TDS Targets For Water-Fed Pole Pure Water

Why this matters

Total dissolved solids (TDS) is the number that decides whether a water-fed pole job dries spot-free or leaves a haze the customer photographs and sends back to you. The working rule in the trade is that any dissolved-solid load above the single digits will deposit on the glass as the water evaporates, because evaporation leaves behind everything that was dissolved in the drop. Hit the wrong TDS and every pane on a 40-window commercial route comes back as a callback. This article gives you the inlet-to-outlet target chain, the meter procedure, and the resin/membrane swap triggers so the truck leaves the shop with water that is actually pure, not just filtered.

What TDS measures

TDS is reported in parts per million (ppm) and measured with a conductivity meter calibrated against a 342 ppm NaCl standard. A handheld TDS pen reads the electrical conductivity of the water and converts to ppm using a fixed factor (typically 0.5 to 0.7). The reading is not selective: calcium, magnesium, sodium, chloride, sulfate, silica, and dissolved iron all add to the total. For window cleaning, what matters is the residue left when the water evaporates - the lower the TDS, the less residue.

Target values

Use these targets at the outlet of the final purification stage, measured at the brush head if possible:

• Pure-water target for final rinse: 0 ppm. Working ceiling is 10 ppm before spotting risk rises sharply on dark glass and tinted panes.
• Inlet from municipal supply: typically 50 to 500 ppm depending on region. Anything above 300 ppm shortens DI resin life and justifies an RO prefilter.
• Well water inlet: often 300 to 1,000 ppm with elevated hardness; RO is mandatory, not optional.
• Softener output (where the house is plumbed through a softener): TDS is unchanged or slightly higher, because two sodium ions have replaced each calcium or magnesium ion. Understand what that does and does not buy you. It does NOT extend DI resin life; the resin works on total ionic load and the load is the same, so a softened supply exhausts a DI vessel on the same schedule an unsoftened one would. Techs expect a softener to stretch resin and it does not. It DOES help an RO membrane, because the hardness that would scale the membrane is gone. Softener ahead of RO is good practice; softener as a substitute for purification is not.

Inlet test procedure

1. Calibrate the TDS meter in 342 ppm NaCl solution before each route day. A meter that reads 339 to 345 is in spec.
2. Open the hose bib and run for 30 seconds to flush stagnant pipe water.
3. Catch a sample in a clean cup and read at 70 to 75 F (meters are temperature compensated but extremes skew).
4. Log inlet TDS in the job record alongside the outlet reading - this is your trend data for predicting resin exhaustion.

DI resin and RO membrane swap triggers

Mixed-bed DI resin produces 0 ppm output until the column is exhausted, then breaks through quickly. The trigger is outlet TDS rising above 5 ppm with a stable inlet. Do not wait for the visual color change of the resin (color-changing indicator resins are useful but lag the actual exhaustion curve on heavy-use days).

RO membranes do not give zero output - they reject roughly 95 to 98 percent of dissolved solids. A 200 ppm inlet should produce a 4 to 10 ppm permeate. A four-stage RO/DI cart with DI polish after the membrane delivers 0 ppm to the pole. Membrane replacement triggers: permeate TDS climbs past 15 percent of inlet, or production rate (gpd) drops below 80 percent of nameplate at the rated pressure (typically 60 psi).

Reading the pole

Take a TDS sample at the brush head, not at the cart. Hose runs, in-line filters, and quick-disconnect fittings can pick up residue between the cart outlet and the head. A 0 ppm cart with a 6 ppm head reading means the contamination is downstream - replace the hose, not the resin.

Spot-free verification

After the first window of the day, walk to the dried glass at a 45-degree angle in direct light and inspect for spotting or filming. If clean, the system is operating in spec. If filmed, recheck head TDS, then check that the brush was not contaminated from a previous high-TDS job (rinse the brush at the start of each route). A water spot on day one is almost always an equipment issue, not a technique issue.

References

1. IWCA - International Window Cleaning Association, Water-Fed Pole Best Practices guidance.
2. ANSI/IWCA I-14.1 - Window Cleaning Safety Standard. Note that this governs the safety side of pole and height work, not water quality; nothing in it sets a TDS number.
3. EPA Secondary Drinking Water Regulations - guidance values for TDS at 500 ppm aesthetic.
4. ASTM D5907 - Standard Test Methods for Filterable and Nonfilterable Matter in Water (laboratory reference for TDS gravimetric verification).