An important ingredient of "The ISI Advantage" is the vast storehouse of knowledge and on-site
experience accumulated by ISI's people during more than 25 years of visiting extrusion plants which,
by any measure, are among the most hostile environments for equipment and instrumentation,
including our own melt pressure sensors.
Observing firs-hand how these conditions can effect operating efficiency has given ISI personnel the
experience to advise plant operators on ways to make their process environments safer and more
efficient by avoiding or eliminating conditions that create downtime and erode productivity and
The purpose of this discussion is to familiarize you with these conditions so that you can be a more
effective problem-solver. ISI personnel are experts in solving problems which commonly plaque
extrusion plant operations in these five areas:
1. Noise Interference
2. Mounted Holes
3. Transducer Installation
4. Diaphragm Considerations
5. Transducer Calibration
6. Transmitter Calibration
Noise interference is often misunderstood or misdiagnosed because it seems so mysterious and
complex. In reality, the problem is quite easy to solve by following well-known and simple
Noise problems caused by electrical interference are usually traceable to poor grounding,
inadequate and/or insufficient electromagnetic protection in the signal processor and/or the readout
Every single instrument and transducer (transmitter) that carries the ISI name has received the C.E.
mark of approval. This means that ISI products are immune to electromagnetic energy and noise
caused by motors, switches, heaters, compressors, walkie-talkies and unbalanced factory loads. DC
motors are particularly troublesome, but independent outside testing has shown that ISI products
are exceptionally well protected against energy spikes induced by rapid switching of the DC motor
control current. (Pulse width modulation.)
Because all melt pressure sensors are grounded at the tip, they are in fact grounded when they
come into direct contact with the extruder. For this reason, it is imperative that the shield of the cable
which connects the transducer to the instrument be attached to the sensor via the connector. This
can be accomplished simply by attaching the shield under on of the screw terminals located on the
transducer. Do not connect the shield to the readout instrument.
It is important that the shield be grounded at one end. Otherwise, a ground loop may be created
through the shield which would defeat the purpose of a single-ended grounded shield called a
"Faraday Cage," (for its developer Michael Faraday.) This common technique is used to extract
noise, collect it via the shield and bring it harmlessly to ground.
There are some instances where grounding at both ends of the cable seems to be the only solution.
In these cases, care must be taken to ensure that a ground loop does not exist. In order for a ground
loop to exist, a potential difference must exist at the transducer and the instrument. (See Figure 1)
If V1 = V2 there is no current flow. If V1 is greater or less than V2, current Ig will flow through the shield.
UNBALANCED Y CONNECTION:
Many extrusion plants use a power service known as a Y connection to power up the plant and its
L1, L2 and L3 represent the loads that are spread throughout a plant. If they are unbalance, as often
happens, a current (In) will flow through the neutral. This current is, in fact, a sine wave but its
frequency is three times the line frequency, or 180 cycles. Depending upon the amplitude, this
neutral current can raise havoc with instruments, since the ground is varying, and not rock-stable as
it should be. This current can be read with a conventional current meter.
It is always good practice to use twisted pairs for all excitation and/or signal leads. A magnetic field
can induce current flow in the signal/excitation leads if a good shield (Faraday Cage) is not present.
Consequently, twisting the wire leads provides a second line of defense by causing these induced
currents to be phase-shifted by 180°, essentially canceling out the induced currents.
2. MOUNTED HOLES back to top
It is extremely important that the threads and mounting wells be concentric to within 0.002". This
means that on a diameter line, 0.002" per side must be maintained.
If the edge of the transducer comes into contact with the side of the hole, a significant amount or
torque could be developed when inserting or removing the transducer, with the result that the tip will
be crushed and hence could cause damage to the diaphragm. It could even cause the weld at the
diaphragm to fail.
Mounting holes should always be cleaned prior to installation. They must be free of burrs and any
degraded material from prior extrusions.
3. TRANSDUCER INTALLATION back to top
The installation of a transducer is always a consideration and certain rules must be followed:
ISI recommends that the transducer be set back about 0.02" by using washers or shims. This will
prevent damage to the diaphragm caused by high shear stress along the face of the transducer,
which can actually push the diaphragm forward. This damage will show up as a wave along the front
of the diaphragm. Care must be taken to ensure that this set back is not greater than 0.02".
Otherwise, degraded polymer can collect inside the depression and interfere with the pressure
Nylon and polycarbonate are particularly troublesome if the set back is greater than 0.02. As the
material cools down, it tends to stick to the diaphragm and, as it shrinks, it actually pulls the
diaphragm into a bowed-out position. Damage will occur when the transducer is removed or when
the next extrusion cycle begins. In this situation, the diaphragm will most likely be pulled off.
If possible in nylon and polycarbonate extrusion, it is better to have the diaphragm as close to flush
as possible, providing the transducer is not directly over a screw flight which puts it into the path of
high shear stress. When installing or removing the transducer, always keep the extruder at
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. Un-melted pellets that are close to the hopper can inflict severe diaphragm damage by
producing stress cracks caused by point loading of the pellet.
TRANSDUCER STAIN 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 stain 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°F to
4. DIAPHRAGM CONSIDERATIONS back to top
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.; (An 0.008" thick diaphragm is recommended,
but zero shift, which is normally 15 PSIG/100°F, will now double to 30 PSIG/100°F.)
Many extrusion products require that corrosive materials be introduced into process. Needless to
say, these acids raise 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 material such as 30% glass-filled nylon is so abrasive that it wears down the transducer so
that it take 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.
The selection of a coating is determined by the abrasiveness or corrosiveness of the material
(Consult the Diaphragm Material & Coating Selection Chart which can be found on the following
5. TRANSDUCER CALIBRATION back to top
If possible, a calibration check of the transducer/transmitter should be performed on a regular basis.
ISO 9000 standards dictate frequent calibration checks.
When a transducer is shipped from ISI, the user receives a certificate that records three parameters