Published 11 July 2026 at 06:54
A hydroponic tomato grower takes a drop from the nutrient solution circulating among 400 tomato plants in his greenhouse. He dips the conductivity meter into the liquid, and the screen displays 1,850 μS. He immediately knows the solution is UNDERFED — his target is 2,200–2,500 μS for fruiting tomatoes. He adds more NPK concentrate, measures again: 2,350 μS. Perfect. The plants will receive adequate nutrition until the next water change.
This is the primary use case for a portable conductivity meter in the high μS range: hydroponic cultivation. However, the same principle is used in many other applications where salt or nutrient concentration must be monitored quickly and portably.
What conductivity actually measures
Electrical conductivity is the inverse of resistance — a measure of how well electric current flows through a material. In water, metal electrons are NOT the charge carriers (pure water is a poor conductor). Instead, current is carried by IONS — charged atoms that can move freely in the liquid.
Common ions in aqueous solutions and their sources:
- Na+ and Cl−: From table salt, sea salt. Most common in natural waters.
- Ca²+ and Mg²+: “Hardness” ions from limestone rocks (limestone, dolomite).
- K+: Potassium from fertilizers, plant ash.
- NO₃−, PO₄³−, SO₄²−: Nitrate, phosphate, sulfate from fertilizers, agricultural runoff.
- NH₄+: Ammonium from fertilizers, biological decomposition.
- H+ and OH−: From acids and bases (affects pH).
Pure H₂O without dissolved ions has a conductivity of ~0.055 μS/cm at 25 °C (almost zero). Pure rainwater: 5–20 μS/cm. Tap water: 100–500 μS/cm. Hydroponic nutrient solutions: 1,500–3,000 μS/cm. Seawater: 45,000–55,000 μS/cm.
Why the COND4 starts at 1,000 μS
The ETI COND4 has a measurement range of 1,000–19,900 μS. This is a HIGH μS range — not suitable for drinking water analysis or rainwater. The reason for this range is that the instrument is specifically designed for applications with high conductivity:
- Hydroponic nutrient solutions (1,500–3,000 μS).
- Marine aquariums (35,000–45,000 μS, but the 19,900 μS limit is sufficient for brackish water and nitrogen-based systems).
- Coolant manufacturing (variable).
- Industrial process control (variable depending on the process).
- Agricultural runoff analysis (500–5,000 μS depending on fertilizer levels).
For low-conductivity measurements (drinking water, rainwater, deionized water), a LOW-range meter (0–1,000 μS or 0–100 μS) is required. ETI has other models for these applications.
The 100 μS resolution is also tailored for the high range: when total conductivity is 2,000 μS, 100 μS = 5% of the reading. This is sufficient for most practical QC decisions but not for research use where high resolution is required.
Automatic temperature compensation — necessary because ions move faster in warmer water
The conductivity of an ionic solution is TEMPERATURE-DEPENDENT. The reason is that ion mobility (how fast ions move in an electric field) increases with temperature:
- At 20 °C: a 2,000 μS solution reads as ~2,000 μS.
- At 25 °C: the same solution reads ~2,200 μS.
- At 30 °C: ~2,400 μS.
Without temperature compensation, a cold lab measurement and a warm greenhouse measurement would yield different values for the same solution. This would compromise its utility as a QC instrument.
The ETI COND4 features ATC over the 0–50 °C range. The instrument measures both conductivity and temperature, and converts internal values to a “25 °C equivalent” display regardless of the actual sample temperature. The standard is 25 °C as the reference (used in all scientific and industrial specifications).
From μS to ppm: The TDS conversion
The COND4 can switch between μS mode and ppm mode (parts per million total dissolved solids — TDS, Total Dissolved Solids). The conversion is:
TDS (ppm) ≈ conductivity (μS) × K
Where K is a conversion factor depending on the ionic composition. For typical hydroponic solutions, K ≈ 0.5–0.7 (so 2,000 μS ≈ 1,000–1,400 ppm TDS). For marine aquariums, K ≈ 0.64 (seawater standard). For industrial solutions, K varies more widely.
Ppm mode is practical when the user is accustomed to TDS terminology (typical in the US market and water treatment plants). μS mode is more scientifically precise because it is the DIRECTLY MEASURED parameter.
Hydroponic cultivation: The core application
Hydroponic cultivation replaces soil with a nutrient solution that circulates around the plant roots. The composition and concentration of the nutrients are entirely the operator’s responsibility. The conductivity meter is the first critical QC instrument for hydroponics.
