MesuLab Conductivity Meter

Conductivity Meter

The Conductivity Meter is an online water quality analysis instrument designed for high-temperature applications, including medical multi-effect distillation water systems, boiler feedwater and condensate, heat exchange systems, industrial thermal cleaning of mechanical components, and industrial circulating water systems. It supports advanced water quality management and automated control across a wide temperature range.

Introduction

The Conductivity Meter is a multi-range instrument capable of meeting various application requirements, from deionized water to seawater. It features automatic temperature compensation and adjustable temperature coefficients, enabling accurate measurements of liquid samples with temperature characteristics different from water. With three measurement ranges and automatic range selection, it ensures optimal range selection during testing.

The instrument comes equipped with a double-prong stainless steel conductivity electrode with a built-in temperature sensor (for automatic temperature compensation), a non-slip rubber sleeve, four AAA alkaline batteries, an operation manual, and a portable carrying case.

Working Principle

Conductivity is a numerical measure of a solution's ability to conduct electric current. The conductivity of water is related to the concentration of inorganic acids, bases, and salts. At lower concentrations, conductivity increases with rising concentration, making it a common indicator for estimating the total ion concentration or salinity of water.

Conductivity and Water Hardness
The conductivity of an aqueous solution is directly proportional to the concentration of dissolved solids. Higher dissolved solids concentrations result in higher conductivity. The relationship between conductivity and dissolved solids concentration is approximately represented as: 1.4 μS/cm = 1 ppm or 2 μS/cm = 1 ppm (in CaCO₃ equivalents). Conductivity meters or total dissolved solids (TDS) meters can indirectly estimate water hardness. For approximate conversion, 1 μS/cm conductivity ≈ 0.5 ppm hardness.

However, note the following:

Indirect hardness measurements via conductivity may have a theoretical error of approximately 20–30 ppm.

Conductivity depends on molecular movement, which is influenced by temperature. For comparative measurements, the standard test temperature is typically set to 20°C or 25°C.

Reagent-based testing provides more accurate water hardness values.

Soft Water and Hard Water
Water is classified as soft or hard based on its calcium and magnesium ion content. Water with low or negligible levels of these ions is soft, while water with high levels is hard. Hardness caused by bicarbonates of calcium or magnesium is temporary (removable by boiling, which precipitates carbonates). Hardness caused by sulfates or chlorides of calcium or magnesium is permanent. Based on total hardness:

0–30 ppm: Soft water

Above 60 ppm: Hard water

High-quality drinking water: ≤25 ppm

High-quality soft water: ≤10 ppm
Natural soft water includes unpolluted rainwater and snowmelt, while temporary hard water includes springs, streams, rivers, and reservoir water. Some groundwater is highly hard.

Common Units for Water Hardness Conversion

1 mmol/L (1/2 Ca²⁺, 1/2 Mg²⁺) = 50 ppm (as CaCO₃)

1 mmol/L (1/2 Ca²⁺, 1/2 Mg²⁺) = 2.92 grains/gallon

1 grain/gallon = 17.1 ppm (as CaCO₃)

1 m³ = 264 gallons (US) = 220 gallons (UK)

1 kg = 2.2 pounds

1 ppm = 1 mg/L

TDS (Total Dissolved Solids) measures the total ion content in water, typically in ppm. In pure water applications, conductivity is often used to indirectly represent TDS. The conductivity of a solution equals the sum of the conductivities of all ions present.

Empirical formula: TDS (ppm) ≈ Conductivity (μS/cm) / 2
TDS may also be expressed in other salts, such as CaCO₃ (using a conversion factor of 0.66). The conversion factor between TDS and conductivity can be adjusted between 0.4 and 1.0 to account for different types of electrolyte solutions.

Conductivity
Conductivity is the ability of a substance to conduct electric current, inversely related to resistance. The unit is Siemens/cm (S/cm).

Water Hardness
Water hardness refers to the concentration of calcium and magnesium ions, measured in ppm. 1 ppm corresponds to 1 mg/L of calcium carbonate (CaCO₃).

Conductance (G) is the reciprocal of resistance (R). When two electrodes (typically platinum or platinum-black) are inserted into a solution, the resistance R between them can be measured. According to Ohm's law, at a constant temperature, R is proportional to the distance between the electrodes (L, in cm) and inversely proportional to the cross-sectional area (A, in cm²) of the electrodes:

R=ρ×(L/A)R=ρ×(L/A)

where ρρ is the resistivity, a material-specific constant representing the resistance of a 1 cm long conductor with a 1 cm² cross-sectional area.

Thus, conductance (G) can be expressed as:

G=1/R=(1/ρ)×(A/L)=K×(1/J)G=1/R=(1/ρ)×(A/L)=K×(1/J)

where K=1/ρK=1/ρ is the conductivity, and J=L/AJ=L/A is the electrode constant. The conductivity of an electrolyte solution is defined as the conductance between two parallel electrodes 1 cm apart, filled with 1 cm³ of the solution. By knowing the electrode constant (J) and measuring the resistance (R) or conductance (G), the conductivity (K) can be calculated.