How to determine the hardness of water?

Water hardness is one of those water quality issues that can look harmless at first but create serious operational headaches over time. A kettle with scale, a blocked nozzle, a boiler tube with mineral build-up, a cooling system losing efficiency, or a water treatment plant struggling to maintain stable quality can all point back to the same underlying concern: minerals in the water.

For South African industries, mines, municipalities, commercial facilities, food and beverage plants, laboratories, farms, hospitals, and water treatment operations, understanding hardness is not only about knowing whether water is “hard” or “soft”. It is about protecting equipment, improving process control, supporting compliance, reducing downtime, and making confident decisions about treatment.

That is where water hardness tests become important.

Prei Instrumentation works with water analytics and monitoring solutions that help facilities understand what is happening in their water systems. Hardness is often assessed alongside related parameters such as alkalinity, conductivity, dissolved solids, pH, turbidity, iron, and silica, because water rarely behaves according to a single measurement alone. The better your data, the better your treatment decisions.

This guide explains how to determine the hardness of water, what different water hardness tests measure, which testing methods are commonly used, and how to choose the right approach for your application.

What is water hardness?

Water hardness refers mainly to the concentration of dissolved calcium and magnesium minerals in water. These minerals occur naturally as water moves through soil, rock, and underground formations. In areas where water passes through mineral-rich geology, the hardness level can be higher.

Hardness is usually expressed as milligrams per litre as calcium carbonate, often written as mg/L as CaCO₃. This standard expression makes it easier to compare results, even when calcium and magnesium occur in different forms.

In simple terms:

• Soft water contains lower concentrations of calcium and magnesium.
• Hard water contains higher concentrations of calcium and magnesium.

Very hard water can lead to scale formation, poor soap performance, higher chemical consumption, reduced heating efficiency, and increased strain on pumps, valves, heat exchangers, and other process equipment.

For domestic users, hard water may be noticed as scale on taps or appliances. For industrial users, it can be far more expensive. Scale can reduce heat transfer, restrict flow, interfere with instruments, damage equipment, and increase maintenance demands.

Why water hardness matters in South Africa

South Africa has diverse water sources, from municipal water and borehole water to surface water, process water, industrial effluent streams, and treated water used in manufacturing. Water quality can vary significantly between provinces, catchments, seasons, and sources.

A facility using borehole water in Gauteng may face very different mineral conditions from a coastal plant, a mining operation in the Northern Cape, a food processing facility in KwaZulu-Natal, or a water treatment site in the Western Cape. Even within the same site, hardness can shift depending on rainfall, abstraction depth, treatment changes, or blending between supply sources.

That variability makes water hardness tests valuable for:

• Boiler feedwater monitoring
• Cooling tower control
• Reverse osmosis pre-treatment
• Borehole water assessment
• Industrial water treatment
• Drinking water process checks
• Food and beverage production
• Mining and mineral processing
• Laboratory water analysis
• Wastewater and effluent monitoring
• Municipal and commercial water systems

For many operations, hardness is not a once-off question. It needs to be checked, recorded and interpreted as part of a wider water quality programme.

What do water hardness tests measure?

Water hardness tests measure the amount of hardness-causing minerals in a water sample. In most cases, the focus is on calcium and magnesium.

There are different types of hardness that may be considered:

• Total hardness measures the combined concentration of calcium and magnesium.
• Calcium hardness measures the calcium portion of the hardness.
• Magnesium hardness can be calculated when total hardness and calcium hardness are known.
• Temporary hardness is linked mainly to bicarbonates and can often be reduced by boiling or softening processes.
• Permanent hardness is associated with minerals such as sulphates and chlorides and generally needs chemical or physical treatment to remove.

For industrial applications, total hardness is often the starting point, but it may not be enough on its own. A boiler system, for example, may also require alkalinity, silica, conductivity, pH, iron, and total dissolved solids to be monitored. A reverse osmosis system may require hardness data to protect membranes from scaling. A cooling tower may require hardness testing as part of cycle-of-concentration and scale-control management.

Common signs that water may be hard

Before formal testing, hard water often reveals itself through visible or operational symptoms. These signs are useful warnings, although they should never replace proper testing.

Common signs include:

• White or chalky deposits on taps, valves, or fittings
• Scale build-up inside kettles, pipes, boilers, or heat exchangers
• Reduced flow through nozzles, spray heads, or small pipework
• Poor soap or detergent performance
• Higher chemical dosing requirements
• Reduced boiler or heater efficiency
• Increased maintenance on pumps and process equipment
• Spots or residue on washed surfaces
• Scaling around cooling tower components
• Membrane fouling in filtration or reverse osmosis systems

In industrial settings, waiting for visible scale is risky. By the time scale appears, efficiency losses or equipment damage may already have started. Regular water hardness tests help identify the issue earlier.

