AirSprayTech Academy Certificate Program
2K and 3K Coating Systems for OEM Product Finishers
Article 07 of 24
Metering, Measurement, and Component Dosing
A plural-component system must know how much of each material is
moving through the process and must control that flow accurately.
Meter selection, dosing-valve performance, calibration, and
operating range determine whether the programmed ratio becomes the
actual ratio.
Learning Objectives
After completing this article, the reader should be able to:
- Explain the difference between measuring flow and controlling flow.
- Describe gear-meter, pump-stroke, and Coriolis measurement methods.
- Recognize the strengths and limitations of each measurement technology.
- Explain how dosing valves maintain the required component ratio.
- Understand meter factors, calibration, resolution, and operating range.
- Identify common causes of inaccurate or unstable flow measurements.
Measurement and Control Are Different Functions
A flow meter or stroke sensor reports how much material has moved.
A dosing valve, controlled pump, regulator, or other device changes
the material delivery. The controller compares measured flow with
the programmed recipe and commands the dosing equipment to correct
the delivery.
A meter does not create the correct ratio by itself. Likewise, a
dosing valve cannot maintain a dependable ratio without accurate
information about component flow.
Reliable proportioning requires a coordinated system of supply
equipment, measurement devices, dosing hardware, software,
calibration data, alarms, and maintenance procedures.
The Basic Electronic Proportioning Sequence
- The operator or production control selects an approved coating recipe.
- Component A begins flowing toward the mix manifold.
- The measuring system reports Component A delivery to the controller.
- The controller calculates the required quantity of Component B and, where applicable, Component C.
- The appropriate dosing valve or controlled pump delivers the additional component.
- Each component’s measured delivery is compared with the target ratio.
- The controller makes corrections and monitors the permitted tolerance.
- An alarm or shutdown occurs if the process cannot remain within its programmed limits.
Gear Flow Meters
A positive-displacement gear flow meter contains precision gears
that rotate as material passes through the meter. Sensors detect
gear movement and send pulses to the controller. Each pulse
represents a calculated quantity of material.
Potential Advantages
- Direct volumetric measurement in the material stream.
- Rapid response to changing flow.
- Useful resolution when properly sized for the required flow.
- Commonly available for industrial coating systems.
- Can support component totals and material-usage records.
Important Limitations
- The moving gears contact the coating and may wear.
- Abrasive pigments or fillers can accelerate wear.
- Very low-viscosity material may leak through internal clearances.
- High-viscosity material can create excessive pressure loss.
- Cured residue or contamination can restrict or lock the gears.
- The meter must be cleaned with materials compatible with its construction.
Pump-Stroke Measurement
A stroke-measurement system determines flow by monitoring the
movement of a positive-displacement pump. The controller uses the
pump’s effective displacement and the measured stroke movement to
calculate material delivery.
Potential Advantages
- The sensor can measure movement without contacting the coating.
- Useful with abrasive, filled, or difficult materials.
- Avoids placing a separate measuring gear set in the fluid stream.
- Can reduce cleaning requirements associated with a separate meter.
- Can provide component-flow information for compatible pump designs.
Important Limitations
- Calculated flow depends on the pump’s actual effective displacement.
- Worn seals, check valves, or pump components can reduce delivered volume.
- Pump movement does not always guarantee that the expected quantity reached the outlet.
- Air, cavitation, incomplete filling, or leakage can create misleading results.
- The system must account properly for pump reversal and changeover behavior.
Coriolis Mass Flow Meters
A Coriolis meter measures mass flow directly by detecting changes in
one or more vibrating measuring tubes as material moves through
them. Depending on the instrument, it may also provide density and
temperature information.
Potential Advantages
- Direct mass-flow measurement.
- No meshing measuring gears in the fluid path.
- Can provide density and temperature as additional process information.
- Useful where mass-based measurement or changing density is important.
- Can process a broad range of suitable liquid properties.
Important Limitations
- Initial equipment cost may be higher than other approaches.
- Meter size and measuring range must match the actual component flow.
- Entrained gas or an incompletely filled measuring tube can affect performance.
