What Can Cause Transducer Field Returns

This article will explore the different causes of transducer field returns and suggestions for how these issues could be resolved.

Ensure that the Mounting Hole Thread Size is Correct

The instrument’s threads can be damaged if a transducer with the wrong thread size is screwed into the mounting hole. If the threads are damaged it may not be possible to form a good seal, meaning the instrument will not work correctly and material could leak through the gap.

To prevent this from happening the mounting hole must have the correct dimensions. In general, threads are of the industry standard dimensions - ½ - 20 UNF 2B. A mounting well gage plug can be used to determine if the mounting hole has been cleaned and machined correctly.

The Transducer Must Be Mounted Correctly

Installing the pressure transducer into a hole which has not been machined properly can result in system damage. The transducer’s diaphragm can be damaged by forcing the system into a hole that is either too small or of irregular shape, and this can result in the system not working correctly.

Using a tool kit to create the mounting hole can help ensure that the cavity is of the right dimensions. The system should be mounted at a torque of 100 to 200 inch pounds (for ½-20 UNF) to ensure an adequate seal is formed, however if this force is too great seizing can occur.

To avoid seizing-related problems, a high-temperature anti-seize compound can be applied to the threads of the transducer before mounting. If the transducer is mounted at a torque that exceeds 500 inch pounds it will be difficult to un-install regardless of if an anti-seize compound has been used.

Select a Good Location

Transducers can be located either side of the melt pump, in the die, in the barrel or before a screen changer; however, a good position should be selected.

If a transducer is located too far upstream in the barrel then un-melted plastic pellets may rub against the tip of the transducer causing damage. If the transducer is located too far back in the mounting hole then it may become surrounded by a pool of melting plastic, which will breakdown into carbon and negatively impact the ability of the transducer to transmit signals.

Conversely, if the transducer is located deep in the barrel the screw flights can shear the tip of the sensor.

Avoid Cold Starts

If the extruder is not warmed up to operating temperature before the machines start running, both the transducer and the extruder could be damaged. Instead, a ‘soak time’ is required over which the plastic can reach its molten state.

It should also be noted that if the transducer is taken out of a cold extruder it could result in material sticking to the transducer’s tip, which could result in the diaphragm being torn. Before removing the transducer ensure that the barrel is warm enough for the plastic to be soft.

The Mounting Holes Must Be Clean

The mounting holes of the transducer must always be clean and free of any plastic debris. Prior to cleaning an extruder, all of the transducer should be removed to ensure that they are not damaged. Following transducer removal plastic may flow into the cavities where it can set, if this set plastic is not removed the tips could be badly damaged when the transducers are re-inserted.

Any unwanted plastic residue can be removed using a cleaning tool kit. It should also be noted that extensive cleaning can result in the holes becoming too deep, which can also result in transducer damage. In these instances, spacers should be used to raise the system.

Clean the Transducer with Care

Before cleaning the extruder barrel with cleaning solutions or a wire brush, all of the transducers should be removed to ensure their diaphragms are not damaged.

Removal of transducers should occur whilst the barrel is hot and they should be cleaned using a non-abrasive cloth. The hole where the transducer sits should also be cleaned using a cleaning drill/guide sleeve.

Don’t Overpressure the Transducer

Even though transducers are designed to endure 1.5 times overpressure, avoid the risk of applying too much pressure by ensuring that you are using the correct model designed for your range of extrusion operation pressures.

A good approximation is that transducers should be used that can withstand double the pressure that an application requires. This means that the extruder would have to be operating at an unusually high (and very unsafe) pressure for transducer failure to occur.

Diaphragm Ground Off

Causes:

  • Cleaning using a cleaning wheel or wire brush
  • Contact with abrasive material in process

Solutions:

  • Use Dymax coating to protect from abrasive materials
  • Wipe tip while hot with cloth to remove plastic
  • Avoid using cleaning wheel or wire brushes

Diaphragm Damaged by a Sharp Edge

Causes:

  • System dropped
  • System damaged by a sharp object, e.g. knife or screwdriver
  • System came into contact with degraded polymer in hole

Solutions:

  • Check hole with gauge plug for burrs or hardened plastic
  • Clean the hole using a Dynisco cleaning tool
  • Protect system using a cap during transport or storage

Diaphragm is Missing

Causes:

  • Shrinking of adhesives
  • The diaphragm has bonded to the processing material
  • Removal in cold material condition

Solutions:

  • Remove while hot to avoid adhesion
  • Double-strength Diaphragm (T80) or TiN coated diaphragm

Diaphragm Torn

Causes:

  • Removal when the material was in a cold or hardened state - Shrinkage of adhesive materials
  • Sensor located over flite (fatigue)

Solutions:

  • Remove while hot in order to avoid adhesion
  • Relocate sensor
  • TiN coated diaphragm or double-strength diaphragm (T80)

Diaphragm is Wavy; the Edge is Crushed on the Sensor Tip

Causes:

  • Lateral crushing of the system during installation (cross thread)
  • Non-concentric mounting hole
  • High shear tension

Solutions:

  • Use double strength (T80) diaphragm
  • Inspect the install location
  • Check hole with gauge plug for burrs or hardened plastic
  • Clean the hole using a Dynisco cleaning tool

Seal Surface is Damaged

Causes:

  • Poor cross-threading or mounting hole

Solutions:

  • Inspect hole with gauge plug for burrs or hardened plastic
  • Check thread integrity using a gage plug
  • Clean the hole using the Dynisco cleaning tool

Flexible Connection (Capillary) Broken

Causes:

  • Damage during installation
  • Bend radius too tight
  • Accidental breakage of capillary from overstretching  or cutting

Solutions:

  • Lengthen flex or use 435XL version
  • Handle the exposed capillary with care

Shaft Bent

Causes:

  • External mechanical effect on stem

Solutions:

  • Shorten the stem to match the depth in cases where the stem is too long
  • Over-torqued

Shaft Torn Off

Causes:

  • Mechanical effect
  • Galling – Thread incorrectly fused in the hole

Solutions:

  • Torque requirements should be adhered to
  • Hastalloy threads can be used to minimize galling
  • Anti-seize coating should be applied before installation

Thread Damaged

Causes:

  • Thread forcibly inserted or removed
  • Poor mounting hole

Solutions:

  • The hole should be cleaned using the Dynisco cleaning tool
  • Inspect hole with gauge plug for burrs or hardened plastic

This information has been sourced, reviewed and adapted from materials provided by Dynisco.

For more information on this source, please visit Dynisco.

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