The main methods used to determine water hardness

There are several ways to determine the hardness of water. The right method depends on the level of accuracy required, the application, the testing environment, and whether results are needed occasionally or continuously.

Test strips

Test strips are one of the simplest forms of water hardness tests. A strip is dipped into a water sample, and the colour change is compared with a reference chart.

They are useful for quick screening and basic checks, especially where a fast indication is enough. However, they are not ideal for critical industrial decisions because colour interpretation can be subjective and the result may not be precise enough for treatment control.

Best for:

• Quick indication of hard or soft water
• Non-critical checks
• Initial screening
• Educational or domestic use

Limitations:

• Lower accuracy
• Subjective colour matching
• Limited measurement range
• Not ideal for process control

Drop-count titration kits

Titration-based water hardness tests are widely used because they provide a more practical and repeatable result than basic strips. In a typical test, a reagent is added drop by drop to a measured water sample until a colour change occurs. The number of drops used is then converted into hardness.

This method is common in water treatment, boiler rooms, cooling tower management, and field testing. It is more accurate than strips when performed correctly, although operator technique still matters.

Best for:

• Field testing
• Boiler and cooling tower checks
• Routine treatment verification
• Service technicians
• Sites needing portable test methods

Limitations:

• Manual process
• Reagent handling required
• Operator-dependent results
• Not continuous

Laboratory analysis

Laboratory testing is used when higher confidence, formal reporting, or detailed water chemistry is required. A properly collected sample is sent to a laboratory, where hardness and related parameters can be analysed using controlled methods.

Laboratory analysis is useful for baseline assessments, compliance support, new water source investigations, troubleshooting, and system design. For example, if a South African facility is commissioning a new borehole, laboratory water analysis can help determine whether softening, filtration, dosing, or additional treatment is needed.

Best for:

• Baseline water quality assessment
• Formal reporting
• New borehole or source evaluation
• Troubleshooting complex water problems
• Multi-parameter water analysis

Limitations:

• Slower turnaround time
• Requires correct sample collection
• Not real-time
• May need repeat testing for variable sources

Online and continuous monitoring

In industrial environments, occasional manual testing may not be enough. If water hardness can change quickly, or if the consequences of scale are severe, online monitoring can provide more consistent process visibility.

Online monitoring helps operators track trends, identify changes earlier, and respond before problems escalate. Continuous or regular monitoring is especially useful where water treatment systems protect high-value equipment.

Best for:

• Industrial process water
• Boiler and cooling systems
• Water treatment plants
• High-risk scaling applications
• Automated process control
• Sites needing trend data

Limitations:

• Requires correct specification
• Needs calibration and maintenance
• Must be matched to the application
• Should be integrated into a broader monitoring strategy

How to determine the hardness of water step by step

The testing process should be practical, repeatable, and suited to the water system being assessed. Here is a clear approach.

Step 1: Identify why the water is being tested

Before choosing a test method, define the reason for testing. The correct approach for a household borehole is not necessarily the correct approach for a boiler plant, cooling tower, food production line, or municipal treatment process.

Ask:

• Is the water used for drinking, production, heating, cooling, or cleaning?
• Is the concern scale, treatment performance, membrane protection, or compliance?
• Is this a once-off test or part of routine monitoring?
• How accurate does the result need to be?
• How quickly are results needed?
• What action will be taken based on the result?

Testing should always support a decision. If the result will influence dosing, equipment protection, or process control, a more robust method is usually needed.

Step 2: Collect a representative sample

A poor sample can lead to a misleading result. When performing water hardness tests, collect water from a point that represents the system you want to understand.

For example:

• Test raw water before treatment to understand incoming hardness.
• Test after a softener to confirm treatment performance.
• Test boiler feedwater before it enters the boiler.
• Test cooling tower make-up water and recirculating water separately.
• Test borehole water after flushing stagnant water from the line.

Use clean sampling containers and avoid contamination. Where laboratory analysis is required, follow the laboratory’s sampling instructions.

Step 3: Choose the correct testing method

The choice of method depends on the application.

• For a quick indication, a test strip may be enough.
• For routine plant checks, a titration kit may be more suitable.
• For detailed analysis, use laboratory testing.
• For process-critical systems, consider online or regular instrument-based monitoring.

