Transducer Definition

Question: How can a transducer be defined?

Answer: A transducer is a device with the capacity to convert one kind of energy to another. This conversion can be to/from electrical, electro-mechanical, electromagnetic, photonic, photovoltaic, or any other form of energy.

Example: Via electromagnetic conversion, a guitar amplifier takes the vibrations of the guitar’s strings and converts this to a sound frequency that is heard through the speaker.

Transducer Technology

Question: How does a pressure transducer function?

Answer: By integrating the foil strain gauge, a filled capillary and diaphragms to acquire a controlled pressure change which is electronically converted to signal output, that is then scaled to a distinct pressure range.

Signal Outputs

Available signal outputs for Dwel sensors:

• Millivolt (one thousandth of a volt) symbol = mV
• Milliamps (one thousandth of an ampere) symbol = mA
• Volt (the measure of electric potential) symbol = V

Available ranges for the various signal outputs: 

• Millivolt = 2 mV/V, 3 mV/V, 3.33 mV/V (other ranges available upon request)
• Milliamps = 4-20 mA, 4-20 mA with HART (pressure ranges can be assigned)
• Volt = 0-5 V, 1-6 V, 0-10 V, 1-11 V (other ranges available upon request)

Benefits of Pressure Transducers 

In Plastic Extrusion, pressure fluctuations at the die entrance could be the result of either an actual rate fluctuation coming from the extruder or a variation in temperature impacting viscosity. 

• Pressure fluctuations are generally controlled using gear pumps
• Gear pumps can be made to have pressure transducer ports
• Transducers will allow the operator to make changes to process variables based on pressure readings identified via signal output to instrumentation.
• Instrumentation can be programmed to stop the extruder if a pressure condition occurs that may compromise the equipment.
• Pressure signals to instrumentation allow management of screen changers that filter out particle contaminants in the melt.
• Instruments will monitor and log pressure variations at the die head, offering the operator the appropriate data to improve the process

When melt temperature is stable, pressure variations are often a result of: 

• Barrel Temperature change
• Inconsistent screw speed
• Operating conditions
• Poor screw design
• Variations in Feed Stock

Transducer Location 

• Before screen changer
• Before melt pump
• After melt pump
• In the dies 

Location Benefits 

Appropriate position of the transducer avoids unnecessary pressure build-up while ensuring adequate melt feed to pump. As well as ensuring adequate pump pressure to die and adequate pressure for the product, it ensures safety. 

Location 

Too close to the feed zone can result in damage from unplasticized pellets. Mounting too shallow in the hole leads to excessive deterioration of the material and freeze-off. 

Mounting too deep can cause damage to the tip resulting in measurement errors.

Repeated hole cleaning can result in holes being ‘too deep,’ as well as possible tip damage. Therefore, make use of spacers. 

Dry Location 

Standard transducers are not water-tight. Dwel circuitry can withstand extrusion plant environments but will not function when wet. Therefore, it is crucial to specify water-tight transducers when wet operation is unavoidable. 

Transducer Mounting 

Mounting holes are important as they cannot be eccentric, too small, or too large. 

Mounting Depth 

• Usually recessed around 0.010” this depth will help avoid any unnecessary diaphragm wear and prolong the life of the pressure transducer: Kit modelto make the holes is KT set.

Mounting Conditions 

When mounting transducers, always use a high temperature anti-seize compound. It is important to also use proper mounting torque: 

• 100-200 inch-pounds (8-16 ft lbs.) may vary on transducer type
• 500 inch-pounds Maximum (250 for push rod)
• Rule of thumb is finger tight plus 1/4 turn 

Transducer Mounting Techniques 

For the machining of a new hole, use Dwel Machining Tool KT set. The following considerations when mounting torque upon installation should also be made:

• Too little causes leaking
• Too much results in seizing and damage to the transducer 

Dwel machining tool kit KT series

Removal of the Pressure Transducer 

• Always remove the transducer while the equipment is hot and clean away any polymer residue from the tip by using a cotton rag. 
• Do not use the wire wheel on the bench grinder!
• Always clean the hole before reinstalling the transducer
• Use the DwelCleaning Kit
• Dwel Cleaning tool kit CT series

Cleaning the Mounting Hole 

Always remove the transducer prior to cleaning the barrel with abrasives, cleaning compounds, or a wire brush. Remove the transducer while the barrel is still hot and wipe the transducer tip clean. Remember to return and clean the hole with the cleaning drill/guide sleeve 

Clean Mounting Hole

Mounting Hole Problems 

• Problem: Damaged tip – Reason: Improper hole
• Problem: Galled threads – Reason: No anti-seize used
• Problem: No response – Reason: Over-pressured

Selecting Pressure 

Transducers operate best at midrange: 

• Transducers can handle 1.5 times overpressure without damage.
• Overpressure can occur when cold material inhibits the flow.
• Verify that the operation of the transducer is within the rated range.
• Pay special attention to the pressure range when extruding varying materials.
• Like people, over-pressured transducers cannot carry out their job properly. 

Calibrating a Transducer 

• When? When it’s up to operating temperature and at zero pressure.
• How often? When your ISO9000 standards dictate, or if the zero appears to have shifted. 

It’s simple: Dwel transducers have a built-in 80% R-Cal, and indicators that utilize R-Cal to calibrate the span. 

Shunt Calibration ‘R-CAL’ 

All Dwel Melt Pressure Transducers incorporate a built-in 80% of full scale ‘R’-CAL. This is achieved using an internal resistor to electrically unbalance the bridge rather than strain-induced via applied pressure. ‘R’-Cal supplies a rapid and accurate method of transducer-to-instrumentation calibration.