Pressure transducers are an advanced form of pressure sensor element. The smallest possible component defined by the measuring principle is the pressure sensor. It is the pressure sensor that changes the physical variable 'pressure' into a quantity that can be processed electrically. A pressure transducer is the next level of sophistication. In a pressure transducer the sensor element and casing are in electrical contact and have a pressure connection.
The typical output signals from pressure transducers lie between 10 mV and around 100 mV, depending on the sensor principle in question. These signals are not standardised, however, nor are they compensated. With pressure transducers of the thin-film type it is customary for just the sensor element to be welded to the pressure connection and then bonded electrically. Priezoresistive pressure transducers, on the other hand, need to run through far more production steps since the semiconductor sensor element has to be protected from the influence of various media by a chemical seal.
Pressure transducers do not contain any active electronics and hence no components which stabilise the supply voltage. This means that the output signal is always proportional to the supply voltage. Typical output signals are, for example 2 mV/V with thin-film pressure transducers and 100 mV/V with piezoresistive pressure transducers. The output signal is proportional to the supply voltage.
Every pressure transducer has not only sensor-specific output signal but also a temperature error that is typical for the particular sensor principle. Figure 1 shows the typical characteristic curves for the temperature errors of
a) thin-film pressure transducers
b) piezoresistive pressure transducers.
As this diagram shows, thin-film pressure transducers have a characteristic curve that it is far easier to compensate than that of pressure sensors made of semiconductor materials.
The linearity figures specify the value of the maximum deviation of the characteristic curve from the maximum line through zero and the end point.
Pressure measuring converters
Pressure measuring converters represent the next stage of assembly of pressure measuring instruments. At their electric outputs they have standardised output signals that can be relayed directly to follow-up control units, eg, 4 to 20 mA.
They consist of:
* An elementary sensor (a. piezo, b. thin-film strain gauge).
* Pressure connection.
* Signal processing electronics.
* Electrical connection.
No typical designs have crystallised on account of the large number of different applications, but a certain trend is visible:
Pressure connection: G 1/2" (in the USA: 1/2" NPT), CrNi steel
Output signal: 4 to 20 mA, 2-wire
Casing material: CrNi steel
The specifications and definitions of characteristic data for electric pressure measuring systems have been standardised in DIN 16 086 since 1992. Unfortunately, not all the data sheets for pressure transducers and pressure measuring converters comply with this DIN standard, leaving the user with no more than a distorted picture for comparing technical specifications.
It is customary to quote the following data in technical descriptions of pressure sensors, pressure transducers and pressure measuring converters.
Measuring range/measuring span
The measuring range is the span between the start and the end of the measurements. The error limits quoted in the data sheet apply within this measuring range. The type of pressure, eg, positive relative pressure (overpressure), negative relative pressure (underpressure), absolute pressure or pressure difference, is also specified in order to define the measuring scope.
The overload range defines the pressures to which a pressure measuring converter can be exposed without it suffering permanent damage, eg, zero-point offset. The pressure measuring converter is allowed to deviate from its specified technical data in this span between the measuring range and overload limit. The overload limit is usually defined by the sensor principle.
With piezoresistive pressure measuring converters the overload pressure is identical with bursting pressure. In the case of thin-film pressure measuring converters, on the other hand, there are notable differences between overload pressure and bursting pressure. This is owing to the sensor materials silicon and stainless steel. While steels enter a flow phase when they exceed their resilient range, brittle materials display a linear characteristic up to a bursting point (Figure 2).
Bursting pressure defines the pressure range which, if exceeded, can lead to the total destruction of the pressure measuring converter.
An auxiliary energy supply is required to operate pressure transducers and pressure measuring converters. The most widespread solution is a direct voltage of 24 V. Thanks to the use of voltage-stabilising components, pressure measuring converters usually have a bigger supply voltage range of eg, 10 to 30 V d.c. Within this range there should be no effects of supply voltage variation.
For example, if the pressure measuring converter is calibrated in the factory with an auxiliary voltage of 24 V d.c., there should be no change of accuracy if the converter is actually operated with just 12 V d.c. The effects of supply voltage variation should not greatly exceed 0,1% per 10 V.
Output signal (analog, digital)
Current and voltage signals have become established as the main alternatives for output signals. Exactly which output signal is selected will depend mainly on the indicating and control devices that follow on after the pressure measuring converter. As a general rule it can be said that current signals are more immune to interference during transmission than voltage signals.
Systems with 4 to 20 mA in two-conductor technology are becoming increasingly popular in industrial plants. The main feature of this technology is that the power supply and measuring signal are both carried over the same connection lines, which for elaborate installations signifies a notable saving in wiring costs.
At the same time the higher zero point (4 mA) enables the measuring position to be monitored for open circuits and device failure, because in these cases the flow of current would drop below 4 mA. This change can be used in the evaluation units, eg, to trigger an alarm.
From the automotive industry there is also an increasing demand for so-called ratiometric voltage signals. This means, for example, that the supply voltage range is defined as 5 V ±10% and the output signal specified as 0,5 to 4,5 V d.c. ±10%. If the supply voltage amounts for example, to 4,5 V (-10% of the nominal range), the output signal will equal 4,0 V (4,5 V less 10%). This output signal saves several electronic components and enables cheaper pressure measuring converters.
With current outputs the permissible load is specified in addition to the output value. This load is called the maximum permissible load impedance.
RA (Ω) = (UB [V] - UBmin)/0,2 [A]
RA = Load impedance in Ω.
UB = Actual supply voltage in volts.
UBmin = Minimum permissible supply voltage quoted in the data sheet.
In practical applications the actual load used should be far smaller than the maximum permissible load in order to rule out measuring errors due to excessively high contact resistances.
There is also a certain demand for frequency outputs in special applications, eg, mining.
Yet another form of signal now gaining importance is the digital output signal, which with the right software can then be processed directly in microprocessor-controlled systems.
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