Prei Instrumentation focuses on helping clients look at water quality in context. Hardness is often linked to other water quality measurements, so the best testing plan may include conductivity, pH, ORP, turbidity, dissolved oxygen, suspended solids, dissolved solids, alkalinity, silica, or ion-specific monitoring, depending on the application.

Step 4: Express the result correctly

Hardness results are commonly expressed as mg/L as CaCO₃. This is important because different instruments, kits, or reports may use different units.

Common units include:

• mg/L as CaCO₃
• ppm as CaCO₃
• mmol/L
• German degrees
• French degrees
• grains per gallon

For South African industrial and treatment environments, mg/L as CaCO₃ is widely understood and useful for comparison. When comparing data from different sources, always check the unit before making decisions.

Step 5: Interpret the result according to the application

A hardness result does not mean much without context. The same hardness level may be acceptable in one system and problematic in another.

For example, moderately hard water may not create an urgent concern in a general domestic supply, but it may be unsuitable for a high-pressure boiler without proper treatment. Water that looks acceptable for washing may still create scaling risk in heat exchangers or membranes.

Interpretation should consider:

• Temperature
• pH
• Alkalinity
• Conductivity
• Total dissolved solids
• Silica
• Iron
• Flow rate
• Treatment method
• Equipment sensitivity
• Operating pressure
• Chemical dosing strategy

This is why hardness is best viewed as part of a complete water analysis programme rather than an isolated number.

Step 6: Track trends over time

A single test gives a snapshot. Trends give insight.

If your facility relies on borehole water, seasonal changes may affect hardness. If you blend water sources, hardness may shift when the blend changes. If a softener begins to fail, hardness may creep upward before visible scale appears. If a dosing programme is not correctly controlled, related parameters may start moving out of range.

Tracking water hardness tests over time helps identify patterns and prevent surprises.

Water hardness and industrial equipment protection

Hard water is a major concern in industrial systems because calcium and magnesium can form scale when water is heated, concentrated, or chemically altered. Scale acts as an insulating layer, which means heat transfer becomes less efficient.

In boilers, scale can increase fuel consumption and create dangerous hot spots. In cooling systems, it can reduce efficiency and restrict flow. In pipes and valves, scale can narrow passages and increase pressure losses. In reverse osmosis systems, hardness can contribute to membrane scaling, reducing performance and increasing cleaning requirements.

For this reason, hardness testing is closely linked to asset protection.

Facilities that benefit from hardness monitoring include:

• Boiler houses
• Cooling towers
• Manufacturing plants
• Food and beverage facilities
• Mining operations
• Laboratories
• Hospitals and healthcare facilities
• Laundries
• Water treatment companies
• Municipal plants
• Commercial buildings
• Agricultural processing sites

In these environments, water hardness tests can support better maintenance planning, lower chemical waste, improved treatment performance, and fewer unplanned interruptions.

The relationship between hardness, conductivity, and dissolved solids

Conductivity is not the same as hardness, but it can provide useful supporting information. Conductivity measures how well water conducts electricity, which is influenced by dissolved ions in the water. Hardness minerals contribute to conductivity, but so do many other dissolved substances.

That means conductivity can help show overall changes in water quality, but it should not be used as a direct replacement for water hardness tests when calcium and magnesium levels matter.

For example, a rise in conductivity may indicate more dissolved material in the water, but it does not automatically confirm that hardness has increased. Further analysis may be needed to understand what has changed.

A strong water monitoring programme often combines hardness testing with conductivity, pH, and other parameters to build a more complete picture.

Why pH and alkalinity matter when testing hardness

Hardness, pH, and alkalinity are closely connected in many water systems. Together, they influence whether water is likely to be corrosive, stable, or scale-forming.

High hardness with certain pH and alkalinity conditions can increase scaling risk. Low hardness with low alkalinity may create different concerns, such as corrosive behaviour in some systems. For boilers and cooling towers, these relationships are especially important.

This is why water hardness tests should not be viewed in isolation. If your facility is dealing with scale, corrosion, poor treatment performance, or inconsistent water quality, hardness is only one part of the investigation.

Water hardness in boiler systems

Boilers are particularly sensitive to hardness. When hard water enters a boiler, calcium and magnesium salts can precipitate as scale under high temperature conditions. Even a thin layer of scale can reduce heat transfer and increase energy consumption.

In boiler applications, hardness control is usually linked to:

• Softener performance
• Feedwater quality
• Blowdown control
• Alkalinity monitoring
• Silica monitoring
• Conductivity measurement
• Total dissolved solids management
• Iron testing
• Chemical dosing

For South African facilities facing rising energy costs and pressure to reduce downtime, boiler water quality cannot be treated casually. Regular water hardness tests help confirm that treatment systems are performing as expected.