- Pressure loss and cleaning requirements must be evaluated.
- Installation, vibration, support, wiring, hazardous-location approval, and configuration must follow the manufacturer’s instructions.
Volumetric Measurement Versus Mass Measurement
Gear meters and displacement calculations commonly report volume.
Coriolis meters measure mass directly. Both approaches can support
accurate proportioning when properly selected, configured, and
maintained.
The controller must use the same ratio basis required by the coating
specification. If the coating ratio is by volume and the system
measures mass, current component-density information and an approved
conversion method may be required.
Software should not silently convert between mass and volume without
documented values, units, and authorization.
Comparison of Measurement Technologies
|
Consideration
|
Gear Meter
|
Stroke Measurement
|
Coriolis Meter
|
| Primary measurement |
Volume |
Calculated displacement volume |
Mass |
| Material contact |
Measuring gears contact material |
Sensor does not contact material |
Material passes through measuring tube |
| Moving measuring parts in fluid |
Yes |
No separate meter |
No meshing gears |
| Abrasive-material concern |
Potential wear |
Avoids separate fluid-contact meter |
Evaluate tube material and pressure loss |
| Additional process data |
Volume and total |
Pump movement and calculated volume |
May include mass, density, and temperature |
This comparison is general. Actual capabilities and limitations
depend on the selected equipment, meter size, material, flow range,
installation, software, and maintenance condition.
Meter Size and Operating Range
A meter should not be selected merely by matching the fluid-line
connection. Every measuring device has an approved operating range.
Performance can decline when actual flow is below or above that
range.
An oversized meter may not provide adequate resolution at low flow.
An undersized meter may create excessive pressure loss or exceed its
maximum flow and speed.
Selection should consider minimum flow, normal flow, maximum flow,
gun triggering, number of applicators, component ratio, viscosity,
pressure, temperature, abrasiveness, cleaning requirements, and
possible future production changes.
Resolution Matters at Small Mixing Ratios
At a high ratio such as 20:1, the hardener or catalyst quantity may
be very small compared with the resin flow. The measuring and dosing
equipment must resolve that small quantity accurately.
If one meter pulse or one dosing-valve opening represents too much
material, the controller may alternate between delivering too
little and too much. The accumulated average may appear acceptable
while short sections of material remain unevenly proportioned.
Meter resolution, dosing-valve size, injection frequency, mixer
volume, gun flow, and acceptable ratio tolerance must be evaluated
as one system.
Dosing Valves
A dosing valve opens and closes in response to commands from the
controller. In many systems, Component A flows continuously while
Component B is injected in controlled increments. Other systems
control multiple components or pumps differently.
Dosing accuracy is influenced by:
- Valve size and flow coefficient.
- Opening and closing response time.
- Component pressure and pressure balance.
- Material viscosity and temperature.
- Air-supply stability for pneumatically actuated valves.
- Seal condition and internal leakage.
- Contamination or cured material on the valve seat.
- Controller timing and feedback speed.
A valve that leaks when commanded closed can continue adding a
component and produce an off-ratio mixture without an obvious
external leak.
Meter Factor and Calibration
A meter factor tells the controller how much material is represented
by a pulse, pump movement, frequency, or other measurement signal.
An incorrect meter factor produces an incorrect calculated flow even
when the sensor operates perfectly.
Meter factors may vary by meter model, size, component, material,
wear condition, and measurement method. They should not be copied
from a different machine without technical verification.
A controlled calibration procedure should identify:
- Meter and component being calibrated.
- Material, batch, viscosity, and temperature.
- Test pressure and flow rate.
- Collection or reference-measurement method.
- Original and adjusted meter factor.
- Required verification runs and acceptance limits.
- Technician, date, authorization, and next required verification.
Changing a meter factor to make a ratio check pass without finding
the cause of the disagreement can hide pump wear, valve leakage,
trapped air, an incorrect test, or a failing meter.
