Melt pressure is one of the most important process variables in an extrusion system. Excessive pressure upstream of the screen, die or nozzle can indicate blockage, incorrect process conditions or mechanical overload and, in the worst case, may result in damage to the extruder, tooling or downstream components.
If melt pressure is therefore used not only for process control but also for a safety-related shutdown, a standard pressure sensor is not automatically sufficient. Sensor, evaluation, safety logic, final element and recurring tests must be considered together as a safety function.
A SIL2-capable melt pressure sensor such as a Dynisco PT46X4 or a correspondingly certified SPX version can form the sensing element of the safety function.
However, the decisive point is:
A SIL2-certified or SIL2-assessed pressure sensor does not automatically make the complete extruder safety function a SIL2 safety function.
For the actual safety integrity, the following must be considered, among other things:
- the safety function must be clearly defined,
- the hazardous process condition must be specified,
- sensor, logic and final element must be assessed together,
- diagnostic capabilities must be taken into account,
- proof-test coverage and proof-test interval must be defined,
- failures and repair times must be considered,
- the complete function must be tested regularly
.
Suitable melt pressure sensors and transmitters can be found under Dynisco products at ICS Schneider as well as under melt pressure transmitters.
Table of Contents
- Why can melt pressure be a safety-related variable?
- What does SIL2 mean for a melt pressure sensor?
- Do not confuse the sensor with the safety function
- Clearly define the safety function
- What does a typical safety chain look like?
- Separate process control and safety function
- Selecting measuring range and trip point correctly
- Which faults must diagnostics detect?
- 4–20 mA signal for the safety function
- What does the Dynisco R-Cal function test?
- Why R-Cal is not a complete proof test
- What is a proof test?
- How is the proof-test interval determined?
- What does proof-test coverage mean?
- Why As-Found values are important
- Actually testing the mechanical pressure response
- Testing the complete safety chain
- Checking zero at operating temperature
- Process connection as part of the measuring chain
- Considering diaphragm and pressure transmission
- Detecting pressure spikes and overload
- Screen monitoring and melt pressure
- When are two sensors useful?
- Distinguishing online diagnostics from proof testing
- Replacement strategy for safety-related sensors
- Practical example: high-pressure shutdown on an extruder
- Typical fault patterns
- What should be included in proof-test documentation?
- Recommended procedure for planning and testing
- Suitable Dynisco sensors from ICS Schneider
- Conclusion
- FAQ
Why can melt pressure be a safety-related variable?
During extrusion, the polymer is conveyed through the screw, barrel, screen pack and die.
The pressure is generated, among other things, by:
- flow resistance of the material,
- melt viscosity,
- screen pack,
- die geometry,
- throughput,
- temperature,
- screw speed.
If flow resistance increases
the melt pressure can also rise significantly.
Typical causes include:
- clogged screens,
- blocked flow channels,
- melt temperature too low,
- incorrect material,
- excessive throughput,
- die problems.
A defined high-pressure limit
can therefore form part of a protective function.
For example:
Melt pressure > safety limit → bring extruder to a safe state
What this safe state actually looks like must be defined for the specific machine.
Depending on the system, this may include, for example:
- stopping the screw drive,
- interrupting material feed,
- shutting down other hazardous functions,
- triggering a defined machine response.
What does SIL2 mean for a melt pressure sensor?
SIL stands for:
Safety Integrity Level
.
SIL describes the required reliability of a safety-related function with regard to dangerous failures.
For a pressure sensor, a SIL2 assessment
does not mean:
This sensor automatically guarantees SIL2 for the machine.
Instead, it provides safety-related device data and/or corresponding systematic capability that can be used when assessing the complete safety function.
The safety function additionally requires
- sensing,
- signal transmission,
- safety logic,
- output stage,
- final element,
- wiring,
- diagnostics,
- proof-test concept.
The assessment must therefore always be performed for the complete safety function.
