AirSprayTech Academy Certificate Program
2K and 3K Coating Systems for OEM Product Finishers
Article 16 of 24
Flow Monitoring, Pressure Monitoring, Alarms, and Interlocks
Detecting abnormal conditions and preventing questionable material
from reaching production
The Control System Must Do More Than Display Numbers
A production proportioner must measure what the components are doing,
compare actual performance with the approved recipe, identify
abnormal conditions, and take the appropriate action before
questionable coating reaches a production part.
Flow sensors, pressure sensors, valve feedback, gun-trigger signals,
alarms, and interlocks work together to provide this protection.
Their value depends on proper selection, installation, calibration,
programming, testing, and operator response.
Learning Objectives
After completing this article, the learner should be able to:
- Explain the difference between monitoring, an alarm, and an interlock.
- Identify important flow and pressure conditions in a 2K or 3K system.
- Recognize common ratio, dose-time, pot-life, purge, and supply alarms.
- Explain how gun-trigger and material-flow signals should correspond.
- Describe the role of the proportioner, PLC, robot, booth, and conveyor controls.
- Develop an alarm-response and product-containment procedure.
- Recognize why bypasses and automatic resets require strict control.
Monitoring, Alarms, and Interlocks
Monitoring measures or observes a process
condition. Examples include component flow, fluid pressure,
material temperature, valve status, gun trigger, and elapsed
pot-life time.
An alarm communicates that a measured or
calculated condition has moved outside an established limit or
that an expected action did not occur.
An interlock prevents or stops an action until
defined conditions are satisfied. An interlock may prevent
spraying, mixing, color change, electrostatic operation, robot
movement, or production release.
An alarm that only flashes on a screen is not equivalent to an
interlock that stops off-ratio material from being applied.
Define What Must Happen When a Fault Occurs
Every important fault requires a predetermined response. Depending
on the risk, the system may warn the operator, stop component
dosing, close the material valves, disable the applicator, command
the robot to a safe position, stop the conveyor, initiate a purge,
or place affected production on hold.
The correct response must be established during system design and
process qualification. It should not be invented by an operator
while the alarm is active.
Component Flow Monitoring
Flow monitoring determines whether Components A, B, and C are being
delivered in the expected amounts. Depending on the proportioner,
this may be measured by gear meters, Coriolis meters, pump-stroke
sensors, dosing-cylinder position, or another approved measurement
method.
Flow information can be used to evaluate:
- Total mixed-material flow rate
- Individual component delivery
- Calculated mixing ratio
- Dose completion time
- Minimum and maximum operating flow
- Material consumption by recipe, shift, part, or work order
- Whether material is moving when the gun is expected to be spraying
No-Flow and Unexpected-Flow Conditions
No Flow When Flow Is Expected
If the applicator is commanded to spray but the system detects no
material movement, the cause may be an empty supply, closed valve,
clogged filter, blocked tip, failed pump, lost pressure, air in the
supply, defective sensor, or incorrect control signal.
Flow When No Flow Is Expected
Material movement while the gun should be closed may indicate a
leaking applicator valve, leaking dose valve, open dump valve,
unintended circulation, hose expansion, pressure decay, or an
incorrect input signal.
Both conditions require investigation. Unexpected flow may waste
material, contaminate a purge, create an incorrect ratio, or allow
coating to enter an unintended location.
Dose-Time Monitoring
Sequential-dose systems often deliver one component and then inject
the other component in controlled increments. Each dose must be
completed within an expected period.
A dose-time alarm indicates that the expected quantity was not
registered within the programmed time. Possible causes include low
supply pressure, an empty container, a clogged filter, air
entrainment, high viscosity, a sticking valve, a failed pump, a
meter problem, or leakage.
A dose-time alarm is evidence that the system could not confirm
proper delivery. Clearing the message without identifying the cause
does not establish that the material in the mixed path is acceptable.
Pressure Monitoring
Pressure sensors provide information about the energy moving each
component through the system. Pressure alone does not prove correct
flow, but it can identify conditions that threaten ratio control,
equipment integrity, or application performance.
Useful monitoring locations may include:
- Component supply outlets
- Pump or pressure-pot discharge
- Before and after filters
- At metering or dosing equipment
- Immediately upstream of the mix manifold
- In the mixed-material line
- Near the applicator when required for flow control
What Pressure Behavior Can Indicate
| Pressure Condition |
Possible Meaning |
| Pressure below the approved range |
Empty supply, pump starvation, air entrainment, regulator problem, leak, or inadequate supply capacity. |
| Pressure above the approved range |
Restriction, closed valve, incorrect regulator setting, cured material, undersized passage, or thermal pressure increase. |
| Pressure fluctuates rapidly |
Pump pulsation, unstable air supply, cavitation, sticking regulator, intermittent restriction, or rapidly changing gun demand. |
| Pressure rises while flow falls |
A downstream restriction, blocked filter, plugged mixer, advancing material, obstructed hose, or worn tip being replaced by an incorrect size. |
| Pressure falls while demand rises |
The material-supply system may not have sufficient capacity for the production demand. |
Pressure Balance and Ratio Control
The component streams do not always require identical pressures, but
each must remain within the operating range required by the dosing
method. Excessive pressure imbalance can affect valve response,
injection stability, meter performance, check-valve sealing, and
the ability of the system to make small corrective doses.
