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Compound Pressure Ranges

Compound pressure ranges include both a positive and negative pressure range in one pressure measurement device.

Compound pressure range -1-5bar to 4-20ma

Compound pressure devices measure both vacuum and positive pressure in a single instrument, simplifying installation and reducing costs compared to using two separate devices. They are ideal for processes requiring pressure fluctuations across zero, like vacuum packaging, sterilization, leak testing, and pneumatic conveying. While convenient, compound pressure devices present an accuracy trade-off: increasing the positive pressure range can reduce the accuracy of the vacuum measurement. Careful consideration of the application’s specific accuracy needs is essential when selecting a compound pressure instrument.

  • Single instrument for measuring both vacuum and positive pressure.
  • Simplifies installation and reduces costs.
  • Ideal for processes with pressure fluctuations across zero.
  • Accuracy trade-off: Larger positive ranges can reduce vacuum accuracy.
  • Compound Range Pressure Gauges - Compound range pressure gauges are designed to measure both positive and negative pressures within a single instrument, making them ideal for applications where vacuum and pressure conditions exist.
  • Compound Range Pressure Transmitters - Compound range pressure transmitters for measuring plus and minus pressures using the same device.

Compound pressure measurement devices, also known as vacuum-to-pressure transmitters or gauges, offer a unique solution for applications requiring the monitoring of both positive and negative pressures within a single instrument. This capability is crucial in processes where pressure fluctuates across the zero point, such as in tank filling and emptying, or pneumatic systems with varying supply and exhaust pressures. A typical example is a process involving purging a vessel under vacuum and then pressurizing it for a subsequent operation. Instead of employing two separate instruments – one for vacuum and another for positive pressure – a single compound pressure device can accurately measure across the entire pressure range.

Consider a scenario where a vessel needs to be evacuated to -1 bar gauge for content removal and then pressurized to +5 bar gauge for a subsequent process. Using two separate pressure instruments increases the risk of damaging the vacuum sensor during the pressurization phase if it’s not adequately protected. Furthermore, the cost and complexity of installation increase with two devices. A compound pressure transmitter, on the other hand, seamlessly measures both vacuum and positive pressure, simplifying the setup and reducing the risk of instrument damage.

Featured compound pressure range products

For instance, a compound pressure transmitter with a 4-20mA output signal can be configured to represent the -1 to +5 bar gauge range. In this configuration, 4mA corresponds to -1 bar gauge, while 20mA represents +5 bar gauge. This linear relationship allows for precise monitoring and control of the pressure throughout the entire process cycle. This type of setup is common in applications like these:

  • Vacuum Packaging: This process often involves drawing a vacuum to remove air and then sometimes backfilling with a modified atmosphere gas at a slight positive pressure. A typical compound range here might be -1 bar to +0.5 bar.
  • Medical Sterilization: Autoclaves and other sterilization equipment often use a vacuum stage to remove air before pressurizing with steam. A compound range of -0.8 bar to +2 bar could be typical.  
  • Leak Testing: Systems designed to detect leaks in sealed containers might use a vacuum to initially evacuate the part and then apply a slight positive pressure to see if the pressure decays. A range like -0.5 bar to +1 bar might be used.
  • Dehydration/Drying: Vacuum drying processes often involve pulling a deep vacuum to remove moisture and then sometimes using a slight positive pressure of dry gas to assist in the final drying stages. A possible range could be -0.95 bar to +0.2 bar.
  • Pneumatic Conveying: Some pneumatic conveying systems might use a vacuum to draw materials into a pipeline and then positive pressure to transport them. The specific range would depend heavily on the material and distance, but it could involve both vacuum and positive pressure.
  • Industrial Furnaces (Vacuum Furnaces): While I mentioned these before, it’s worth reiterating. Vacuum furnaces often use a vacuum for degassing and then might use a positive pressure of inert gas for specific heating or cooling cycles.

