Pressure measurement
- Principle of pressure measurement
- Pressure transducer construction
- Application of pressure transducers
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1. Principle of pressure measurement
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Pressure measurement like any other physical quantity measurement is converting this quantity
into other measurable quantity mostly voltage or current.
Pressure transducers made by Peltron measure pressure by converting it to voltage 0÷10V, or current 4÷20mA.
Output signal from a transducer is very often converted to digital signal of any standard and after digital conversion displayed on
a computer's screen. Peltron delivers both, pressure transducers and signal conditioners. Software is also made by Peltron.
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2. Pressure transducer construction.
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a. How it works.
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Pressure measurement in each pressure transducer made by Peltron
relates to change in resistance of piezoresistors. The change is result of pressure which is put on
the piezoresistors and is proportional to the pressure. Piezoresistors are arranged in bridge shape
and are supplied by constant current 4 mA. Each change of pressure results in change of resistance of the bridge
which results in change of voltage drop on the bridge. The voltage is amplified and converted
to output voltage signal which is in tight relation to the applied pressure. Pressure transducers made by Peltron
consist of two main parts tightly connected with each other:
measure head and electronic board.
- Measure head.
There is a piezoresistive sensor in measure head shaped in Wheatstone bridge
and diaphragm which main task is separation sensor from medium. Volume between
diaphragm and sensor is filled with silicone oil (or other). Diaphragm and other
wetted parts are made from steel 316 or other steel appropriate to medium
which can be chemically aggresive.
- Electronic board.
The electronic board of a pressure transducer consists of a current source and an amplifier. The current source supplies the sensor.
The measure amplifier converts signal from the sensor to output transducer's signal which is a function od applied pressure and is
a voltage or current signal. The amplifier has two potentiometers one to regulate zero ( output signal when no pressure is applied )
and the second one to regulate range (output signal when max pressure is applied ). The electronic board has safety module against
overvoltage. Electrical connector ends the electronic part of a transducer and commonly it is a connector according to DIN43650
or sometimes cable.
The pressure which is measured is applied to the diaphragm and through silicon oil acts on the piezoresistive sensor. It results
in change of piesoresistors' values. There is a voltage signal which appears as sensor's output signal
proportional to pressure which is measured.
Electronic board converts the signal to standard output voltage or current signal.
There are two types of pressure transducers which lack electronic boards only with sensor inside: PT and PTD.
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b. Pressure transducers division.
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Pressure transducers made by Peltron can be classified into four groups:
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| ▪ Absolute pressure transducers |
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Transducer od the group provides an output voltage which is proportional to the difference between
the applied pressure and a perfect vacuum. In such a unit, the piezoresistive sensor is completely evacuated and
sealed. This group is marked by letter A, for example NPXA10 which means:
- NPX marks a series
- A marks absolute pressure to measure
- 10 marks ten bar range
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| ▪ Gage pressure transducers |
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Transducer of the group provides an output voltage which is proportional to the difference between the applied
pressure and the ambient pressure. This group is markes by letter G, for example NPXG10 which means:
- NPX marks a series
- G marks pressure related to ambient
- 10 marks ten bar range
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| ▪ Level probes |
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Level probes are pressure transducers dedicated to measure liquid pressure which is calculated to level height. |
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| ▪ Differential pressure transducers |
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Transducer of this group provides an output signal which is proportional to the difference between two
applied pressures. In such a unit, there are two pressure connectors:
- marked as + to receive the higher of the two pressures
- marked as - to receive the lower of the two pressures
This group is marked by letter D, for example NPXD1 which means:
- NPX marks a series
- D marks differential pressure to measure
- 1 marks one bar range
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c. Ranges.
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Pressure transducers made by Peltron cover pressure ranges from ±1,25 mbar to 1000 bar.
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| Lp | Range min. | Range max. | Transducer type |
| 1 | ±1,25 mbar | ±25 mbar | PXM |
| 2 | 0÷50 mbar | 0÷1000 bar | NPX,PX,PXW,PT |
| 3 | 0÷200 mbar | 0÷2 bar | PMS |
| 4 | 0÷10 bar | 0÷1000 bar | PXT |
| 5 | 0÷1 bar | 0÷1000 bar | GPX |
| 6 | 0÷1 bar | 0÷200 bar | MPX |
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Differential pressure transducers mad by Peltron measure pressure in ranges from ±1,25 mbar to 0÷20 bar.
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| Lp | Range min. | Range max. | Transducer type |
| 1 | ±1,25 mbar | ±25 mbar | PXMD |
| 2 | 0÷50 mbar | 0÷20 bar | NPXD, PXD, PXWD, PTD |
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d. Accuracy of pressure transducers and differential pressure transducers.
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There is one main error of transducers - nonlinearity error.
There are following values of nonlinearity errors:
0,5% MR, 0,25%MR, 0,1%MR, it is calculated from output voltage
to input pressure characteristic.
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3. Application of pressure transducers.
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| Pressure transducers are used in a vast area of applications. There several of them listed below: |
| ▪ | Pressure measurement of fluid sulphur in pipeline supplying boiler KS-13. |
| ▪ | Water pressure measurement in boiler OR-32. |
| ▪ | Brake fluid pressure measurement in brake system of Coal mine lifting machine.
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| ▪ | Mazout pressure measurement in Coal-fired Power Station. |
| ▪ | Brake fluid pressure measurement in car brake system. |
| ▪ | Air wave pressure measurement in acetylen blast test. |
| ▪ | Fluid pressure measurement in two-phases flows lab-test. |
| ▪ | Water pressure measurement in resistance testing in tubes. |
| ▪ | Measure reducers calibrating for liquid. |
| ▪ | Pressure transducer NPX used in water pressure measurement. Telemetric system of a city water network |
| ▪ | Water pressure measurement in a boiler WR-2 |
| ▪ | Pressure measurement in the fire-chamber of a boiler WR-25 |
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