Typical target values for different vegetables:
- Lettuce and other leafy greens: 800–1,500 μS/cm. Low-level plants.
- Herbs (basil, mint, coriander): 1,000–1,800 μS/cm.
- Tomatoes in the vegetative phase: 1,500–2,000 μS/cm.
- Tomatoes in the fruiting phase: 2,000–3,000 μS/cm.
- Peppers and eggplants: 2,000–2,500 μS/cm.
- Cucumbers: 1,700–2,500 μS/cm.
- Strawberries: 1,000–1,800 μS/cm.
- Cannabis (medical cultivation): 1,000–2,500 μS/cm depending on the phase.
Below range = starvation (nutrient deficiency manifests as pale color, slow growth). Above range = salt toxicity (roots cannot absorb water, plants wilt despite the presence of solution). Conductivity measurement reveals both problems before they become visible.
Other applications
Marine aquaristics (saltwater): Seawater salinity is measured as conductivity. Standard 35,000 μS for fully marine water. Brackish water 5,000–25,000 μS. The COND4’s 19,900 μS upper limit covers brackish water but not fully marine water (for that, a specialized salinity refractometer or high-range conductivity meter is needed).
Cleaning verification (Clean-in-Place, CIP): In the food industry, verification is required that rinse water does NOT contain residues of cleaning agents. Conductivity measurement below 500 μS confirms that the rinse phase is complete.
Agricultural water QC: The salt content of irrigation water affects long-term soil quality. Runoff water is checked for leaked fertilizer.
Pond and lake water monitoring: Fish farms regularly measure water conductivity. Abnormal values indicate pollution or degradation of water quality.
Industrial process fluids: Cooling loops, washing solutions, semiconductor production (deionized water verification). The COND4’s high range covers many industrial applications but not all — specialized instruments exist for specific chemicals.
Pools and spas: Levels of salts and chlorine can affect conductivity.
Car battery acid concentration: Lead-acid battery electrolyte (sulfuric acid) has extremely high conductivity (> 500,000 μS — outside the COND4’s range), but workshop testers for VALUE-diluted acid work with this type of meter.
Janitorial service QC: During large-scale floor cleaning with floor wax, the cleaning solution has specific conductivity targets. Conductivity measurement verifies correct dilution.
Battery and practicality
The COND4 is designed for field use:
- 100 hours of battery life = weeks to months with intermittent use.
- Automatic shutdown to save battery.
- IP65 protection — withstands rain, coolant spray, rinse water.
- Pocket size — fits in the breast pocket of a workshop work jacket.
- Protective cap for the sensor included — prevents drying of the calibration electrode between uses.
Calibration
Conductivity meters drift slowly over time due to:
- Electrode surface degradation from exposure.
- Lime deposits on the electrode when measuring in hard water.
- Fat and protein residues from food applications.
Standard calibration cycle: monthly or every 100 measurements (whichever comes first) with a reference solution. Most common is KCl 1,413 μS solution (potassium chloride at a known concentration). Reference solutions cost 100–300 SEK per bottle and last for 6–12 months of normal use.
What the COND4 does NOT replace
- pH meter: Conductivity and pH are different parameters. High conductivity tells you nothing about whether the liquid is acidic or basic. For full hydroponic QC, BOTH meters are needed.
- Single-ion meters: NO₃ meters, Ca²+ meters, K+ meters show SPECIFIC ion concentrations. The COND4 shows total ion concentration.
- DO meter (dissolved oxygen): For aquariums and fish farming, dissolved oxygen is critical.
- Chlorine tester: For pools and cleaning verification.
- Extreme-range measurement: Seawater (35,000+ μS) and deionized water (below 100 μS) require other instruments.
What you get for the money
ETI COND4 pocket-format conductivity meter, measurement range 1,000–19,900 μS/cm, resolution 100 μS, accuracy ±2% of full scale, automatic temperature compensation 0–50 °C with 25 °C standard display, switchable between μS and ppm (TDS), 6 mm easy-to-read LCD display, IP65 waterproofing, 100 hours of battery life, automatic shutdown, protective cap for sensor. 1,224 SEK.
The standard instrument for hydroponic cultivation, marine aquaristics, industrial process QC, agricultural control, and other applications where conductivity monitoring in the 1,000–20,000 μS range is critical. Portable, robust, economical. For extreme ranges or precision research work, dedicated alternatives exist, but for field use, this instrument is the economical sweet spot.
Read more: ETI COND4 conductivity meter in the shop →