Do not confuse the sensor with the safety function
A safety-related function is often referred to as:
SIF – Safety Instrumented Function
.
In simplified form, it consists of:
Sensor → logic → final element
For an extruder, this could be, for example
Melt pressure sensor → safety PLC → safe shutdown of the extruder drive
.
The melt pressure sensor is therefore only one part of the chain.
The complete function must work
A perfect pressure sensor is of no help if:
- the input signal is scaled incorrectly,
- the safety PLC does not evaluate the limit correctly,
- an output is stuck,
- the contactor or drive system does not shut down.
A meaningful proof test therefore does not consider only the sensor but – where required by the test strategy – also the actual response path of the safety function.
Clearly define the safety function
Before selecting a sensor, it should first be established which function is actually to be implemented as a safety-related function.
A definition could, for example, be
If the melt pressure upstream of the die exceeds the defined safety limit, the extruder drive must be safely shut down within the defined time.
For this, the following must be known, among other things
- measuring point,
- maximum permissible process pressure,
- trip limit,
- permissible response time,
- safe machine state,
- reset conditions,
- behavior in the event of a sensor-signal fault.
These requirements belong in the Safety Requirements Specification or the corresponding machine safety concept.
What does a typical safety chain look like?
A simplified safety chain may, for example, consist of:
SIL2 melt pressure transmitter
→
safety-related analog input
→
safety PLC
→
safe output
→
drive shutdown
.
The design must take the actual risk into account
This includes, among other things:
- required SIL or Performance Level,
- sensor architecture,
- diagnostic coverage,
- failure rates,
- proof-test interval,
- proof-test coverage,
- repair time,
- architecture of the safety PLC and final element.
Separate process control and safety function
The same melt pressure is often required simultaneously for:
- process indication,
- control,
- alarm functions,
- safety shutdown
.
Control and protective functions pursue different objectives
Process control may, for example, try to:
keep pressure constant
.
The safety function, on the other hand, is intended to:
reliably detect and control dangerous overpressure
.
Shared components can create common-cause problems
If the same sensor, the same input module and the same logic are used completely for both tasks, a common fault can impair both functions at the same time.
The necessary independence must therefore be assessed within the safety concept.
Selecting measuring range and trip point correctly
The measuring range of a SIL sensor should not be selected solely on the basis of normal operating pressure.
Example
Normal process pressure:
120 … 180 bar
.
Safety shutdown:
250 bar
.
A sensor with a measuring range of:
0 … 200 bar
would obviously be unsuitable because the safety-related limit lies outside its measuring range.
An extremely large measuring range is not automatically optimal either
A:
0 … 1,000 bar
sensor may offer sufficient reserve, but for a 250 bar limit it provides different usable resolution and measurement uncertainty than a more appropriately selected narrower range.
The following should therefore be considered together when selecting the range:
- normal operating range,
- safety limit,
- possible pressure spikes,
- overpressure capability,
- measurement accuracy at the trip point.
Which faults must diagnostics detect?
A safety-related sensor can fail in different ways.
Possible faults include, for example
- cable break,
- short circuit,
- loss of supply,
- electronics fault,
- large zero error,
- loss of sensitivity,
- damaged diaphragm,
- blocked pressure channel,
- mechanically blocked pressure transmission.
Not every fault is detected automatically
A current loop can appear electrically completely plausible even though the process pressure is no longer being transmitted correctly to the measuring diaphragm.
These dangerous faults that are not automatically detected are a key reason for carrying out proof tests.
4–20 mA signal for the safety function
The Dynisco PT46X4 series provides a:
4 … 20 mA
output signal.
Typically:
4 mA = lower range value
and:
20 mA = upper range value
.
The current loop provides an important advantage
A completely missing signal can generally be distinguished from a normal zero signal.
However, the safety evaluation must be designed accordingly.
It should not only consider the high-pressure limit, but should also define how to respond to:
- implausibly low current,
- implausibly high current,
- signal loss,
- diagnostic indication.
