Placement:

Positioning of the transducer should always be a consideration. The best position is forward of the screw where the polymer is in its most molten state. This also happens to be the point at which shear stress is minimal.

When transducers are used in reclaimed extrusion lines, they should be downstream of the feed hopper. Unmelted pellets that are close to the hopper can inflict severe diaphragm damage by producing stress cracks caused by point loading of the pellet.

Transducer Strain Gauge Housing:

The transducer strain gauge housing should always be away from the hot zone. In general, if you can place your hand on the strain gauge housing and leave it there, it is not too hot. Short-stem rigid transducers should not be mounted on top of the extruder. Rising heat from the extruder (the chimney effect) will invariably increase the housing temperature past its acceptable limits (170 degrees F to 200 degrees F).

4. DIAPHRAGM CONSIDERATIONS

High shear stress: One additional solution to high stress on the diaphragm is to make it thicker. However, if it becomes too thick, it gets stiffer and loses all flexibility and becomes less pressure sensitive and more temperature sensitive.

Corrosive Material

Many extrusion products require that corrosive materials be introduced into the process. Needless to say, these acids create havoc with stainless steel to a point where the diaphragm fails. In these corrosive situations it is best to use Hastelloy, a nickel based alloy. It is always good practice to manufacture both the threads and the diaphragm out of Hastelloy.

Abrasive Materials:

Abrasive materials such as 30% glass-filled nylon is so abrasive that it wears down the transducer so that it takes on the shape of the internal curvature of the barrel. The best protection is ISI's Modification 261, which was designed specifically for ceramic extrusion, ceramic being a more abrasive material than glass filled nylon.

Other Types:

The section of coating is determined by the abrasiveness or corrosiveness of the material.

5. TRANSDUCER CALIBRATION

If possible, a calibration check of the transducer/transmitter should be performed on a regular basis. ISO 9000 standards dictate frequent calibration checks.

Transducer Calibration

When a transducer is shipped from ISI, the user receives a certificate that records three parameters as shown:

PARAMETER	TYPICAL
Zero Balance	0.3 mv/v
Full Scale Sensitivity	3.33 mv/v
R-Cal @ 80%	2.66 mv/v

A full 10 point calibration is not normally needed by customers but it can be provided, if required, at an additional cost. These three points may be used as a reference to determine if the transducer is still within specification.

A visual examination of a transducer diaphragm should be done before proceeding with the calibration check to determine if the diaphragm is flat and free from any damage. A quick check of the three parameters may be done now. The data should be recorded and compared against the data of the transducer as received. The following table illustrates the data presentation:

DATA POINT	AS CHECKED	AS RECEIVED	DEVIATION
Zero	0.3 mv/v	0.3 mv/v	0
FS	3.33 mv/v	3.33 mv/v	0
80% FS	2.66 mv/v	2.66 mv/v	0

The deviation in this illustration is zero, indicating that the transducer is fine. A deviation of +- 0.25% is acceptable. Please note that the data are given in millivolts/volts and not millivolts. This is done in order to eliminate the possibility of the use of different power supply voltage, which will give different output voltages. Outputs in millivolts/volts is independent of the supply voltage. The user may want to go further and check the linearity of the transducer if a complete calibration system such as the ISI CST2000 is available. Generally speaking, the shape of the curve is not important as long as the non-linearity is within the original tolerance of the transducer as shipped. Figure 4 depicts the normal definition of a calibration curve.

Transmitter Calibration

The transmitter calibration is not as straightforward as calibrating a transducer. This is due to the fact that an amplifier is built into the transmitter. The purpose of the amplifier is to give the user the ability to re-zero the transmitter at temperature. This means that zero at room temperature will be negative after the transmitter is removed from the extruder, making it difficult to check the original zero balance, which was, prior to installation, noted and recorded at room temperature. The user should do a visual diaphragm inspection and then perform a 5 or 10 point calibration check.

DIAPHRAGM MATERIAL & COATING SELECTION CHART

The standard ISI transducer diaphragm is machined out of a single piece of type 15-5PH stainless steel, heat treated and then Armoloy coated. This material gives ISI transducers the traverse strength and toughness needed for most standard applications.

There are, however, certain extrusion processes that require different types of diaphragm materials and/or coatings. ISI is able to supply customers with a diaphragm and coatings specifically suited to their needs and applications.