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Portable Plural-Component Coating Systems for Contractors

Article 09 of 24

Proportioning Pumps, Ratio Control, and Pressure Balance

Plural-component equipment must deliver the correct quantity of each component throughout application. Balanced pressures support stable operation, but pressure readings alone do not prove that the coating is being proportioned correctly.

Ratio Is a Quantity Relationship

A coating ratio describes the required quantity of Component A compared with Component B. Depending on the product, that relationship may be specified by volume or by weight. The proportioner must reproduce that relationship within the coating manufacturer's allowable tolerance.

Pressure is the force used to move the components through the system. Pressure affects pump loading, flow, mixing, and spray performance, but equal or similar pressure readings do not establish that the correct quantities were delivered.

Learning Objectives

After completing this article, the contractor should be able to:

• Explain how positive-displacement pumps proportion coating components.
• Distinguish fixed-ratio mechanical systems from electronically controlled systems.
• Explain why pressure balance is important without treating it as proof of ratio accuracy.
• Recognize mechanical and material conditions that can produce off-ratio delivery.
• Perform and document a manufacturer-approved ratio verification.
• Respond appropriately to ratio alarms and abnormal pressure conditions.

Understanding the Specified Mix Ratio

A 4:1 ratio by volume requires four volume units of Component A for every one volume unit of Component B. The total mixed quantity contains five units.

Example: 4:1 by Volume

• Total parts: 4 + 1 = 5
• Component A: 4 ÷ 5 = 80 percent of mixed volume
• Component B: 1 ÷ 5 = 20 percent of mixed volume
• For 10 gallons mixed: 8 gallons A and 2 gallons B

The component identified as A is not universally the resin, base, or isocyanate. Manufacturers use different designations. The contractor must follow the component identity stated in the current product instructions.

Weight Ratio and Volume Ratio Are Not Interchangeable

Components commonly have different densities. A coating specified at 4:1 by weight may require a different proportion by volume. Entering a weight ratio into a volume-metering machine without conversion can produce off-ratio material.

Use the coating manufacturer's published volume ratio whenever one is provided. If a conversion is necessary, use verified component densities and a written calculation approved by the coating manufacturer or another responsible technical authority.

Density values should apply to the specific product, component, batch conditions, and temperature. Do not use an assumed density based only on generic resin type.

How Positive-Displacement Proportioning Works

Positive-displacement pumps move a designed volume of material during each completed stroke or cycle. A plural-component proportioner establishes ratio by controlling the displacement, stroke relationship, measured flow, or timed delivery of the individual components.

Theoretical displacement assumes that each pump chamber fills completely, its inlet and outlet valves seal properly, and the displaced material reaches the mix point. Wear, leakage, cavitation, air, restriction, or incomplete filling can make actual delivery differ from theoretical delivery.

This is why ratio assurance requires equipment condition, calibration, operational monitoring, and physical verification—not pump geometry alone.

Common Electronic Dosing Methods

Pressure Balance Supports Ratio Control—But Does Not Prove It

Component pressures should remain within the operating relationship specified by the equipment manufacturer. Large or unstable pressure differences can affect valve operation, pump loading, check-valve performance, component delivery, and mixing behavior.

However, two pressure gauges showing the same value do not prove equal flow, and equal flow would not be correct for a ratio such as 2:1 or 4:1. Pressure gauges report pressure at their installation points. They do not directly report the quantity of material delivered.

Treat pressure balance as an operating condition and diagnostic indicator. Confirm ratio through the machine's approved monitoring and physical test procedure.

What Creates Unequal Component Pressure?

• Different component viscosities or material temperatures
• Different hose diameters, lengths, or internal conditions
• A restricted filter, valve, heater, hose, or manifold passage
• Incorrect regulator or feed-pump setting
• A leaking, worn, contaminated, or slow-acting check valve
• Cavitation, air entry, low container level, or an empty supply container
• Pump packing leakage or internal bypass
• A plugged mixer or mixed-material restriction downstream
• Incorrect component connections or improperly configured equipment
• Pressure gauges that are damaged, uncalibrated, or unsuitable for comparison

Do not immediately adjust one component pressure to make two gauge needles look alike. Find the reason for the difference and follow the equipment manufacturer's troubleshooting procedure.

Ratio Accuracy and Ratio Tolerance

The nominal ratio is the intended relationship between the components. Ratio tolerance is the permitted deviation from that relationship. The coating manufacturer—not the contractor—determines how much deviation the chemistry can tolerate.

An equipment manufacturer may publish a machine accuracy under defined test conditions. That rating does not automatically establish that every installed system will deliver every coating within that accuracy. Material properties, component condition, calibration, flow range, pressure, temperature, maintenance, and field configuration all affect performance.

The contractor should know both the coating's acceptable ratio tolerance and the equipment's alarm or shutdown limits. A wide alarm setting may permit material outside the coating manufacturer's acceptable range.

Conditions That Can Produce Off-Ratio Material

Do Not Spray Through a Ratio Alarm

A ratio alarm, component-pressure alarm, loss-of-supply condition, unexpected shutdown, or repeated dosing fault is a stop-work event. Continuing to spray can place uncured or improperly cured material on the substrate.

Stop application, identify the affected area and time period, isolate questionable material, correct the equipment condition, complete the required verification, and obtain direction concerning material already applied.

