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Compound Pressure Ranges
Compound pressure ranges are ones that combine positive and negative pressure ranges into one pressure measurement device.
For example you may want to purge a vessel with a vacuum pressure down to -1 bar gauge to empty it of its contents and then fill the vessel again using a pump which has a positive pressure of 5 bar gauge.
If you fit 2 pressure sensors or gauges to measure the pressure, the vacuum range could be over pressured and damaged when the +5 bar gauge is applied unless it is protected in some way. Also it is more expensive to purchase two pressure instruments.
So the solution is to the combine both pressure ranges into one pressure instrument. So in the above example if a 4 to 20mA output signal is required 4 mA would equal -1 bar gauge and 20 mA would equal +5 bar gauge.
The main disadvantage of using compound ranges is that the accuracy is compromised when compared to two individual ranges.
Firstly this is because generally 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.
Secondly the accuracy of the lesser range typically the negative one is driven by the greater range typically the positive one since there is only one sensing diaphragm which is rated to the greater of the 2 ranges.
Thirdly a sensing diaphragm will have slightly different mechanical characteristics in either direction which are difficult to eliminate in analogue circuits.
Fourthly when the sensor is relaxed there is a small degree of mechanical slack 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.
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Site Last Updated: 17/05/08
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