Pressure should not be adjusted merely to make the display appear
stable. The correct settings must support accurate delivery across
the full production flow range and must remain within the ratings of
every wetted component.
Gun-Trigger and Flow Confirmation
The proportioner needs to know when material is expected to move.
Depending on the application equipment, this signal may come from an
automatic-gun command, an air-flow switch, a fluid-flow switch, a
robot output, a PLC signal, or another approved monitoring device.
The controls should distinguish among at least four conditions:
- Gun commanded on and material flow confirmed
- Gun commanded on but material flow not confirmed
- Gun commanded off and no material flow detected
- Gun commanded off but material flow detected
Comparing command and confirmation signals can expose failed gun
valves, broken cables, closed material valves, incorrect PLC logic,
and leaks that would otherwise remain hidden.
Common Process Alarms
- Ratio alarm: Calculated component delivery moved outside the approved tolerance.
- Dose-time alarm: The required dose was not registered within the permitted time.
- No-flow alarm: Material movement was expected but not detected.
- Unexpected-flow alarm: Material movement was detected when flow should have stopped.
- High- or low-pressure alarm: A monitored pressure moved outside its approved range.
- Pot-life alarm: Activated coating has remained in the mixed path beyond the programmed limit.
- Purge alarm: The required purge sequence, time, volume, valve action, or gun position was not confirmed.
- Supply alarm: A material source is low, empty, unavailable, or not supplying acceptable pressure.
- Meter or sensor alarm: A required signal is missing, invalid, or outside its operating limits.
- Valve alarm: A commanded valve did not reach or confirm the required state.
- Communication alarm: Required data exchange among the proportioner, PLC, robot, or line controls has been lost.
Warnings, Process Stops, and Emergency Stops
Not every abnormal condition requires the same response.
Classification should reflect the potential effect on people,
equipment, coating quality, and production.
- Warning: Calls for attention while controlled operation may temporarily continue.
- Process stop: Stops mixing or spraying because the process can no longer be confirmed.
- Protective shutdown: Places equipment in a defined state to prevent damage or uncontrolled material delivery.
- Emergency stop: Addresses an immediate hazardous condition through the machine's designed emergency-stop system.
A normal process stop is not a substitute for an emergency stop, and
an emergency stop does not automatically complete the flushing,
pressure-relief, product-hold, or recovery steps required after the
immediate hazard is controlled.
Interlocks That Protect the Process
Depending on the system and hazard assessment, interlocks may
prevent production unless:
- The correct recipe and permitted material combination are selected.
- Component supplies are available and within the required pressure range.
- Flow meters, stroke sensors, and controllers are ready.
- The system has completed the required fill or purge sequence.
- The ratio is within the approved tolerance.
- The pot-life timer has not expired.
- The gun is correctly positioned for spraying or flushing.
- Spray-booth ventilation is operating as required.
- Required grounding and electrostatic permissives are satisfied.
- Robot, conveyor, booth, and line controls are in the permitted state.
- No unacknowledged stop-level fault remains active.
PLC and Production-Line Communication
In an automated finishing line, the proportioner may exchange
signals with a programmable logic controller, robot controller,
conveyor, booth controls, applicator controller, color-change
system, and plant data system.
Typical signals may include:
- System ready
- Mix enabled
- Recipe request and recipe confirmation
- Gun trigger command
- Flow or spray confirmation
- Color-change or purge request
- Purge complete
- Alarm active and alarm classification
- Production permissive
- Material consumption and process data
Each signal should have a documented definition. The integrator must
define which device owns the decision, how confirmation is returned,
what timeout applies, and what happens if communication is lost.
Design for Loss of Signal
A disconnected cable, failed sensor, frozen data value, lost network,
or failed controller should not appear to be a normal production
condition. Critical signals should be designed so that a missing or
invalid signal is detectable.
The required response to signal loss must be based on the system
design and risk assessment. Where continued operation would make
material quality uncertain or create a hazard, the system should
move to its approved protective state.
Alarm Priorities and First-Out Information
One initiating fault can create several secondary alarms. For
example, an empty hardener container may cause low pressure, slow
dosing, an off-ratio condition, and a process stop.