What does the Dynisco R-Cal function test?
Many Dynisco melt pressure sensors feature an internal:
R-Cal
or:
Shunt Calibration
.
On the PT46X4, a signal of approximately:
80 % FS
is typically simulated.
For a measuring range of 0 … 500 bar
this corresponds, for example, to approximately:
400 bar
on the connected display, provided that the complete signal chain is correctly scaled.
This can be used to check
- whether the sensor signal is transmitted electrically,
- whether the display is scaled correctly,
- whether the analog input processes the value correctly,
- whether a connected evaluation system responds to the simulated signal.
R-Cal is therefore a very useful diagnostic and commissioning function.
Why R-Cal is not a complete proof test
The R-Cal function generates a defined electrical output signal.
However, it does not generate real process pressure at the measuring diaphragm.
It therefore does not fully verify, for example
- whether the process diaphragm responds mechanically correctly,
- whether the pressure transmission system is working correctly,
- whether the pressure channel in front of the diaphragm is clear,
- whether sensitivity to real pressure is still correct.
Dynisco explicitly points out that R-Cal is used to verify sensor/instrument scaling and does not confirm the mechanical pressure response of the sensor.
To verify actual pressure measurement
a traceable or calibrated pressure source is required.
R-Cal can therefore form part of a test strategy, but does not automatically replace a complete proof test.
What is a proof test?
A proof test is intended in particular to detect dangerous faults that are not identified by automatic diagnostics during normal operation.
A proof test is therefore more than a functional check
The question is not only:
Does the sensor show any value?
but:
Can the safety function still detect and control a dangerous process condition with the required reliability?
A test concept may – depending on the safety manual and SRS – include, for example
- visual inspection of the sensor,
- checking cable and connector,
- checking the zero signal,
- R-Cal functional test,
- testing with a calibrated pressure source,
- checking the safety limit,
- checking the safety logic,
- checking the final element.
The steps actually required must be derived from the safety manual of the device used and the machine’s Safety Requirements Specification.
How is the proof-test interval determined?
There is no universal value such as:
SIL2 sensor = test every year
.
The required interval depends on the safety function.
Relevant parameters include
- dangerous undetected failure rate,
- automatic diagnostics,
- proof-test coverage,
- repair time,
- architecture,
- required SIL,
- operating mode of the safety function.
For a low-demand safety function, the proof-test interval directly influences the average probability of a dangerous failure on demand.
A longer interval
reduces maintenance effort.
However, it may allow a dangerous undetected fault to remain unnoticed for longer.
The interval should therefore be defined and documented as part of SIL verification.
What does proof-test coverage mean?
A proof test does not necessarily detect every conceivable sensor fault.
The:
Proof-Test Coverage
describes what proportion of the relevant dangerous, non-automatically diagnosed faults can be detected by the selected test procedure.
Example
A purely electrical R-Cal test can test part of the signal chain very effectively.
However, it may not detect a completely blocked mechanical pressure path.
An additional test with real reference pressure therefore provides greater confidence regarding the complete pressure measuring chain.
The assumed proof-test coverage must not be estimated arbitrarily. For a SIL calculation, reliable device data or manufacturer information and the actual test procedure performed must be used.
Why As-Found values are important
Before performing zero adjustment or calibration, the current condition should first be documented.
This is often referred to as:
As Found
.
Why?
If the sensor is immediately re-zeroed, it is no longer possible to determine afterward whether, for example, significant drift had occurred.
Recommended sequence
- document the current measured value,
- evaluate the deviation,
- perform the test,
- only then adjust if necessary,
- document the final condition.
Historical As-Found data are particularly valuable for safety-related sensors because they allow drift and sensor ageing to be assessed.
Actually testing the mechanical pressure response
To completely verify the measuring function, the sensor must respond to a known real pressure.
Outside the process
a suitable calibrated pressure source or deadweight tester can be used for this purpose.
Several points across the relevant measuring range may, for example, be tested.