The ability to monitor both vacuum and pressure with one device not only simplifies the instrumentation but also reduces potential leak points and maintenance requirements. This is particularly beneficial in critical applications where system integrity is paramount.

However, a key consideration with compound pressure ranges is the inherent trade-off between the span of the pressure range and the accuracy, particularly within the vacuum or negative pressure portion. As the positive pressure range of the instrument increases, the resolution and accuracy of the negative pressure measurement can be compromised. This is because the sensor’s full-scale range is being stretched to accommodate a larger overall pressure span. Consequently, the relative change in signal for a given change in vacuum pressure becomes smaller, leading to reduced accuracy. Therefore, it is common practice to select compound pressure ranges where the positive pressure range is typically two to five times the negative range to achieve a reasonable compromise in accuracy across both ranges. While ratios of 10, 20, or even 30 times are not uncommon, it’s crucial to understand that the vacuum range accuracy performance will be significantly lower than the positive range accuracy in such cases. Careful consideration of the specific accuracy requirements for both the vacuum and positive pressure portions of the application is essential when selecting a compound pressure instrument.

Featured compound pressure range products

Help

Accuracy compromise

How does the accuracy of a compound pressure range instrument compare to the performance of individual single range devices?

The accuracy of an analogue pressure sensor is specified as a percentage of the full span which would be 6 bar in the above example. The accuracy of the lesser range, which is typically the negative one, is driven by the accuracy of the greater positive range. Since there is only one sensing diaphragm, it has to be pressure rated to the highest of the 2 pressure ranges.

A sensing diaphragm will have slightly different mechanical characteristics in either direction which are difficult to eliminate in analogue circuits.

When the sensor is relaxed there is a small degree of mechanical play, especially with more mechanised sensing technology that produces what is called a dog leg characteristic around the null point. This dog leg effect creates a zero offset in either direction which cannot be trimmed out by analogue circuitry easily.

The impact of the last two effects can be reduced if the pressure measurement instrument is digitally compensated but this is only worthwhile with sensing technology that has good long term repeatability.

Dog leg shift

What is meant by a “dog leg” shift that occurs with compound pressure ranges, is there an obvious shift in the output when transitioning from positive to negative gauge pressure?

Yes there is a shift in output across the null position of the sensing diaphragm, there is always a region of diaphragm movement around the null with very little change in output. This effect varies depending on different sensor technology, and it should always be included in the overall specification error.

Is negative part of compound range absolute or gauge

Please could you advise us for a pressure sensor with a -1 to 4 bar, which we believe has a negative rating, and therefore needs to be pressure absolute, but on the part number and data sheet says it is a gauge pressure. Could you clarify if this is correct?

Part of the range is negative, so it will also measure in the vacuum range. Absolute does not necessarily mean a vacuum range though, but refers to the type of pressure reference.

For a -1 to 4 bar pressure range, it is only possible to have a gauge reference or differential type with a negative range. It is not possible to have a negative absolute pressure because the reference is a perfect vacuum.

Alternatively the closest absolute range would be 0 to 5 bar, but this is not exactly the same unless atmospheric pressures happens to be exactly 1 bar, since a gauge reference tracks with changes in atmospheric pressure. Absolute reference is fixed to a perfect vacuum.

-1 to 4 bar compound range pressure transmitter

  • SKU ID: s1-dmp331-0006
  • Part No: 110-V402-1-3-100-100-1-000
  • Pressure Range: -1 to 4
  • Units: bar
  • Range Type: Compound Gauge (-P1 to +P2)
  • Output Signal: 4-20mA (2 wire)
  • Accuracy: 0.35%FS >= 0.4 bar, 0.5% < 0.4bar (11-12bit resolution)
  • Electrical Connection: DIN43650A plug IP65
  • Process Connection: G1/2 DIN3852 male
  • Process Connector Material: Stainless steel 316L
  • Diaphragm Material: Stainless steel 316L
  • Media Exposed Seals: FKM

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