Installing a melt pressure sensor with a flexible capillary initially appears straightforward: the rigid sensor stem is screwed into the mounting hole of the extruder or injection moulding machine, the sensor housing is mounted at some distance, and the flexible connection between them is routed according to the available installation space. However, this seemingly simple installation step can have a decisive influence on the service life and reliability of the sensor.
The flexible connection of a Dynisco melt pressure sensor is not an ordinary electrical cable. Depending on the sensor design, it contains the capillary of the closed pressure transmission system. The process pressure acts on the flush diaphragm and is transmitted through this system to the actual measuring cell. If the capillary is kinked, crushed, subjected to excessive tension or repeatedly bent at the same point, not only can the outer protective armour be damaged. In the worst case, the pressure transmission itself can also be impaired.
At the same time, the flexible design serves a second important purpose: it allows the strain gauge or electronics housing to be positioned away from the particularly hot area of the machine. The diaphragm of a suitable melt pressure sensor can be exposed to significantly higher temperatures than the rear sensor housing. Unfavourable routing directly over heater bands or along very hot machine components can negate this design advantage.
The most important rule is therefore: The flexible capillary must be routed free from mechanical stress, with a sufficiently large bend radius and in such a way that the sensor housing remains thermally isolated. In addition, the housing must be mounted separately and the flexible connection supported so that vibration and cable tension do not continuously act on the capillary and process connection.
What is the purpose of the flexible capillary?
Many Dynisco melt pressure sensors operate with a closed, fluid-filled pressure transmission system. The pressure of the polymer melt first acts on the process diaphragm at the sensor tip. From there, the pressure change is transmitted through the internal transmission system to the actual measuring cell.
With a rigid sensor design, the process connection, rigid sensor stem and measuring housing are arranged directly one behind the other. This is compact and sufficient for many installation situations. However, at high local temperatures or where installation space is limited, it can be advantageous to separate the measuring housing spatially from the measuring point. Sensor versions with a flexible connection are used for this purpose.
A typical example is the Dynisco PT462. In this design, a flexible capillary connection is located between the rigid sensor stem and the strain gauge housing. This allows the housing to be mounted in a more favourable thermal and mechanical position while the diaphragm remains in direct contact with the polymer melt.
The flexible connection must therefore be treated as a functional part of the measuring system. It should not be handled like an ordinary cable that can be bent or coiled arbitrarily, tightened with cable ties or trapped between machine components.
| Component | Function | What should be considered during installation? |
|---|---|---|
| Process diaphragm | Directly senses the melt pressure | Do not damage mechanically; keep the mounting hole clean |
| Rigid sensor stem | Positions the diaphragm at the measuring point | Observe the correct bore geometry and insertion depth |
| Flexible capillary / flexible stem | Transmits the pressure and enables spatial separation | Do not kink, crush, overstretch or install under tension |
| Strain gauge/electronics housing | Contains the measuring cell or electronics | Mount separately and observe the permissible housing temperature |
| Electrical connection | Transmits the measuring signal or power supply | Provide strain relief and route in accordance with EMC requirements |
Maintaining the correct bend radius
The most common mechanical error when installing a flexible capillary is using a bend that is too tight. Particularly in compact extruder or injection moulding installations, there is a strong temptation to accommodate excess length in a tight loop directly behind the rigid sensor stem or to route the flexible connection immediately around a machine edge.
For the relevant sensor designs, Dynisco explicitly states that the flexible connection must not be kinked or crushed. Various Dynisco manuals specify a minimum bend radius of 25 mm for protected or armoured capillaries. However, this value must not be applied indiscriminately to every Dynisco design because there are also special sensors with exposed capillaries designed for particularly tight bends.
In practice, this means that a larger bend radius is normally preferable to a radius that only just meets the minimum requirement. If sufficient space is available, the flexible connection should therefore be routed in a smooth, generous curve. After installation, the capillary should rest in its natural position and should not visibly try to spring back into another position.
