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 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 equipment.
Here L1, L2, and L3 represent the loads that are spread throughout a plant. If they are unbalanced, 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 wreak 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.
Twisted Pairs
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 degrees, essentially canceling out the induced currents.
2. MOUNTING HOLES
Concentricity:
It is extremely important that the threads and mounted wells be concentric to within 0.002". This means that on a diameter line, 0.002" per side must be maintained.
Cleanliness:
Mounting holes should always be cleaned prior to installation. They must be free of burrs and any degraded material from prior extrusions.
3. TRANSDUCER INSTALLATION
The installation of a transducer is always a consideration and certain rules must be followed:
Set Back:
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 sheer 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 measurement.
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.