Sensors - July 2001 - Extrusion Melt Pressure Measurement for Food and Medical Applications

PRESSURE

Extrusion Melt
Pressure Measurement
for Food and Medical Applications

A new version of the standard mercury-filled pressure sensor uses mineral oil as the pressure transfer medium and a corrugated sensing diaphragm that compensates the effects of oil expansion at high temperatures and pressures.

Mark Caldwell, Gefran ISI, Inc.

Extrusion processes, whether performed on food or polymers including medical products such as tubing, bags, and packaging, are characterized by temperature as high as 750 F and pressures up to 15,000 psi. The environment in the extrusion machinery becomes even more hostile when corrosive or abrasive substances are added to the melt. The industry standard technology for accurate melt pressure measurement is the mercury-filled pressure sensor, which uses a small column of mercury as a pressure transmission medium. Pressure applied to the sensing diaphragm is transferred by the mercury to a bonded-foil full-bridge (Wheatstone) strain gauge located outside the high-temperature zone of the process. The strain gauge produces an output proportional to the applied pressure. The isolation provided by the mercury is required because standard foil strain gauges are limited to temperature environments <250 F.

After several decades of use by the polymeric extrusion industry, mercury-filled melt pressure sensing technology remains the benchmark for performance, cost, and reliability. Although standard mercury-filled sensors contain very little mercury—typically <0.003 in.³—they are generally unacceptable for applications involving food or medical devices because of the toxic nature of mercury. Various alternative technologies (e.g., the push-rod sensor) have been developed for food and medical device production, but none has performed so well and reliably as the filled sensor design. This fact has long posed a challenge to melt pressure sensor designers.

The ultimate design goal has been to develop a similar, well-proven “filled” design that uses a nontoxic pressure transmission medium. The design would ideally have the same components, manufacturing equipment, assembly processes, and calibration techniques as the standard mercury-filled sensor design. The oil-filled 3000 Series of melt pressure sensors developed by Gefran ISI’s engineering department satisfies these criteria.

Design Evolution
Our first design step was to identify, evaluate, and ultimately select a nontoxic fill medium that would conform to the federal Food and Drug Administration’s standards governing food purity. Furthermore, the fill would have to survive high-temperature environments without charring or breaking down. Certain physical properties were also critical, in particular a low volumetric thermal expansion and low compressibility (the reciprocal of a fluid’s bulk modulus).

After extensively testing many candidate heat transfer fluids, we selected a highly refined mineral (paraffin) oil that is approved by the FDA for contact with and inclusion in food as defined by 21 CFR 172.878. But this wasn’t the only hurdle we had to jump. The oil’s

Photo 1. The new corrugated diaphragm is shown with its tough physical vapor deposited titanium nitride coating for protection against abrasion and adhesion.

thermal expansion and compressibility properties were poor compared to those of mercury. In short, the oil’s high expansion and compressibility at high temperatures yielded a poorly performing sensor.

To compensate for the negative high-temperature behavior of the oil, we developed a corrugated sensing diaphragm (see Photo 1). This design is more compliant than a standard flat diaphragm, and easily compensates the effects of the expanding oil. These properties reduce internal sensor pressure and thus minimize the change of the sensor’s zero output with changes in process temperature. An additional benefit was that being more compliant, the sensor’s change in full-scale sensitivity (or span) with temperature was reduced. Flat sensing diaphragms, if used with the oil we chose, typically “stiffen” with the internal pressure generated by the fill material’s expansion with temperature. The results are poor linearity performance in the lower presıure areas and a decrease in full-scale sensitivity (or span) between room and process temperatures. The corrugated sensing diaphragm minimizes both of these negative effects while at the same time operating at lower stress levels, thus ensuring long-term reliability and overload capability. We also complemented the diaphragm with a tough coating of physical vapor deposited titanium nitride for protection against abrasive processes.

As the 3000 Series design was evolving, we investigated an area of melt pressure performance that has long been problematical—full-scale pressure ranges <1500 psi. Sensors operating at these lower ranges have characteristically yielded poorer linearity and zero change with temperature performance. To ensure optimum linearity, the recognized rule of thumb for sensor design has been to keep the maximum center deflection of the sensing diaphragm to <¼ the thickness of the deflecting plate. Although this was a simple mechanical matter, we had to use a thicker strain sensing element but at the same time maintain the full-scale sensitivity to within the melt pressure industry standard of 3.3 mV/V.

We solved this problem with a unique foil alloy having a gauge factor that is 50% greater than the standard constantan foil. With its slightly higher bridge resistance (550 vs. 350 ), the strain gauge provides higher outputs with less diaphragm deflection. The ultimate benefit was a thicker sensing element design for low pressure ranges, one that offers improved linearity at pressure ranges <1500 psi, with the possibility of producing sensors with full-scale pressure ranges down to 250 psi.

The performance specs of the 3000 Series melt pressure product line are given in Table 1.

TABLE 1

3000 Series Melt Pressure Sensor Performance Specs

Fill medium
Single-phase paraffin oil approved for food contact per:
USDA: H1 Status
FDA: 21 CFR 172.878

Available pressure ranges
250-10,000 psi

Combined error (sum of linearity, hysteresis and repeatability error)
±0.5% F.S. for ranges 5000 psi
±1.0% F.S. for ranges <5000 psi

Maximum process temperature
600ºF (315ºC)

Zero change with process temperature
±0.3 psi/ºF

Sensing diaphragm
Titanium nitride-coated 17-7 PH SS

Zero drift on electronics
<0.01% F.S./ºF

Span drift on electronics
<0.01% F.S./ºF

Internal R-cal circuit
Factory set at 80% F.S.

Overload capacity
2 × rated F.S. pressure range

Outputs available
Ratiometric 3.3 mV/V
4–20 mA
0–10 VDC

Photo 2. The 3000 Series melt pressure system shown here includes the 1/8 DIN pressure indicator and the cable assembly.

The 3000 Series is available in system packages (see Photo 2) with a cable and either a ¼ DIN pressure controller or ¹/₈ and ¼ DIN pressure indicators with alarms and analog retransmission.

Mark Caldwell is Vice President, Operations, Gefran ISI, Inc.

For more information, contact Matthew Carrara, Vice President, Sales, Marketing, and Finance, Gefran ISI, Inc., 8 Lowell Ave., Winchester, MA 01890; 888-888-4474, fax 781-729-1468, matt@industrial-sensors.com.