Tight bends directly at the transition from the rigid sensor stem to the flexible connection, and at the transition to the measuring housing, are particularly critical. At these points, bending forces act directly on structurally sensitive connection areas. Even if the actual curve meets the nominal minimum bend radius, permanent lateral loading of these transition points should be avoided.
Why the same bend radius does not apply to every Dynisco sensor
Dynisco uses different designs for flexible stems and capillaries. It would therefore be technically incorrect to state that “25 mm applies to every Dynisco sensor”.
| Design / example | Capillary design | Typical manufacturer value | Practical recommendation |
|---|---|---|---|
| Various PT/MDT versions with protected capillary | Armoured or protected flexible connection | Minimum bend radius of 25 mm | Use significantly larger, smooth bends wherever possible |
| PT467 versions with exposed capillary | Unprotected / exposed capillary | Approximately 2 mm depending on the version | Use only the value specified in the relevant data sheet |
| PT435A | Exposed capillary designed for particularly tight bends | 1/16" according to the product specification | Do not apply this value to protected capillaries from other series |
The structural difference is significant. A special exposed capillary for very confined injection moulding applications is not the same component as the protected flexible connection of a conventional melt pressure sensor.
Before installation, the complete sensor type designation should therefore first be identified. Only then can the data sheet or operating instructions be used to determine which bend radius, length and mechanical installation are specified for that particular version.
Strain relief and housing mounting
A second common error is allowing the flexible capillary alone to support the sensor housing. Mechanically, this may initially appear to work with a small housing. During machine operation, however, vibration, temperature fluctuations and movement of the connecting cable occur. This creates continuously changing loads on the flexible connection.
For the relevant sensors, Dynisco therefore provides for separate mounting of the electronics or strain gauge housing. It is also recommended that the flexible connection between the sensor and housing be secured using a suitable clamp.
The strain relief should support the capillary, not compress it. A metal clamp that is tightened too firmly or an excessively tight cable tie can deform the protective armour and cause exactly the type of damage that the support is intended to prevent.
A good installation therefore separates three mechanical functions:
- The process connection secures only the rigid sensor stem at the measuring point.
- The measuring or electronics housing has its own stable mounting.
- The flexible connection is guided and supported while remaining free from tension, crushing and sharp bends.
The electrical connecting cable must also not pull on the sensor housing. If a heavy cable is allowed to hang freely downwards, its weight is transferred through the connector to the sensor housing and therefore indirectly to the capillary. Separate cable strain relief is therefore also advisable.
Clearance from heater bands and hot surfaces
Extruders, injection moulding units and polymer lines frequently have several electrical heater bands. The area around a melt pressure measuring point can therefore be considerably hotter than the general ambient temperature in the production hall or control cabinet.
Depending on the type, the process diaphragm of a suitable Dynisco melt pressure sensor may be designed for very high media temperatures. However, this must not be taken to mean that the complete sensor housing can withstand the same temperature. For the PT462, for example, the typical maximum diaphragm temperature is up to 400 °C, while the permissible housing temperature is considerably lower.
This is precisely where the flexible capillary performs one of its key functions: it creates distance between the hot process measuring point and the more temperature-sensitive measuring housing.
The flexible connection should therefore not be placed directly on a heater band unless absolutely unavoidable, trapped between the heater band and the barrel, or pressed tightly against a highly heated machine surface. Likewise, the housing bracket should not simply be installed at the hottest available location.
However, specifying a universal minimum clearance of, for example, 20, 50 or 100 mm would not be technically sound. The relevant factors are heater band temperature, ambient air, radiant heat, sensor version, capillary length and permissible housing temperature.