The range around the safety limit is particularly relevant
If the safety shutdown is set, for example, at:
300 bar
, the test strategy should ensure that the measuring chain operates reliably in this relevant range.
However, the applicable test points and tolerances must be defined in the test procedure.
A mechanical test can detect faults
that may remain hidden during purely electrical simulation.
These include, for example:
- loss of pressure sensitivity,
- larger characteristic deviation,
- mechanical damage,
- problems with internal pressure transmission.
Testing the complete safety chain
A sensor calibration initially confirms only the measuring function of the sensor.
The actual protective function, however, only ends when the safe machine state is reached.
A complete functional test can therefore additionally verify
whether, during a simulated or controlled safety event:
- the sensor generates the signal,
- the analog input detects it correctly,
- the safety logic recognizes the limit,
- the safety outputs respond,
- the final element establishes the defined safe state.
A test across the complete chain
is particularly valuable because it can also detect faults in:
- scaling,
- programming,
- wiring,
- limit configuration,
- output circuitry.
The specific test must be performed under safe, controlled conditions and according to the approved test procedure for the system.
Checking zero at operating temperature
Melt pressure sensors operate at very high process temperatures.
Temperature influence can therefore affect the zero point.
For melt pressure sensors, Dynisco recommends
checking or adjusting zero at:
0 bar process pressure
and:
operating temperature
.
This takes into account the temperature influence of the actual installation condition.
Important
A zero adjustment must not be used simply to hide a genuine mechanical fault or unusually large drift.
The original value should therefore be documented before adjustment.
Process connection as part of the measuring chain
For a melt pressure sensor, the process connection is not merely a mechanical detail.
An incorrectly machined mounting hole can
- mechanically load the diaphragm,
- stress the sensor,
- cause zero shifts,
- damage the diaphragm,
- promote polymer deposits.
The classic Dynisco design often uses:
1/2-20 UNF
.
Before installation, the following should therefore be checked
- thread,
- sealing surface or 45° seat,
- bore depth,
- cleanliness,
- alignment.
Dynisco offers inspection and cleaning tools for sensor mounting holes for this purpose.
Considering diaphragm and pressure transmission
The melt acts directly on a thin isolation diaphragm at the sensor tip.
In conventional filled sensors, this pressure is transmitted via an internal transmission medium to the actual measuring cell.
The measuring chain therefore includes not only
electronics + strain gauge
, but also:
- process diaphragm,
- pressure transmission system,
- measuring diaphragm,
- signal conditioning.
A proof-test concept should therefore also consider faults that may occur before electrical signal processing.
Detecting pressure spikes and overload
Extrusion processes can generate short pressure spikes that may barely be visible on a slow process display.
Such spikes can be caused by
- starting against material that has not yet fully melted,
- sudden blockages,
- die problems,
- rapid process changes.
They can place a heavy load on the sensor even though the subsequently visible steady-state pressure appears uncritical.
After an unusual overpressure event
an additional sensor check may therefore be advisable.
A sensor that still shows a plausible zero after a pressure event is not automatically fully intact.
Screen monitoring and melt pressure
A typical application is monitoring a screen changer or screen pack.
As contamination increases
the pressure upstream of the screen rises.
A pressure sensor can therefore perform several tasks:
- process monitoring,
- maintenance indication,
- alarm function,
- where applicable, safety shutdown.
With two sensors upstream and downstream of the screen
the pressure difference:
Δp = pupstream − pdownstream
can additionally be evaluated.
However, this process diagnostic function must be distinguished from an independent safety-related high-pressure function if such a function is required by the risk assessment.
When are two sensors useful?
A safety function does not necessarily have to be redundant.
However, depending on the required safety integrity and device characteristics, an architecture with two sensors may be useful or necessary.
Possible advantages
- comparison of two measured values for fault detection,
- higher availability,
- detection of certain sensor faults,
- higher hardware fault tolerance.
