In water treatment, chemical, power and other industries, water quality total hardness is a critical indicator. Excessively high hardness tends to cause pipeline scaling, while large hardness fluctuations will affect process control. Although the traditional manual titration method is accurate, it cannot provide real-time feedback and is labor-intensive. Therefore, an increasing number of enterprises choose online total hardness sensors.
With many products available on the market, how to select a stable, durable and easily integrated sensor? Taking the ZW-TH-PR online total hardness sensor as an example, this article explains the key points for sensor selection.
Why is Online Total Hardness Monitoring Needed?
Total hardness mainly refers to the concentration of calcium and magnesium ions in water. The benefits of real-time monitoring are obvious:
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Scaling prevention: Timely adjust the dosage of scale inhibitor in boilers and heat exchangers.
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Process control: In food, beverage and pharmaceutical industries, water hardness directly affects product quality.
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Early warning: A sudden rise in hardness allows quick detection of water source pollution or equipment leakage.
Online sensors can output data continuously and realize automatic control when connected with PLC or host computers.
Sensor Principle: Ion-Selective Electrode (ISE) Method
Most online hardness sensors adopt the Ion-Selective Electrode (ISE) method. The ZW-TH-PR is based on a PVC membrane calcium-magnesium selective electrode, with a reference system as its core.
The ZW-TH-PR is equipped with patented microporous salt bridge technology. The reference electrolyte slowly seeps out under pressure to form a stable liquid junction potential. Compared with ordinary electrodes, this design delivers a more stable reference system and longer electrode service life.
It also has a built-in Pt1000 temperature sensor for automatic temperature compensation, eliminating errors caused by seasonal changes within the range of 0~40℃.
Key Selection Indicators
1. Measuring Range and Resolution
The ZW-TH-PR has a measuring range of 0~1000.0 mg/L (calculated as CaCO₃), a resolution of 0.1 mg/L and a minimum detection limit of 0.1 mg/L. This range covers most industrial water applications (drinking water < 150 mg/L, circulating water 300~800 mg/L). If the on-site hardness often exceeds 1000 mg/L, a wider-range model or diluted sampling is required.
2. Accuracy and Response
Accuracy: ±10% of reading; Response time (T90): < 60 seconds.±10% accuracy is sufficient for process trend monitoring, and higher precision can be achieved by regular comparison and calibration with laboratory methods.
3. Operating Conditions
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Temperature: 0~40℃
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Pressure: ≤ 0.2 MPa
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pH range: 4~10
If the water pH exceeds the limit or contains high-concentration interfering ions (e.g., sulfides, heavy metals), electrode poisoning may occur, requiring pretreatment or special electrodes.
4. Material and Protection
The ZW-TH-PR offers two housing materials:
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Standard version: POM + ABS, cost-effective, suitable for general industrial water.
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Enhanced version: POM + 316L stainless steel, corrosion-resistant, suitable for seawater or saline wastewater.
It features IP68 protection grade and can be immersed for long-term use.
5. Communication Interface
Standard configuration: RS485 + Modbus RTU protocol (default baud rate 9600, adjustable address); 4-20mA output is optional. Hardness and temperature values can be read in one command for easy system integration.
Installation and Maintenance Tips
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Installation angle: The sensor must be installed at an inclination (≥15°), and inversion or horizontal installation is strictly prohibited to avoid bubble retention.
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Interface: 3/4 NPT thread, supporting submerged or pipeline installation (flow cell required for pipeline installation).
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First use: Activate the electrode by immersing it in tap water with power on for 48 hours, then rinse with deionized water.
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Long-term storage: Take the electrode out of water, cover with a protective cap and store dry to prevent dehydration of the sensitive membrane.
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Cleaning: Rinse the PVC membrane with deionized water if deposits form; avoid contact with organic silicone oil or grease.
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Calibration: Two-point calibration (zero point + slope). Pre-calibrated at the factory, on-site calibration can be performed via Modbus commands with standard solutions.
The electrode is a consumable with a 6-month warranty. Replace it when response is slow or calibration fails.
Selection Summary
Five key points for online hardness sensor selection:
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Range matching: Whether it covers the actual hardness range.
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Operating adaptability: Whether pH, temperature and pressure are within the allowable range.
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Corrosion resistance: Select the housing material according to water corrosiveness.
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Communication compatibility: Whether the protocol is compatible with the existing system.
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Maintenance cost: Electrode service life and replacement frequency.
The ZW-TH-PR is a mature product within the 0~1000 mg/L range. Its patented reference system, IP68 protection and dual material options make it suitable for most industrial scenarios. Comprehensive judgment based on on-site conditions is essential to obtain stable and reliable monitoring data.https://www.zonewuiot.com/water-quality-sensors_c1