In critical applications, the actual housing temperature should therefore be checked after the machine has reached thermal steady state. If it is too high, possible measures include increasing the spatial separation, changing the routing, choosing a thermally more favourable mounting point or, where appropriate, using suitable thermal shielding.
| Installation situation | Assessment | Better solution |
|---|---|---|
| Capillary lies directly on the heater band | Unfavourable additional thermal load | Route past the heater band with clearance and a free bend |
| Housing mounted directly above the hot barrel zone | Thermal isolation is partially negated | Mount the housing at a cooler, mechanically stable location |
| Capillary trapped between two machine components | Risk of crushing and vibration damage | Create a free routing path with suitable guidance |
| Generous bend with separate housing mounting | Mechanically and thermally favourable | Preferred installation arrangement |
Preparing the mounting hole and process connection correctly
A perfectly routed capillary cannot compensate for an incorrect process connection. Damage to melt pressure sensors is frequently caused by unsuitable, contaminated or non-concentric mounting holes.
The sensor tip and, in particular, the thin process diaphragm must not mechanically strike the bore during installation. Hardened polymer residues, burrs or incorrectly machined bore geometry can cause lateral loading of the diaphragm or damage to the sealing surface.
The mounting hole should therefore be inspected before installation and cleaned with the appropriate tools if necessary. Particular care is required when reusing existing measuring points because polymer melt may enter the bore after a sensor has been removed and harden there.
The complete geometry of the mounting hole must match the process connection used. Typical Dynisco sensors use, for example, 1/2-20 UNF or, depending on the version, M18 × 1.5. However, the thread alone is not sufficient as a selection criterion; the seating surface, bore depth and diaphragm position must also be correct.
Installing the sensor without twisting the capillary
When screwing in the sensor, the installation torque must not be applied through the flexible connection or housing. The wrench must be applied to the designated wrench flats or hexagon of the process connection.
If the housing is rotated instead, the flexible capillary can be subjected to torsional loading. This type of twisting is particularly critical because it may be distributed over the entire flexible length and may not always be clearly visible after installation.
A sensible installation procedure is therefore:
- Inspect the mounting hole and clean it correctly if necessary.
- Inspect the sensor and diaphragm for visible damage.
- Position the capillary before screwing in the sensor so that it can be routed into the intended bend without twisting after installation.
- Screw in the sensor using only the designated hexagon and tighten it to the torque specified for the particular sensor type.
- Then route the capillary in a generous bend.
- Mount the measuring housing separately.
- Lightly support or provide strain relief for the flexible connection.
- Provide mechanical strain relief for the electrical connecting cable as well.
- Check clearance from heater bands, protective covers and moving machine components.
The last point is particularly important. A capillary may have sufficient clearance while the machine is cold and uncovered, but after a protective cover is installed or the machine expands thermally, it may suddenly contact another component.
Distinguishing between the capillary and electrical signal cable
During troubleshooting, the flexible capillary and electrical connecting cable are sometimes confused with each other. Both extend away from the sensor area, but they perform completely different functions.
The capillary is part of the mechanical-hydraulic pressure transmission system. The electrical cable, by contrast, connects the measuring cell or transmitter to the display, controller, PLC or DCS.
With conventional mV/V melt pressure transducers such as various PT models, the electrical measuring signal is comparatively small. Clean cable routing in accordance with EMC requirements is therefore additionally necessary. Signal cables should not be routed unnecessarily over long distances directly alongside heavily loaded heater cables, motor cables or variable-frequency-drive cables.
With 4–20 mA versions, signal transmission is more robust against electromagnetic interference, but the fundamental requirements for correct wiring, shielding and strain relief still apply.
It is therefore important to distinguish between two different tasks: The capillary must be routed correctly from a mechanical and thermal perspective; the electrical cable must be routed correctly from an electrical and EMC perspective.
Practical example on an extruder
On an extruder, the melt pressure sensor is installed directly behind a heated process zone. The rigid sensor stem is screwed horizontally into the extruder barrel. However, because of a protective cover, the sensor housing cannot be mounted directly behind the sensor.
An unfavourable solution would be to bend the flexible capillary through 180 degrees immediately after the process connection, route it closely back over the heater band and allow the sensor housing to hang freely from the capillary. Mechanical stress, temperature and vibration would then all act simultaneously on the same connection.