Redundancy alone is not sufficient
Two identical sensors can be affected simultaneously by the same cause.
Examples:
- same pressure channel blocked,
- common power supply failed,
- both sensors damaged by the same overpressure spike,
- same incorrect parameterization.
Common-cause failures must therefore also be considered.
Distinguishing online diagnostics from proof testing
Continuous diagnostics operate during normal machine operation.
They may detect, for example:
- signal loss,
- wiring faults,
- certain internal electronics faults,
- implausible output values.
A proof test, on the other hand, detects faults
that are specifically not detected automatically.
Both methods complement each other.
Good online diagnostics reduce the number of dangerous undetected faults but do not automatically eliminate the need for a specified proof test.
Replacement strategy for safety-related sensors
With normal process sensors, action is often only taken when the measurement becomes abnormal.
For safety-related sensors, an additional documented lifecycle strategy should exist.
This can take into account
- manufacturer information on service life,
- number of operating hours,
- temperature exposure,
- pressure cycles,
- overpressure events,
- historical calibration data,
- proof-test results,
- mechanical condition.
Increasing drift
can, for example, be a reason to replace the sensor preventively even if it still narrowly meets the current test limit.
Historical data are therefore particularly valuable for replacement decisions.
Practical example: high-pressure shutdown on an extruder
An extruder has a melt pressure sensor with:
measuring range 0 … 500 bar
.
The normal process operates at:
180 … 230 bar
.
The risk assessment defines a safety-related trip point at:
320 bar
.
Safety chain
Dynisco SIL2 melt pressure sensor → F-analog input → safety PLC → safe drive shutdown
During normal operation
the 4–20 mA signal is continuously monitored for:
- overpressure,
- signal faults,
- plausible limits.
During the planned intermediate check
R-Cal can be used to verify the electrical signal chain and scaling.
At 80 % FS, this corresponds to:
400 bar
.
The safety logic must respond according to the specified test procedure.
However
this does not yet confirm that the sensor responds correctly to a real pressure of, for example:
320 bar
.
During the complete proof test
the actual pressure response of the sensor is therefore additionally checked using a suitable reference pressure source, in accordance with the approved test procedure.
The measured values are documented before any adjustment as:
As Found
.
The safety chain is then tested
up to the intended safe machine state.
This covers different fault levels:
| Test | Detects, among other things |
|---|---|
| Signal diagnostics | cable faults and certain electronics faults |
| R-Cal | scaling and electrical signal chain |
| Reference pressure test | actual pressure sensitivity and characteristic deviation |
| SIF functional test | faults in logic, limit setting and shutdown chain |
Only the combination of the intended diagnostic and test measures allows a reliable assessment of the safety function.
Typical fault patterns
| Observation | Possible cause | Recommended check |
|---|---|---|
| 4–20 mA signal completely missing | Cable break, supply or electronics | Check supply, wiring and sensor |
| R-Cal correct, but actual pressure measurement incorrect | Mechanical measuring system or pressure transmission impaired | Test with calibrated pressure source |
| Zero point changes significantly | Temperature, mechanical stress or sensor damage | Document As-Found condition and check installation |
| Zero point shifts when the sensor is screwed in | Incorrect mounting hole or excessive installation torque | Check bore and installation |
| Pressure signal barely responds to process changes | Blocked pressure channel or sensor damage | Check mounting hole and actual pressure response |
| Sensor shows plausible value, but safety shutdown does not respond | Scaling, logic or output chain faulty | Perform complete SIF functional test |
| Shutdown occurs at the wrong pressure | Incorrect PLC scaling or limit parameterization | Check R-Cal and reference pressure together |
| Sensor drifts after an overpressure event | Mechanical overload | Calibrate or replace sensor |
| Pressure rises extremely quickly during startup | Material not yet fully melted | Check process heating and soak time |
| Measured value upstream of screen rises continuously | Screen contamination | Monitor process pressure or differential pressure |
| Two sensors increasingly show different values | Drift, installation or process problem | Check both sensors against a reference |
| Sensor is regularly re-zeroed and drift remains unnoticed | As-Found values are not documented | Adjust test procedure |
| Proof test is passed regularly, but a dangerous fault remains undetected | Insufficient proof-test coverage | Review test scope and safety calculation |
What should be included in proof-test documentation?
