDS, you have spent a cartridge and the spotting persists. Anchoring on the meter reading first prevents that. Keep a log of brush-head TDS against production hours on each job; resin exhaustion shows as a slow climb across the day (for example 002 ppm at fill, drifting to 015 ppm by mid-afternoon), and that trend tells you to size the next vessel up before it bites mid-job.

Remediation

For exhausted DI: regenerate the bed if it is regenerable resin and you have the setup, otherwise replace the cartridge. For a fouled membrane: replace it and verify rejection on the new unit. For rinse technique: standardize the final-flush sequence and slow the lift-off. For contaminated hose/tank: drain after each shift, do not store hoses full in heat, and sanitize a tank that shows biofilm. Re-test brush-head TDS as the final sign-off reading on any WFP repair.

References

• International Window Cleaning Association (IWCA), I-14.1 Window Cleaning Safety Standard.
• US EPA, Secondary Drinking Water Standards (total dissolved solids guidance for source-water assessment).
• IWCA technical guidance on water-fed-pole and pure-water systems, including TDS targets for spot-free results.
• ASTM D5907, Standard Test Methods for Filterable and Nonfilterable Matter (Total Dissolved and Suspended Solids) in Water.