A significantly better solution is to first route the capillary away from the hot zone with a generous radius. The housing is mounted on a stable, cooler machine component. Between the rigid sensor stem and housing, the capillary is loosely guided at a suitable point so that its own weight and operational vibration are not transferred entirely to the two end connections.
The electrical cable is then routed separately from the sensor housing to the evaluation device and is likewise provided with strain relief. Where possible, heater and power cables should not be routed directly alongside the sensitive measuring cable over long distances.
Such an installation may appear unspectacular, but it improves precisely those factors that become decisive during many years of machine operation: low alternating mechanical stress, sufficient thermal isolation and reproducible signal quality.
Systematically checking typical fault patterns
If a Dynisco melt pressure sensor begins to show abnormal readings after an extended period of operation, the electrical calibration should not be the only aspect considered. Particularly with sensors that use a flexible capillary, the mechanical installation can also provide important clues to the cause.
A permanent zero shift can, for example, be associated with excessive thermal stress, a damaged measuring diaphragm or other mechanical influences. A clearly deformed or kinked section of the flexible connection, on the other hand, is a serious warning sign.
Dynisco identifies an excessively tight bend radius, installation damage and excessive stretching of the capillary among the possible causes of a broken flexible capillary connection. A visibly damaged flexible connection should therefore not simply be “repaired” by bending it back into shape and then returned to service.
The following procedure is suitable for systematic inspection:
- Identify the complete sensor type designation.
- Inspect the mechanical condition of the diaphragm, stem and capillary.
- Look for kinks, crushing and abrasion points.
- Check the mounting of the sensor housing.
- Check whether the electrical cable or capillary is under tension.
- Assess the clearance from heater bands and hot machine surfaces.
- Check the housing temperature under actual operating conditions.
- Inspect the mounting hole and process connection for correct condition.
- Check the electrical supply and signal wiring.
- Finally, verify the plausibility of the measuring signal against the process condition or a suitable reference.
Common installation errors
Treating the capillary like an ordinary cable
The flexible connection is part of the pressure transmission system. Severe bending, crushing or tensile loading can impair the measuring function.
Applying the smallest Dynisco bend radius to every sensor
Special exposed capillaries can permit significantly smaller bend radii than protected flexible stems. The specific sensor series is always decisive.
Allowing the housing to hang from the capillary
The measuring housing should be mounted separately. The capillary is not a load-bearing suspension element.
Tightening the strain relief too firmly
A clamp should limit movement and support weight, but it must not compress the flexible connection.
Routing the capillary directly on the heater band
This introduces unnecessary heat towards the measuring housing and also creates a mechanically unfavourable contact point.
Considering only the permissible media temperature
A high permissible temperature at the process diaphragm does not automatically mean that the strain gauge or electronics housing can withstand the same temperature.
Rotating the housing while screwing in the sensor
Installation and removal forces must only be applied to the designated wrench flats on the sensor.
Ignoring a contaminated mounting hole
Hardened polymer residues can damage the diaphragm or sealing surface while the sensor is being screwed in.
Suitable Dynisco components
ICS Schneider Messtechnik offers various Dynisco sensors for melt pressure measurement in extrusion, injection moulding and polymer processing. Different designs with flexible connections are available for applications involving high local temperatures or confined installation conditions.
A classic example is the Dynisco PT462. The flexible capillary connection makes it possible to position the strain gauge housing away from the hot measuring point and mount it in a more suitable location.
For particularly confined installation conditions, special sensors with exposed capillaries designed for very tight bends are also available. However, these designs must not be treated in the same way as the protected flexible connections used in other Dynisco series.
A complete measuring chain also includes suitable connecting cables, mating connectors, displays and controllers as well as tools for inspecting and cleaning the mounting hole.
Complete Dynisco range at ICS Schneider
View Dynisco melt pressure sensors
Dynisco cable assemblies and connectors
Conclusion
The flexible capillary of a Dynisco melt pressure sensor makes installation considerably easier and provides important thermal isolation between the hot process measuring point and the measuring housing. However, this flexibility must not be confused with unlimited deformability.