A proof test should be documented in a traceable manner.
At minimum, it is useful to include
- machine and measuring-point identification,
- sensor type,
- serial number,
- measuring range,
- safety function,
- safety limit,
- date,
- tester,
- reference instruments used,
- calibration status of the reference,
- As-Found zero point,
- As-Found values at the test points,
- R-Cal result,
- result of the actual pressure test,
- result of the safety shutdown,
- deviations found,
- adjustments performed,
- As-Left results,
- release or replacement decision.
In the event of deviations, additionally
the following should be assessed:
Since when could the safety function have been impaired?
and:
What impact did this have on safe operation?
Recommended procedure for planning and testing
- Determine the hazard: Define which overpressure condition must be safely controlled.
- Define the safety function: Specify trip criterion, response time and safe state.
- Determine required SIL or PL: Derive from the risk assessment.
- Select the sensor: Check safety assessment and device characteristics.
- Define the measuring range: Consider normal pressure, trip point and pressure spikes.
- Select the measuring point: Position the sensor where the safety-relevant pressure actually occurs.
- Check the process connection: Machine the bore according to manufacturer specifications.
- Install the sensor correctly: Observe installation specifications and permissible torque.
- Define the signal path: Specify safety-related analog input and diagnostic limits.
- Program the safety logic: Clearly define limit and fault responses.
- Assess the final element: Ensure that the defined safe state is actually achieved.
- Perform SIL verification: Evaluate sensor, logic and final element together.
- Define the proof-test procedure: Use manufacturer information and SRS.
- Define proof-test coverage: Use only reliable values.
- Determine the proof-test interval: Derive from the safety calculation.
- Document test conditions: Define responsibility and plant condition.
- Record As-Found condition before adjustment: Do not hide drift by immediately re-zeroing.
- Perform visual inspection: Check diaphragm, cable, connector and installation.
- Check zero: According to manufacturer specifications and under suitable temperature conditions.
- Check R-Cal: Verify electrical signal chain and scaling.
- Check actual pressure response: Use a suitable calibrated pressure source.
- Check the safety limit: Verify input and logic response.
- Check the shutdown chain: Verify the safe machine state according to the approved procedure.
- Evaluate results: Compare limits and historical data.
- Analyze the cause of abnormalities: Do not simply readjust automatically.
- Replace the sensor if necessary: Particularly in the event of mechanical damage or unacceptable drift.
- Document the As-Left condition: Record the final state.
- Archive the test: Retain evidence for the next assessment and trend analysis.
Suitable Dynisco sensors from ICS Schneider
Dynisco PT46X4 – 4–20 mA melt pressure sensor with SIL2 assessment
The Dynisco PT46X4 Series is designed for conventional melt pressure measurement in extrusion and plastics applications.
Key features include:
- 4–20 mA output,
- SIL2 assessment,
- Performance Level “c”,
- measuring ranges from 0…500 to 0…30,000 psi,
- accuracy ±0.5 % FS,
- media temperatures up to 400 °C,
- local or remote zero adjustment,
- internal 80 % shunt calibration,
- 1/2-20 UNF process connection on standard versions.
The series includes versions such as:
- PT4604,
- PT4624,
- PT4634,
- TPT4604,
- TPT4624,
- TPT4634.
This provides different stem and temperature configurations for various extruder and tooling installation situations.
Dynisco SPX Series – smart melt pressure transmitters with SIL2-certified pressure output
For more demanding applications, the Dynisco SPX series are also available.
