Pressure Sensors: Melt Pressure Transducer

A sensor is any device that produces a measurement of a physical value by transforming this value into a signal output that a user can be observed, or one displayed instrumentation.

Pressure Sensors are utilized for numerous applications to help operators control and monitor the stability of various processes while preserving equipment safety.

Here, the focus is on the melt pressure transducer that is applied in plastic extrusion and injection molding. Calibration of the signal output from the sensor must be performed in line with instrumentation.

This is considered crucial to the reliability and accuracy of the pressure measurement.

Appropriate calibration should be conducted at operating temperature with the transducer at rest (no pressure applied) to recompense the temperature effect to zero. All Dynisco pressure transducers come with a rating of temperature effect on zero, and this rating can differ depending on the selected fill media in the transducer capillary.

The zero-output level of the signal will be impacted by the physical properties of the liquid fill media, and this value must be considered when allocating the range with instrumentation and adjustments made for the necessary compensation.

This adjustment would be akin to setting a TARE on a balance, where the temperature effect is analogous to the package of the product being weighed.

The temperature effect published for the three liquid fill options are as follows: 

  • Hg = 15 psi/ 100°F or 27 psi/100°C 
  • Oil = 36 psi/ 100°F or 65psi/100°C 
  • NaK = 30 psi/ 100°F or 54psi/100°C 

According to these values, it is expected that it will be necessary to configure the instrument to accept the signal input as zero by following calibration procedures at the operating temperature of the process.

Example: Process temperature = 400°F, Hg type sensor displays the signal output of 60 psi @ zero pressure

Therefore, depending on the full-scale pressure range of the transducer, this offset would be in addition to the standard combined error specification; so a 10,000 psi range carrying 0.5% combined error (+/- 50 psi) would actually seem to have an offset of 110 psi or more at any given time.

It is critical to bear in mind that when calibration is being performed, there are no alterations made to the signal output, only confirmation that output is still within a tolerance of the model specifications.

Dynisco instruments have been designed to streamline the calibration process of pressure sensors. With minimal keystrokes, Dynisco’s indicators and controllers can test the high and low limit of the signal output.

This information has been sourced, reviewed and adapted from materials provided by Dynisco.

For more information on this source, please visit Dynisco.

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