A generous bend radius, stress-free routing, separate housing mounting and gentle strain relief are the most important mechanical requirements for permanently reliable installation.
For protected capillaries used in various Dynisco sensor series, a minimum bend radius of 25 mm is an important guideline stated in the relevant manufacturer documentation. Special exposed capillaries can permit significantly tighter bends. The specific sensor design must therefore always be checked before installation.
The thermal aspect is equally important. The permissible temperature of the process diaphragm can be considerably higher than the permissible housing temperature. The flexible connection should support this design advantage rather than negate it by being routed tightly over heater bands or by mounting the housing directly at a particularly hot location.
By considering the mounting hole, bend radius, capillary routing, housing bracket, clearance from heater bands and electrical connecting cable together, not only is the risk of premature sensor failure reduced. Zero-point stability, process reliability and maintainability of the complete melt pressure measuring point also benefit.
FAQ on flexible capillaries for Dynisco sensors
What bend radius is required for a flexible Dynisco capillary?
This depends on the sensor design. For various protected or armoured capillaries, Dynisco specifies a minimum of 25 mm. Special exposed capillaries from certain series may permit significantly tighter bends. The operating instructions for the specific sensor are always authoritative.
Can I simply coil up the flexible capillary?
Excess length can generally be routed in a generous loop. Tight coils, sharp kinks or tightly winding the capillary should be avoided.
May the capillary touch a heater band?
Direct and permanent contact with heater bands should be avoided. One of the purposes of the flexible connection is to provide thermal isolation for the measuring housing. The decisive requirement is that the permissible temperatures of the respective sensor sections are not exceeded.
How much clearance is required between the capillary and heater band?
There is no universally applicable value for all machines and Dynisco sensors. Heater band temperature, radiant heat, capillary design, air circulation and sensor version vary. The decisive factors are thermally safe routing and compliance with the permissible housing temperature.
Does the Dynisco housing need to be mounted separately?
For relevant versions with a flexible stem, Dynisco provides for separate housing mounting. The flexible connection should not have to support the weight of the housing.
How should strain relief be provided for the capillary?
It should be guided or lightly secured so that vibration and weight are not transferred entirely to the connection points. The mounting must not crush the flexible connection.
Can I use a cable tie?
Guiding the capillary with suitable fastening material can be useful. The key requirement is that the cable tie or clamp is not tightened so firmly that the protective armour is deformed or the capillary is crushed.
Why does a melt pressure sensor have a flexible capillary?
Among other things, it enables spatial and thermal separation of the measuring cell or electronics from the very hot process measuring point and facilitates installation where space is limited.
Is the flexible capillary the sensor’s electrical cable?
No. In these designs, the capillary is part of the pressure transmission system. The electrical connecting cable has a separate function and transmits the power supply and measuring signal.
What happens if the capillary is kinked?
A severe kink can damage the flexible connection and, in the worst case, the internal pressure transmission system. A visibly kinked capillary should not simply be bent back into shape and reused without further inspection.
Can a damaged capillary be repaired?
Damage to the closed pressure transmission system is not a normal on-site repair. If a damaged capillary is suspected, the sensor should be professionally inspected or returned to the manufacturer or service partner for assessment.
Why must I not turn the sensor housing while screwing in the sensor?
This can mechanically or torsionally load the housing and flexible connection. The installation torque must be applied through the designated wrench flats on the process connection.
How can I identify a thermally unfavourable installation?
Possible indications include a very high housing temperature, strong heating of the electrical connector or capillary routing directly over heater bands. In critical installation situations, the housing temperature should be checked while the machine is operating at thermal steady state.
Can poor capillary routing cause measurement errors?
A mechanically or thermally damaged measuring chain can impair the measuring function. Visible kinks, crushing, excessive tensile loading or impermissible temperatures should therefore not be regarded as merely cosmetic installation issues.
What should be checked before commissioning?
The mounting hole, process connection, bend radius, free movement of the capillary, separate housing mounting, strain relief, clearance from hot surfaces and electrical wiring should all be checked together.