Depending on the model, these offer, among other things:
- 4–20 mA output,
- SIL2-certified pressure output,
- optional HART® communication,
- higher accuracy depending on the series,
- ATEX/IECEx versions,
- different process and electrical connections.
For example, the Dynisco SPX-T Series 3 is available with temperature compensation and DynaLarity™.
Further SIL2-capable versions can be found under Dynisco melt pressure transmitters at ICS Schneider.
Conclusion
A SIL2 melt pressure sensor can be an important component of a safety-related extruder monitoring system.
SIL2 does not automatically apply to the complete machine
Sensor, safety logic and final element must be considered as a complete safety function.
The safety function must be defined first
Measuring point, pressure limit, response time and safe machine state must be clearly specified.
4–20 mA enables robust signal transmission
Fault ranges and signal loss should additionally be evaluated in a safety-related manner.
R-Cal is a valuable diagnostic function
The 80 % shunt calibration is highly suitable for checking scaling and the electrical signal chain.
However, R-Cal does not replace a real pressure test
The mechanical response of diaphragm and pressure transmission is not fully tested by R-Cal.
The proof test must detect dangerous undetected faults
Test scope and interval must therefore be derived from the safety manual, SRS and SIL calculation.
Proof-test coverage is decisive
A frequent but incomplete test can be less effective than a well-designed test with high fault coverage.
As-Found values must not be lost
The actual initial condition should be documented before zero adjustment or calibration.
The process connection is part of the measuring chain
An incorrectly machined or contaminated sensor bore can impair measurement just as much as an electrical fault.
The complete safety chain should be considered
Only if the logic and final element also respond correctly does the safety function fulfill its intended purpose.
For practical applications
Define the dangerous overpressure condition → determine the required safety integrity → select a suitable SIL2 melt pressure sensor → coordinate measuring range and trip point appropriately → install the process connection correctly → define signal diagnostics and fault response → assess safety logic and final element → define proof-test coverage and test interval as part of SIL verification → document As-Found values → use R-Cal to test the electrical signal chain → verify actual pressure response using a suitable reference → verify safety limit and complete shutdown chain → evaluate results as trends → do not simply readjust abnormal sensors, but identify the cause or replace the sensor.
FAQ: SIL2 Melt Pressure Sensor, Proof Test and Extruder Safety
What does SIL2 mean for a melt pressure sensor?
SIL2 describes the safety-related suitability or assessment of the device for use within a correspondingly designed safety function. The sensor alone does not automatically make the complete machine SIL2.
What is a SIF?
SIF stands for Safety Instrumented Function. It typically consists of sensing, logic and a final element and is intended to control a defined hazardous condition.
Can melt pressure be used for safety shutdown?
Yes. A defined high-pressure condition can, for example, be used as the trip criterion for a safety-related shutdown of the extruder, provided this is derived from the risk assessment and safety concept.
Is a SIL2 sensor sufficient for a SIL2 safety function?
No. The safety PLC, final element, architecture, diagnostics, proof testing and other components of the safety chain must also be considered.
Which Dynisco series is suitable for SIL2 applications?
The Dynisco PT46X4 Series is assessed for SIL2. Various SPX series also feature a SIL2-certified pressure output.
Which output signal does the PT46X4 Series use?
The series provides an industrial 4–20 mA output signal for connection to DCS, PLC or safety-control systems.
What is R-Cal?
R-Cal is an internal resistance or shunt calibration function that simulates a defined electrical output signal.
What value does Dynisco R-Cal typically simulate?
For the PT46X4 Series, a signal corresponding to approximately 80 % of full scale is typically generated.
Can I use R-Cal to check PLC scaling?
Yes. R-Cal is very well suited to checking whether the sensor, current loop and downstream evaluation are correctly scaled.
Is R-Cal a complete proof test?
No. R-Cal simulates the electrical output signal but does not apply real pressure to the process diaphragm.
Which fault can still be present despite a successful R-Cal test?
For example, the mechanical pressure path may be impaired or actual pressure sensitivity may have changed even though the electrical shunt calibration works correctly.