Water hardness in cooling towers

Cooling towers lose water through evaporation, which concentrates minerals in the remaining water. As hardness minerals concentrate, scale risk increases. If scale forms on heat transfer surfaces, cooling efficiency can decline and operating costs can rise.

In cooling tower management, hardness testing may be used alongside conductivity, pH, alkalinity, microbiological control, and chemical dosing checks.

Testing helps operators decide when to adjust blowdown, review dosing, inspect treatment performance, or investigate make-up water quality.

Water hardness and borehole water

Many South African homes, farms, lodges, factories, and commercial properties use borehole water either as a primary supply or as a backup to municipal water. Borehole water can be highly variable, and hardness is often one of the first parameters that should be checked.

Borehole water hardness matters because it can affect:

• Pumps and pipework
• Irrigation systems
• Domestic plumbing
• Water heaters
• Filtration systems
• Softener selection
• Reverse osmosis performance
• Taste and aesthetic quality
• Industrial suitability

A proper borehole water assessment should not stop at hardness. Depending on the use, it may also need iron, manganese, pH, conductivity, total dissolved solids, turbidity, microbial testing, and other parameters.

Choosing the right water hardness test for your facility

To choose the right test, start with the risk level.

• For low-risk applications, occasional manual testing may be enough.
• For operational systems, routine titration or laboratory analysis may be more appropriate.
• For critical industrial applications, online monitoring or structured water analytics may be necessary.

Consider these questions:

• What happens if hardness increases unnoticed?
• Could scale damage expensive equipment?
• Is the water heated, pressurised or concentrated?
• Are membranes, boilers or heat exchangers involved?
• Is the water source stable or variable?
• Is the test for internal control, troubleshooting, or reporting?
• Do operators need immediate results?
• Should data be trended over time?

A simple test can tell you whether hardness is present. A well-designed testing strategy tells you what to do about it.

Common mistakes when testing water hardness

Water hardness tests are straightforward in principle, but errors can still happen. Some of the most common mistakes include:

• Testing at the wrong point: Testing treated water when the issue is raw water, or testing raw water when the concern is post-treatment performance, can lead to the wrong conclusion.
• Ignoring units: A result in ppm, mg/L as CaCO₃, mmol/L or grains per gallon must be interpreted correctly. Unit confusion can lead to incorrect dosing or treatment decisions.
• Relying only on visual signs: Scale is a late warning sign. Testing should happen before visible build-up becomes a maintenance problem.
• Using conductivity as a direct hardness result: Conductivity is useful, but it does not measure hardness specifically.
• Testing once and assuming the result is permanent: Water quality can change. Borehole levels, rainfall, municipal blending, treatment performance, and process conditions can all affect hardness.
• Looking at hardness alone: Hardness should often be considered alongside pH, alkalinity, conductivity, dissolved solids, silica, iron, and other relevant water quality parameters.

How Prei Instrumentation supports water hardness testing strategies

We support South African facilities with water analytics products and solutions that help make water quality easier to understand, monitor, and manage.

Our water analytics offering includes instrumentation and monitoring support for key water quality parameters used across industrial, drinking water, process water, and wastewater applications. These parameters may include conductivity, pH, ORP, turbidity, dissolved oxygen, suspended solids, fluoride, ammonium, chlorine, and other application-specific measurements.

When hardness is part of the water quality challenge, we help clients consider the broader picture. In many cases, the best solution is not simply to run one hardness test. It is to understand the water source, the process, the equipment being protected, and the related measurements that influence scale, corrosion, and treatment performance.

We assist clients who need practical support for:

• Water analysis planning
• Industrial water monitoring
• Instrument selection
• Process measurement
• Water treatment applications
• Drinking water and process water quality checks
• Wastewater and effluent monitoring
• Turbidity, conductivity, pH, and dissolved oxygen measurement
• Supporting data for maintenance and treatment decisions

Our goal is to help facilities move from guesswork to reliable measurement.

When should you test water hardness?

The right testing frequency depends on the application, but hardness should be checked whenever water quality affects performance, safety, or cost.

You should consider water hardness tests when:

• Commissioning a new water source
• Installing or checking a softener
• Protecting a boiler or cooling tower
• Investigating scale formation
• Designing a treatment system
• Setting up reverse osmosis pre-treatment
• Using borehole water
• Experiencing unexplained equipment fouling
• Reviewing water treatment chemical use
• Monitoring industrial process water
• Comparing municipal and alternative water sources
• Troubleshooting inconsistent product quality

For high-risk systems, hardness should be tested regularly and trended over time.

