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Proper Care for Tempco Melt Pressure Transducers

Overview
   
The Melt Pressure Transducer is a measuring instrument that is subject to damage if mishandled, improperly installed, or dropped.
   
Sensing Diaphragm
The Tempco fluid transducer is a sealed system. The sensing diaphragm is one of the most important components of the transducer. The diaphragm is a one-piece construction, machined out of 15-5 PH stainless steel and is coated with Armoloy to increase the surface hardness, offering resistance to abrasion. It has a nominal thickness of 0.0045 inches.
If the sensing diaphragm becomes damaged--that is, dented, gouged or deformed in any way, the transducer will no longer provide accurate readings and should be returned for repair/replacement.
 
Transducer Mount
A bad transducer mount is one of the key causes of damage to the transducer. Extruder barrel wear should be monitored. As the extruder barrel wears, the transducer tip will get closer and closer to the barrel bore until flush with the sidewall. Once the tip is flush, it will then wear at the same rate as the extruder barrel. To alleviate this problem, Tempco recommends that the transducer be recessed 0.015 inches from the barrel bore. A copper shim can be manufactured (to seal on the 45º seating surface) that will act as a spacer backing off the transducer tip from the inside barrel bore.
If the transducer tip protrudes into the extruder barrel, it is subject to unnecessary wear and is vulnerable to damage by a passing screw flite.
Another common transducer mount problem that is a result of barrel wear is rollover. Over time the opening into the barrel is reduced from the recommended dimension of .312"/.314". If a transducer is installed in this under-size hole, the tip becomes crimped or forced into a cone shape, deforming the sensing diaphragm and thus damaging the instrument.
 
Gauging the Transducer Mount
The transducer mount should be gauged prior to the installation of a new transducer or replacement of an existing transducer. The proper method for gauging the transducer mount is to use a gauge plug coated with machinists blue. If the blue is removed in an area, then there is a burr or imperfection in the transducer mount that must be repaired.
 
Mounting Threads
The transducer mounting threads should be a 1/2-20UNF-2B and should be cleaned and inspected prior to the installation or reinstallation of a transducer. The use of a high temperature anti-seize compound will help in preventing galling and will allow easy removal of the transducer. Copper graphite anti-seize compounds should be used, such as Never-Seez by Bostik or C5-A by Felpro.
 
Mounting Torque
Although the transducer is capable of being installed at a mounting torque of up to 500 inch-pounds without damage or zero shift, an adequate seal is made with 100-200 inch-pounds. Excessive mounting torque will cause seizing and make the transducer difficult to remove.
 
Operating Temperature
The transducer tip is capable of operating at temperatures up to 750ºF (400ºC). The transducer electronics are capable of operating at temperatures of up to 170ºF (70ºC). It is important that the electronic housing not be placed on top of a heater or a heat source.
 
Cold Starts
One of the most common causes of diaphragm damage is cold starts. A combined temperature and pressure transducer can be used to insure that the extruder barrel is up to temperature prior to the rotation of the extruder screw.
 
Removal/Replacement
The transducer should only be removed or replaced while the machine is at operating temperature and the polymer is liquid. Removal of the transducer from a cold extruder will cause diaphragm damage due to polymer adhesion. A transducer should never be installed into a transducer mount where there is solid polymer. If the diaphragm is forced against a solid, the transducer can easily be overloaded and left with a very high zero offset--too high to be re-zeroed by the zero potentiometer on the instrument.
 
Tip Cleaning
If the transducer is removed warm, the tip can easily be cleaned with a soft cloth. If there is solid polymer present, a fluidized bed cleaning system can be used. Never use a wire brush, knife, or torch to clean the tip of a transducer.

Given proper care, Tempco transducers will provide years of service.