How is the actual pressure response tested?
The sensor is subjected to a suitable calibrated reference pressure source according to the specified test procedure and its output is compared with the reference value.
What is a proof test?
A proof test is intended in particular to detect dangerous faults that are not identified by automatic online diagnostics.
How often must a SIL2 melt pressure sensor be tested?
There is no universally applicable interval. The proof-test interval must be derived from the safety function, device characteristics, required safety integrity and SIL verification.
What is proof-test coverage?
It describes the proportion of relevant dangerous undetected failures that can be detected by the proof-test procedure used.
Why is proof-test coverage important?
An incomplete test can leave certain dangerous faults undetected and therefore affects the safety integrity that can actually be achieved.
What does As Found mean?
As Found describes the condition of the sensor immediately before adjustment or repair. This value should be documented so that drift and any safety-relevant deviations remain traceable.
What does As Left mean?
As Left describes the condition of the sensor after completion of testing, adjustment or repair.
Should I immediately re-zero the sensor before a test?
No. The existing zero value should first be documented and assessed as the As-Found condition. Adjustment should only be performed afterward if necessary.
Why should zero be checked at operating temperature?
The high process temperature can influence the zero point of a melt pressure sensor. Testing under realistic temperature conditions reduces the risk of incorrect assessment.
Can an incorrect sensor bore influence the measurement?
Yes. An incorrectly machined, contaminated or misaligned mounting hole can cause mechanical stress, zero shifts or damage to the diaphragm.
What is a typical process connection for Dynisco melt pressure sensors?
Many classic Dynisco extruder sensors use a 1/2-20 UNF process connection.
Why can a blocked pressure channel be dangerous?
The actual process pressure may no longer be transmitted correctly to the sensor diaphragm. The electrical signal may nevertheless initially still appear plausible.
Can overpressure permanently change the sensor?
Yes. High pressure spikes or mechanical overload can damage the diaphragm or internal measuring system and lead to zero or sensitivity errors.
Should a sensor be tested after a severe overpressure event?
For safety-related applications, an additional check after an exceptional load event is advisable or should be carried out according to the defined maintenance and safety procedure.
Why are pressure spikes during cold startup critical?
If the polymer has not yet fully melted, flow resistance can be very high and can cause significant pressure spikes.
What is screen monitoring?
The pressure upstream of a screen pack or the differential pressure across it is monitored in order to detect increasing contamination or blockage.
Is screen monitoring automatically a safety function?
No. It may be purely process diagnostics. Whether an additional safety-related high-pressure shutdown is required depends on the risk assessment.
Can one sensor be used for both control and safety?
This depends on the architecture and specific safety assessment. Shared components and possible common-cause failures must be taken into account.
Why can two sensors be useful?
A redundant architecture can provide additional fault detection and higher hardware fault tolerance. The specific need results from the safety concept.
What is a common-cause failure?
This refers to a common fault that impairs several redundant components at the same time, for example a shared power supply or a common process cause.
What should be tested in addition to the sensor during a proof test?
Depending on the specified test procedure, the safety PLC, limit logic, wiring, safe outputs and final element may also form part of the test.
Why is an end-to-end test useful?
It verifies the actual signal path from the detected process condition through to the defined safe machine state and can detect faults outside the sensor.
Which data should be stored?
Among other things, sensor identification, measuring range, As-Found values, reference values, R-Cal result, test points, shutdown function, adjustments performed and As-Left condition.
When should a safety-related sensor be replaced?
Replacement may be required particularly in the event of mechanical damage, unacceptable drift, abnormal repeatability, a failed proof test or according to defined lifecycle requirements.
Which Dynisco series additionally offers HART?
Various Dynisco SPX series are available with 4–20 mA output and optional HART® communication.
Where can I find further Dynisco melt pressure sensors?
Further melt pressure sensors, transmitters, indicators and accessories can be found under Dynisco products at ICS Schneider.