Definition

The combined mass of all dissolved inorganic salts and organic matter in a water sample, reported in mg/L (≈ ppm). For steam humidification, TDS is the single-number summary of feedwater quality; combined with conductivity and hardness, it directly drives equipment selection. The very low TDS and conductivity of RO/DI water are not enough for an electrode unit to run, while a resistive humidifier works at any TDS level; TDS only affects maintenance frequency.

Detailed Explanation

TDS (Total Dissolved Solids) is the total mass of all salts, minerals and organic matter dissolved in water (mg/L ≈ ppm). In the laboratory it is determined by evaporating a filtered sample and weighing the dry residue (gravimetric method); in the field it is estimated from conductivity using TDS ≈ conductivity × 0.5–0.7. The factor depends on water composition.

Turkish drinking-water legislation sets no separate TDS limit; the regulation gives an indicator value of 2,500 μS/cm (20 °C) for conductivity. WHO sets no health-based guideline value for TDS; it considers water below about 600 mg/L generally good in taste and above 1,000 mg/L increasingly unpalatable.

Why It Matters

Steam produced by boiling does not carry dissolved solids, so all dissolved solids in the feedwater stay in the tank. Part of them builds up as deposits and part leaves with the drain. High TDS means more salt concentrating in the tank, more frequent draining and a shorter maintenance interval. If foam forms in the tank or the water level rises too high, mineral-laden droplets can be carried over with the steam; good drain and foam control limit this risk.

A resistive humidifier runs at any TDS level; only maintenance frequency changes. In an electrode humidifier, conductivity changes along with TDS, so the unit's operating band is affected too.

Worked Example

Water with a conductivity of 500 μS/cm has a TDS of roughly 250–350 mg/L (≈ 325 mg/L with 0.65). Conversely, water with 325 mg/L TDS would be expected at roughly 465–650 μS/cm. The factor is an estimate; the exact value is determined by laboratory measurement.

Engineering Note

Field TDS pens actually measure conductivity and convert it to TDS with a fixed factor; because different meters may use different factors, results should not be compared directly. For an exact value, a gravimetric analysis by an accredited laboratory is preferred. TDS alone is not enough; it is evaluated together with hardness, alkalinity, chloride and silica.

How Nem Kontrol Teknolojileri Applies It

As Neptronic's authorised distributor in Türkiye, Nem Kontrol Teknolojileri asks the customer for a recent water analysis from an accredited laboratory when selecting a steam humidifier and compares the values with Neptronic's feedwater requirements. The resistive Neptronic SKE4 runs on any water, RO and DI included; water quality only affects maintenance frequency. Pre-treatment systems are designed and installed by specialist water-treatment companies.