Oil-Filled Extrusion Melt Pressure Measurement Technology Eliminates Mercury

April, 2001

Instrumentation:

Oil-Filled Extrusion Melt Pressure Measurement Technology Eliminates Mercury

Gefran ISI has developed an oil-filled melt pressure transducer for extrusion applications where the use of mercury is prohibited or where a processor would rather not have exposure. The initial target markets are medical, food and packaging.

The 3000 Series melt pressure transducers are available in standard transducer (mv/v) and transmitter (4-20 mA, 0-10 v dc) outputs. They are also available in 'System Packages' with a cable and either a 1/4 DIN pressure controller or 1/8 and 1/4 DIN pressure indicators with alarms and analog retransmission.

According to Mark Caldwell, Vice President of Gefran ISI, the industry standard technology for accurate melt pressure measurement has been, and still is, the mercury-filled pressure sensor. And while the standard mercury-filled sensor contains very little mercury, typically less than 0.003 cu in, this sensing technology is typically not used in medical extrusion because of the presence of mercury.

Various non-mercury-sensing technologies (such as the push-rod sensor) have been developed, however none has performed as well and reliably as the filled sensor design, Caldwell adds.

"In developing the 3000 Series, the first design challenge was to select a fill media that is non-toxic and preferably approved by the Food and Drug Administration (FDA)," says Caldwell. "Secondly, the fill had to survive and not char or breakdown in high temperature environments. Specific physical properties were also critical, particularly a low volumetric thermal expansion and low compressibility (numerically the inverse of a fluid's bulk modulus)."

After extensive testing of many different heat transfer fluids, Gefran ISI chose an oil that met the non-toxicity requirements, but more design challenges remained, says Caldwell. "Being a highly refined mineral (paraffin) oil, the thermal expansion and compressibility properties, when compared to mercury, were poor."

To compensate for the negative high temperature behavior of the oil, Gefran ISI engineers developed a corrugated sensing diaphragm. "Compared to a standard flat diaphragm, the corrugated design is more compliant and easily compensates for the effects of the expanding oil. This reduces internal sensor pressure and thereby minimizes the change of the sensor zero output with changes in process temperature."

Caldwell adds that another benefit was found in that, being more compliant, the sensor change in full scale
sensitivity (or span) with temperature is minimized.

"Typically, flat sensing diaphragms, if used with such an oil would 'stiffen' with internal pressure generated by the fill material's expansion with temperature. This results in poor linearity performance in the lower pressure areas and a decrease in full scale sensitivity (or span) between room and process temperature", according to Caldwell.

The corrugated sensing diaphragm is designed to minimize both of these negative effects, while also operating at low edge stress levels ensuring long term reliability and overload capability.

This new corrugated diaphragm was complemented with a tough, PVD (physical vapor deposited) titanium nitride coating, which is said to provide robust performance and protection against abrasive processes.

As the 3000 Series design was evolving, Gefran ISI engineers also investigated an area of melt pressure performance which has long been a challenge: full-scale pressure ranges less than 1500 psi. These lower
pressure ranges typically yield poorer linearity and zero change-with-temperature performance.

Caldwell says: "From a linearity perspective, the recognized rule of thumb for sensor design is to keep the maximum center deflection of the sensing diaphragm to less than one-quarter the thickness of the deflecting plate to ensure optimum linearity."

Although a simple mechanical challenge, Gefran ISI engineers had to work by this rule of thumb (that is, to utilize a thicker strain sensing element) but to also maintain the full scale sensitivity to within the melt pressure industry standard of 3.3 mv/v.

The engineers solved this problem by introducing a higher gage factor strain gage for these low-pressure ranges. A unique foil alloy with twice the gage factor of the standard constant foil was selected. With a slightly higher bridge resistance (550 ohms compared to 350 ohms) this higher strain gage ensures higher outputs with less diaphragm deflection.

The benefit is a thicker sensing element design for low-pressure ranges, which yields improved linearity at pressure ranges below 1500 psi, with the possibility of producing sensors with full scale pressure ranges down to 250 psi.

Gefran ISI cites the following specifications for the 3000 Series:

According to Matthew Carrara, Vice President, the company is offering the new transducers at the same price as its current mercury-filled line. For example a basic melt pressure transducer with a 3.3mv/v output would sell at the same price as the oil-filled equivalent or $665.00. "We did this so the customer could choose either version and not pay a premium for getting non-mercury units, which seems to be the trend elsewhere in the industry," saysCarrara.

Gefran ISI
Winchester, MA