From lab to production,
providing a window into the process
Melt-Pressure Transducers Part II
When selecting a melt pressure transducer, the first factor you should consider is the
pressure range that will be measured. Available in ranges between 0 to 25 psi and 0 to
30,000 psi, transducers must be operated within their stated pressure range to ensure
proper functioning. A transducer can be damaged if the extruder is operated at pressure
levels beyond the transducer's rated capability, resulting in inaccurate pressure readings.
Next, make sure that the transducer's electrical output is compatible with the extrusion
control system. The distance the wire must run, the transducer's proximity to other
equipment, and the amount of electrical noises present in a given system all influence the
type of output that is required to provide usable data.
Durable construction and coating selection are other critical factors to be addressed. The
transducer's lower sensing diaphragm should be between 0.004 and 0.005 inches thick to
ensure durability and remain stable at varying temperatures. Some transducers with thicker
diaphragms are more rugged but less accurate than other designs. On some models, the
diaphragm is attached to the unit by welding across the diaphragm's circumference. This
place stress directly on the diaphragm itself. Other designs employ a diaphragm itself. Other
designs employ a diaphragm cap that shifts all the stresses behind the diaphragm.
Diaphragms can be coated to prevent damage from abrasion and corrosion. The coating
must be chosen for its ability to withstand the effects of the specific process the transducer
is monitoring. Several coatings are available with varying degrees of abrasion and corrosion
resistance, depending on the specific process to be monitored.
A transducer can be damaged if the extruder is operated at pressure levels
beyond the transducer's rated capacity.
In high temperature applications (400 to 600°F), a poorly designed transducer will fluctuate
with temperature changes and give inconsistent readings. A well-built transducer will
compensate for such shifts, providing a reading as close to the measured point as possible.
Melt-Pressure Transducers Part II
Melt-pressure transducers are generally accurate to within ±0.25 to ±1% of full-scale
output. Transducer accuracy is determined by its linearity, repeatability and hysteresis.
Linearity is the maximum deviation of the transducer output from a defined straight line
during a calibration cycle. Repeatability simply refers to the ability of a transducer to
reproduce readings under identical conditions of pressure and temperature. Accuracy is
gauged by totaling all the deviations of a transducer's output from a specified straight line.
Periodically, transducers must be calibrated to ensure precise readings. One way to
accurately calibrate transducers is to select a model with internal calibration, such as an
80-percent-of-full-scale, internal-shunt-calibration resistor that is built into the transducer.
Located in the strain-gage bridge, the compensation resistor offers fast and precise
transducer-to-instrumentation calibration. Internal calibration can be bypassed if external
shunt calibrations at other values are needed.
A final consideration is the ability of the transducer supplier to support its products.
A responsive engineering department and service organization will, for example, help
minimize unproductive downtime if a problem with the transducer occurs. A supplier's
ability to provide custom units can also solve problems associated with specialized
applications.
Installation and Maintenance
Incorrect mounting and installation is one of the most common causes of damage to
transducers. Forcing a transducer into a hole that's too small or has been incorrectly
machined will crush the sensing diaphragm. Make sure that the mounting hole is properly
gauged to prevent thread galling. Attempting to screw a transducer into an incorrectly sized
mounting hole will damage the unit's threads and result in improper functioning.
Forcing a transducer into a hole that's too small or has been incorrectly
machined will crush the sensing diaphragm.
Before starting the extruder drive, turn on the heater bands and allow enough time for
the plastic to reach a molten state. Failure to follow this procedure can damage the
transducer and extruder. The extruder barrel should also be warm before the transducer is
removed. Otherwise, cold melt and material adhering to the transducer tip can damage the
diaphragm as the transducer is removed from the barrel.
Since cleaning materials and procedures can ruin the transducer diaphragm, always remove
transducers from the machine before cleaning the extruder barrel. After the extruder is
cleaned, remove any hardened plastic from the mounting holes before reinserting the
transducers.
Don't use wire brushes or knives to clean solidified polymer from the transducer tip. To
Melt-Pressure Transducers Part II
prevent damage to the diaphragm, heat the probe to the melt temperature, not to exceed
750°F (for most models), and remove the polymer with a soft cloth. Hot fluidized-bed
cleaning systems can also be used to safely clean transducers.
By following these guidelines, the extrusion processor will be assured of years of reliable
service and accurate pressure measurement.
www.dynisco.com
©2016. Dynisco reserves the right to make changes without notice.
Refer to www.dynisco.com for access to Operator Manual and other support documentation.
Hotline1-800-Dynisco
38 Forge Parkway
www.dynisco.com
Franklin, MA 02038 Phone +1 508 541 9400
USA
Fax
+1 508 541 6206
Emailinfoinst@dynisco.com
Dynisco
Dynisco Europe, GmbH
Pfaffemstr. 21
Phone
74078 Hellbronn Fax
Germany
Email
+49 7131 297 0
+49 7131 297 166
dyniscoeurope@dynisco.com
Dynisco Shanghai
Building 7A, No. 568
Longpan Rd Malu
Jiading, 201801 China
Phone +86 21 34074072-819
Toll Free +86 400 728 9117
Fax
+86 21 34074025