The control system should preserve the first detected fault and the
sequence of following events. Without first-out information,
personnel may spend time responding to secondary symptoms while the
original cause remains unresolved.
Alarm history should include time, operating mode, recipe, material
flow, relevant pressures, system response, acknowledgment, reset,
and corrective action when the controller supports those records.
Alarm-Response Matrix
| Alarm |
Immediate Control |
Investigation |
| Off ratio |
Stop questionable material from reaching parts and identify affected production. |
Check supplies, meters, valves, pumps, pressure balance, calibration, and recipe. |
| Dose timeout |
Stop dosing and follow the approved purge or recovery procedure. |
Check component availability, pressure, restriction, air, valves, pump, and meter. |
| Pot life expired |
Prevent spraying and remove or purge aged mixed material. |
Review interruption length, timer settings, renewal volume, and automatic-flush function. |
| Purge incomplete |
Do not release the next recipe to production. |
Check flushing supply, valves, flow, gun position, sequence, and waste path. |
| Communication lost |
Move the process to the approved communication-failure state. |
Check power, cables, network, addressing, controller status, and signal mapping. |
Alarm Acknowledgment Is Not Fault Correction
Acknowledging an alarm confirms that someone has seen the message.
Resetting an alarm allows the controller to re-evaluate its
conditions. Neither action proves that the cause has been corrected.
Restart authorization should require the necessary inspection,
corrective action, purge, ratio verification, test spray, product
containment, and documentation for the specific fault.
Control Alarm Bypasses
A bypass may be necessary for specific commissioning, maintenance,
or troubleshooting work, but uncontrolled bypassing defeats the
protection the system was designed to provide.
Any permitted bypass should be:
- Limited to authorized personnel
- Visible at the operator interface
- Recorded in the event history
- Limited to a defined operating mode or time
- Supported by an approved temporary procedure
- Removed and verified before normal production resumes
Testing Alarms and Interlocks
Commissioning and periodic verification should confirm:
- The sensor detects the intended abnormal condition.
- The controller identifies and displays the correct fault.
- The required output, shutdown, or interlock operates.
- The PLC, robot, booth, or conveyor receives the correct status.
- The process cannot restart while the fault remains active.
- The alarm and response are recorded correctly.
- The approved reset and recovery procedure works as intended.
- Questionable material and affected parts are properly identified.
Testing should use controlled simulations or manufacturer-approved
procedures. Do not create an uncontrolled hazardous condition merely
to prove that an alarm works.
Product Containment and Traceability
When a ratio, flow, pressure, or dosing alarm can affect coating
quality, the facility must determine which parts may have received
questionable material. This requires reliable time stamps and
coordination among the proportioner, line controls, production
records, and part identification system.
The containment boundary should begin at the last verified acceptable
condition and extend until the process has been corrected, verified,
and formally released. Parts should not be released merely because
the alarm was reset.
Key Takeaways
- Monitoring observes a condition, an alarm reports a problem, and an interlock prevents or stops an action.
- Flow and pressure provide different but complementary process information.
- Gun commands should be compared with actual material-flow confirmation.
- Dose-time alarms can reveal supply, restriction, valve, pump, meter, or air-entrainment problems.
- PLC and controller signals require documented ownership, timing, and failure responses.
- The first alarm may identify the initiating fault more accurately than later alarms.
- Acknowledging or resetting an alarm does not correct its cause.
- Questionable material and affected production must remain contained until the process is verified and released.
Knowledge Check
- What is the difference between an alarm and an interlock?
- What can a dose-time alarm indicate?
- Why should gun-trigger commands be compared with material-flow confirmation?
- Does normal pressure prove that the component flow is correct?
- Why is first-out alarm information valuable?
- What must happen before production resumes after a significant ratio fault?
Answer Guide
- An alarm reports an abnormal condition; an interlock prevents or stops an action until required conditions are satisfied.
- Inadequate supply, low pressure, restriction, air entrainment, valve or pump failure, meter problems, or leakage.
- The comparison identifies missing flow when spraying is commanded and unintended flow when the gun should be closed.
- No. Pressure and flow are related, but normal pressure alone does not prove correct component delivery.
- It helps identify the initiating problem before secondary alarms obscure the original cause.
- The cause must be corrected, the fluid path handled as required, ratio and operation verified, affected work contained, and an authorized release completed.
Technical References and Further Study
Professional responsibility: Alarm limits,
interlocks, shutdown responses, and PLC communications must be
designed and validated for the specific coating, equipment,
production line, and hazard assessment. Follow the current coating
manufacturer's technical and safety information and the equipment
manufacturer's installation, operation, and service manuals. When
requirements conflict or remain unclear, obtain written technical
direction before placing the system into production.
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