Whether the water is in a river, an aquaculture pond, a cooling tower, a boiler, a pure-water system or a treatment plant, three measurements come up again and again: pH, conductivity and dissolved oxygen. Together they cover acidity, dissolved ionic content and the oxygen available for biology and corrosion.
pH
pH expresses hydrogen ion activity on a logarithmic scale from 0 to 14, where 7 is neutral; each unit is a tenfold change. It controls corrosion and scaling in cooling and boiler water, the effectiveness of disinfection and coagulation, and whether aquatic life survives.
A glass pH electrode develops a voltage of about 59 mV per pH unit at 25°C across its pH-sensitive glass membrane, measured against a reference electrode. Electrodes drift and slow down as they age, so:
- calibrate with at least two buffers that bracket the expected range (for example pH 4 and 7, or 7 and 10);
- compensate for temperature, because the electrode's slope changes with it;
- keep the electrode wet in storage solution, never in distilled water;
- replace it when the slope falls well below theoretical or response becomes sluggish.
The MEWOI-QHB5 measures pH from 0.00 to 14.00 with ±0.03 pH accuracy, and redox potential (ORP) from −1400 to +1400 mV with an optional ORP probe.
Conductivity, TDS and Resistivity
Conductivity measures how well water carries current, which rises with the concentration of dissolved ions. It is the quickest general indicator of dissolved salts: it controls blowdown in cooling towers and boilers, verifies demineralised and reverse-osmosis water, and detects contamination. Total dissolved solids (TDS) is commonly estimated from conductivity using a conversion factor. Ultra-pure water is often expressed as resistivity (MΩ·cm), the reciprocal of conductivity.
The key choice is the cell constant (K) of the electrode, which must suit the range. Low-K cells (0.01, 0.1) are for pure water; K = 1 for general use; K = 10 for brines and process solutions. Conductivity rises roughly 2% per °C, so readings are compensated to a reference temperature, usually 25°C.
The MEWOI-EDS407 laboratory meter covers 0.00 µS/cm to 100 mS/cm with selectable cell constants of 0.01, 0.1, 1.0 or 10, and is supplied with a K = 1 platinum-black electrode.
Dissolved Oxygen
Dissolved oxygen (DO) is vital in aquaculture and natural waters, central to aeration control in biological wastewater treatment, and a corrosion driver in boiler feedwater. It is reported in mg/L or as percent saturation.
How much oxygen water can hold depends on temperature, barometric pressure and salinity, so a DO meter compensates for all three. Membrane-covered (polarographic or galvanic) sensors consume oxygen as they measure, so the sample has to move past the membrane, either by stirring or by flow, or the reading falls. Membranes and electrolyte need periodic replacement.
The MEWOI-707B reads 0.00–20.00 mg/L with ±0.1 mg/L accuracy and simultaneous % saturation and temperature, with automatic temperature compensation, manual salinity compensation from 0 to 35 g/L, and storage for 4000 data sets.
Good Measurement Habits
- Calibrate on the day, with fresh standards, and record it.
- Measure at the source when you can: DO and pH change as samples warm, degas or absorb CO₂.
- Rinse between samples to avoid carry-over, especially when going from concentrated to dilute.
- Watch temperature — all three results depend on it.
Frequently Asked Questions
Is TDS the same as conductivity? No. TDS is a mass concentration that meters estimate from conductivity using a factor that depends on what the dissolved salts are. For exact TDS, the sample is evaporated and the residue weighed.
Why does my DO reading drift down while I hold the probe still? A membrane sensor uses up oxygen next to the membrane. Without stirring or flow, the reading falls.