Conditions That Can Corrupt a Flow Reading
|
Condition
|
Possible Result
|
| Entrained air or cavitation |
The system may register movement that does not represent a solid liquid stream. |
| Gear wear or internal slip |
Actual volume differs from the volume assumed by the meter factor. |
| Pump seal or check-valve wear |
Pump movement no longer equals expected delivered volume. |
| Flow below measuring range |
Resolution and accuracy may be inadequate for dependable control. |
| Blocked or partially cured meter |
Pressure rises and measurement becomes irregular or stops. |
| Electrical or signal fault |
Pulses or measurement data can be lost, duplicated, or interrupted. |
| Incorrect controller configuration |
Correct meter signals are converted into incorrect displayed quantities. |
Backflow Prevention and Pressure Balance
Pressure differences can force one component backward into another
component circuit if the system does not prevent reverse flow.
Check valves, isolation valves, and properly designed manifolds help
keep the components separated.
Backflow can contaminate a meter, valve, hose, or supply line with
reactive material. The result may be internal curing, blockage,
component damage, and extensive downtime.
Pressure balance must remain within the range specified by the
equipment manufacturer. A large pressure difference can interfere
with dosing-valve response even when backflow does not occur.
Flow Control After Mixing
Ratio control determines the relationship between components. Flow
control determines the total quantity delivered to the applicator.
Some automated systems include a meter and control valve in the
mixed-material line to regulate total flow.
Total-flow control can help coordinate coating delivery with robot
speed, reciprocator movement, gun triggering, part geometry, or
production recipes.
The mixed-material meter and control hardware must remain within the
coating’s pot-life limits and be included in the flushing and
mixed-volume calculations.
Production Verification and Records
Depending on the system and quality plan, records may include:
- Selected recipe and target ratio.
- Actual component totals.
- Calculated ratio and deviation.
- Instantaneous and average flow.
- Material temperature and pressure.
- Alarm events and operator responses.
- Calibration and ratio-check results.
- Material consumption and waste totals.
- Maintenance or component replacement.
Data should be reviewed for trends, not simply stored. A gradual
change in correction frequency, pressure, or meter factor may reveal
wear or restriction before the process fails.
Metering-System Inspection Checklist
- The correct recipe and ratio basis are selected.
- Meter models, sizes, and factors match the approved configuration.
- Actual flow remains within each device’s approved range.
- Component supply pressures are stable.
- Meters and dosing valves show no external leakage.
- Check valves prevent reverse flow.
- Signal cables and connections are secure and protected.
- Calibration and ratio checks are current.
- Alarm limits are approved and have not been bypassed.
- Maintenance history and corrective actions are documented.
Key Takeaways
- Measurement tells the controller what flowed; dosing hardware changes the flow.
- Gear meters provide direct volumetric measurement but contain moving parts in the fluid stream.
- Stroke sensors calculate delivery from pump movement without placing a separate sensor in the coating.
- Coriolis meters directly measure mass flow and may also report density and temperature.
- Meter technology must match the material, flow range, ratio, pressure, and production process.
- Meter factors and calibration values are controlled process data.
- Entrained air, wear, leakage, restriction, and incorrect configuration can corrupt measurement.
- Ratio control and total-flow control are different functions.
Knowledge Check
-
What is the difference between measuring component flow and controlling component flow?
-
What does a gear flow meter normally measure directly?
-
Why can pump-stroke measurement become inaccurate when a pump is worn or starved?
-
What quantity does a Coriolis meter measure directly?
-
Why is meter resolution especially important at a high mixing ratio?
Answer Guide
1. Measurement reports the delivered quantity; control equipment changes delivery to reach the target.
2. Liquid volume.
3. Pump movement may no longer represent the expected delivered volume because of leakage, incomplete filling, or failed check valves.
4. Mass flow.
5. The minor component quantity is small, so each pulse or valve opening represents a larger percentage of the required dose.
Technical References and Industry Resources
Manufacturer products are referenced as technical examples and not
as endorsements. Use the current coating documentation, equipment
manuals, approved system drawings, software configuration records,
and facility quality procedures for the installed system.
Professional responsibility: Meter type, size,
operating range, meter factor, dosing-valve configuration, ratio
limits, calibration values, and software settings are controlled
process parameters. Do not change them without documented technical
authorization and verification of the resulting system performance.
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