How accurate do water hardness tests need to be?

Accuracy depends on the decision being made.

If you only need to know whether water is generally hard or soft, a basic test may be acceptable. If you are adjusting chemical dosing, protecting boiler tubes, or managing membrane scaling risk, you need more reliable data.

Industrial decisions should be based on repeatable, fit-for-purpose measurements. This may require better sampling, calibrated equipment, laboratory analysis, or online monitoring.

In other words, the more expensive the consequence of getting it wrong, the more robust your testing method should be.

Water hardness tests and treatment decisions

Once hardness is known, a facility can decide whether treatment is needed.

Common hardness treatment options include:

• Ion exchange softening
• Reverse osmosis
• Lime softening
• Chemical dosing
• Anti-scalant programmes
• Blowdown control
• Filtration as part of broader treatment
• Process adjustments based on water chemistry

The correct treatment depends on the water quality, required output, flow rate, budget, system design, and operational goals. A water hardness result is the starting point, not the full design.

That is why proper water analysis is so important. A treatment system designed only around hardness may miss other issues such as iron, silica, pH imbalance, turbidity, or high dissolved solids.

Why professional water analytics matter

Water quality decisions affect equipment, budgets, compliance, production, and maintenance. Professional water analytics help facilities avoid assumptions and respond based on evidence.

For South African operations, this matters even more because water reliability and quality can be challenging. Many sites are investing in alternative water sources, backup water storage, boreholes, treatment systems, and process optimisation. Without reliable measurement, these systems are harder to manage.

Professional instrumentation and analysis support can help identify:

• What is in the water
• How water quality changes over time
• Whether treatment is working
• Where scale or corrosion risk may develop
• Which parameters need closer monitoring
• When maintenance or process changes are required

Water hardness tests are an important part of this picture, especially where mineral build-up can affect performance.

FAQs about water hardness tests

What are water hardness tests?

Water hardness tests measure the concentration of hardness-causing minerals in water, mainly calcium and magnesium. Results are usually expressed as mg/L as calcium carbonate, or mg/L as CaCO₃.

How do I determine the hardness of water?

You can determine water hardness using test strips, titration kits, laboratory analysis, or online monitoring. The best method depends on the accuracy required and the purpose of the test.

What is the most common water hardness test?

Titration kits are commonly used for practical field and plant testing because they provide a clearer result than basic test strips. Laboratory analysis and online monitoring may be better for more critical applications.

Can conductivity tell me water hardness?

Conductivity can indicate the amount of dissolved ions in water, but it does not measure hardness specifically. Water hardness tests are still needed when calcium and magnesium levels are important.

Why is hard water a problem in industrial systems?

Hard water can cause scale inside boilers, cooling systems, pipes, valves, heat exchangers, and membranes. This can reduce efficiency, increase maintenance, and shorten equipment life.

Is hard water unsafe to drink?

Hardness is usually an aesthetic and operational concern rather than a direct health concern. However, drinking water quality should always be assessed according to the full range of relevant physical, chemical, and microbiological parameters.

How often should water hardness be tested?

Testing frequency depends on the application. A stable municipal supply may only need periodic checks, while boilers, cooling towers, boreholes, and industrial treatment systems may require regular monitoring.

Why is borehole water often tested for hardness?

Borehole water can contain naturally dissolved minerals from underground rock formations. Testing helps determine whether softening, filtration, or additional treatment is needed.

What unit is used for water hardness?

Water hardness is commonly reported as mg/L as CaCO₃. Some tests may use ppm, mmol/L or other units, so it is important to confirm the unit before interpreting results.

Can Prei Instrumentation help with water hardness testing?

Yes. We support water quality and water analytics applications by helping clients understand the broader measurement requirements around hardness, conductivity, pH, turbidity, dissolved oxygen, dissolved solids, and other key parameters.

Get Reliable Water Quality Insight with Prei Instrumentation

If you need to determine the hardness of water, improve your water testing programme, or understand the wider chemistry affecting your system, we can help.

We provide water analytics solutions for South African industrial, commercial, and treatment environments where accuracy, dependability, and practical support matter. Whether you are dealing with borehole water, process water, drinking water, cooling systems, boiler feedwater, or wastewater, our team can help you identify the right monitoring approach for your application.

Speak to us about water hardness tests, water analysis, and supporting instrumentation that helps protect your equipment, improve decision-making, and keep your water systems under control.