Do not simply reset the alarm and resume spraying without determining its cause. The alarm record and corrective action should become part of the project file.

Physical Ratio Verification

A ratio check collects the unmixed components separately through the equipment's approved test outlets. The collected quantities are then compared by volume or weight according to the specified test procedure.

Use only the procedure in the equipment manual or another written procedure approved for the exact system. Test-outlet location, operating pressure, pump speed, collection time, sample size, temperature, and whether the material is flowing dynamically can affect the result.

General Preparation

1. Confirm the required ratio and whether it is specified by volume or weight.
2. Verify correct component identity, supply level, temperature, agitation, and inlet conditions.
3. Prime the equipment completely and remove trapped air using the approved procedure.
4. Bring the system to the specified test pressure, temperature, and operating condition.
5. Use clean, compatible, accurately graduated containers or calibrated scales.
6. Collect each component simultaneously or in the sequence required by the manufacturer.
7. Collect a sample large enough to reduce reading error without creating unnecessary waste.
8. Record the actual quantities, calculation, result, equipment settings, and operator.

Evaluating a Volumetric Ratio Check

Example

A system intended to deliver 4:1 by volume produces:

• Component A collected volume: 40 fluid ounces
• Component B collected volume: 10 fluid ounces
• Measured relationship: 40 ÷ 10 = 4.0
• Measured ratio: 4.0:1 by volume

This calculation establishes the collected relationship. Acceptance still depends on compliance with the coating manufacturer's tolerance and the equipment manufacturer's test requirements.

Evaluating a Weight-Based Ratio Check

A weight-based check requires clean containers, an appropriate calibrated scale, and correct tare measurements. If the product ratio is specified by weight, compare the net component weights directly.

If the coating is specified by volume but the collected samples are weighed, each component's density must be used to convert weight to volume. Because the components may have different densities, comparing their weights alone would not establish the volumetric ratio.

Remove the container tare weight from each measurement. Do not combine the two components merely to obtain a single total weight; the separate component quantities are required for ratio evaluation.

A Ratio Test Is a Controlled Snapshot

A passing ratio check confirms the collected relationship under the test conditions. It does not guarantee that the system will remain on ratio for the entire project.

Continuous assurance depends on stable material supply, correct temperature, functioning pumps and valves, clean filters, proper calibration, operating within the approved flow range, responding to alarms, preventive maintenance, and repeating verification at the required intervals.

When Should Ratio Be Verified?

Follow the most stringent applicable requirement. Verification commonly occurs:

• During initial equipment commissioning
• At the beginning of a project or production shift
• After changing the material, ratio, recipe, pump size, or mechanical configuration
• After pump, valve, meter, sensor, seal, or controller service
• After loss of prime, an empty container, cavitation, or an off-ratio alarm
• When component pressures, pump movement, color, cure, or spray behavior appear abnormal
• At intervals required by the coating manufacturer, equipment manufacturer, specification, or quality plan

Calibration and Ratio Verification Are Different

Calibration establishes or corrects the relationship between a sensor, meter, pump displacement, or controller value and a known reference. Ratio verification checks the quantity relationship actually delivered during a test.

A system may require calibration before a valid ratio verification can be performed. Likewise, a controller displaying the programmed ratio does not prove that its calibration remains correct.

Calibration should be performed only by qualified personnel using the equipment manufacturer's current instructions, approved test equipment, and required access level.

Documentation That Protects the Project

A ratio-verification record should identify:

• Project, location, date, time, and applicator
• Equipment manufacturer, model, serial number, and configuration
• Coating manufacturer, product, components, batch numbers, and specified ratio
• Ratio basis: volume or weight
• Material and ambient temperatures
• Component supply and operating pressures
• Collected quantities, calculation, result, and acceptance tolerance
• Calibration or test-equipment identification where required
• Alarm status, corrections made, retest result, and authorization to resume spraying

Field Diagnosis: Pressure and Ratio Indicators

Practical Field Rule

Use pressure gauges to understand how the system is operating. Use calibrated metering, approved ratio testing, alarm controls, and documented verification to determine whether the system is delivering the correct component quantities. Never substitute pressure appearance for ratio evidence.

Technical References and Further Reading

These official resources illustrate different approaches to positive-displacement proportioning, ratio monitoring, calibration, pressure control, and equipment verification. Product references are educational examples and do not constitute an endorsement.

• WAGNER, TwinControl Electronically Controlled 2K Systems—Operating Manual: Official operating manual
• WAGNER, TwinControl Mixing and Dosing Systems: Ratio-control and system information
• Graco, XM Plural-Component Sprayers—Operation: Official operating manual
• Graco, Hydra-Cat Fixed-Ratio Proportioning Pumps: Fixed-ratio pump instructions
• Sames, PU3000 Airless Plural-Component Mixing and Dosing Pump: Official equipment information
• ASTM D1475, Standard Test Method for Density of Liquid Coatings, Inks, and Related Products. Use the current published edition when verified density is required for a weight-to-volume conversion.

Professional responsibility: Follow the coating manufacturer's current ratio, tolerance, mixing, application, and cure requirements. Follow the equipment manufacturer's approved pump configuration, calibration, ratio-check, pressure-balance, alarm, operation, and maintenance procedures. Project specifications and applicable regulations also apply. Stop application and obtain written technical direction whenever ratio control cannot be verified or requirements conflict.

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