Correctly interpreting accuracy values for pressure sensors

Correctly interpreting accuracy values for pressure sensors

In the search for a suitable pressure transmitter, various factors will play a role. Whilst some applications require a particularly broad pressure range or an extended thermal stability, to others accuracy is decisive. The term “accuracy”, however, is defined by no standards. We provide you with an overview of the various values.

Although ‘accuracy’ is not a defined norm, it can nevertheless be verified from values relevant to accuracy, since these are defined across all standards. How these accuracy-relevant values are specified in the datasheets of various manufacturers, however, remains entirely up to them. For users, this complicates the comparison between different manufacturers. It thus comes down to how the accuracy is presented in the datasheets and interpreting this data correctly. A 0.5% error, after all, can be equally as precise as 0.1% – it’s only a question of the method adopted for determining that accuracy.

Accuracy values for pressure transmitters: An overview

The most widely applied accuracy value is non-linearity. This depicts the greatest possible deviation of the characteristic curve from a given reference line. To determine the latter, three methods are available: End Point adjustment, Best Fit Straight Line (BFSL) and Best Fit Through Zero. All of these methods lead to differing results.

The easiest method to understand is End Point adjustment. In this case, the reference line passes through the initial and end point of the characteristic curve. BSFL adjustment, on the other hand, is the method that results in the smallest error values. Here the reference line is positioned so that the maximum positive and negative deviations are equal in degree.

The Best Fit Through Zero method, in terms of results, is situated between the other two methods. Which of these methods manufacturers apply must usually be queried directly, since this information is often not noted in the datasheets. At STS, the characteristic curve according to Best Fit Through Zero adjustment is usually adopted.

The three methods in comparison:

Measurement error is the easiest value for users to understand regarding accuracy of a sensor, since it can be read directly from the characteristic curve and also contains the relevant error factors at room temperature (non-linearity, hysteresis, non-repeatability etc.). Measurement error describes the biggest deviation between the actual characteristic curve and the ideal straight line. Since measurement error returns a larger value than non-linearity, it is not often specified by manufacturers in datasheets.

Another accuracy value also applied is typical accuracy. Since individual measuring devices are not identical to one another, manufacturers state a maximum value, which will not be exceeded. The underlying “typical accuracy” will therefore not be achieved by all devices. It can be assumed, however, that the distribution of these devices corresponds to 1 sigma of the Gaussian distribution (meaning around two thirds). This also implies that one batch of the sensors is more precise than stated and another batch is less precise (although a particular maximum value will not be exceeded).

As paradoxical as it may sound, accuracy values can actually vary in accuracy. In practice, this means that a pressure sensor with 0.5% error in maximal non-linearity according to End Point adjustment is exactly as accurate as a sensor with 0.1% error of typical non-linearity according to BSFL adjustment.

Temperature error

The accuracy values of non-linearity, typical accuracy and measurement error refer to the behavior of the pressure sensor at a reference temperature, which is usually 25°C. Of course, there are also applications where very low or very high temperatures can occur. Because thermal conditions influence the precision of the sensor, the temperature error must additionally be included. More about the thermal characteristics of piezoresistive pressure sensors can be found here.

Accuracy over time: Long-term stability

The entries for accuracy in the product datasheets provide information about the instrument at the end of its production process. From this moment on, the accuracy of the device can alter. This is completely normal. The alterations over the course of the sensor’s lifetime are usually specified as long-term stability.  Here also, the data refers to laboratory or reference conditions. This means that, even in extensive tests under laboratory conditions, the stated long-term stability cannot be quantified precisely for the true operating conditions. A number of factors need to be considered: Thermal conditions, vibrations or the actual pressures to be endured influence accuracy over the product’s lifetime.

This is why we recommend testing pressure sensors once a year for compliance to their specifications. It is essential to check variations in the device in terms of accuracy. To this end, it is normally sufficient to check the zero point for changes while in an unpressurized state. Should this be greater than the manufacturer’s specifications, the unit is likely to be defective.

The accuracy of a pressure sensor can be influenced by a variety of factors. It is therefore wholly advised to consult the manufacturers beforehand: Under which conditions is the pressure transmitter to be used? What possible sources of error could occur? How can the instrument be best integrated into the application? How was the accuracy specified in the datasheet calculated? In this way, you can ultimately ensure that you as a user receive the pressure transmitter that optimally meets your requirements in terms of accuracy.

Common errors in pressure measurement and how to correct them

Common errors in pressure measurement and how to correct them

Uncertain output signals, zero offsets or even the complete failure of the measuring instrument are symptoms that can quickly strain the nerves of users. The good news is that when the cause is correctly identified, these errors can often be easily corrected.

 In the following, we show you a number of typical errors that users may encounter in practice, but which can usually be avoided with just a little background knowledge. Incidentally, we have already published detailed articles on many of the topics here, which are linked below at the appropriate point.

Error Cause Troubleshooting
No output signal

Line breakage

 

Check the cable for damage and ensure that it is properly laid.
Wiring error Check the plug-cable assignment and, if necessary, consult the installation and operating instructions.
Wrong polarity
The display indicates too low a pressure Inlet pressure too low due to a blocked port opening
  • Check the port opening for contamination and clean it.
  • If the medium is dirty, a filter should be attached to the process interface.
  • If necessary, use a pressure transmitter with a front-flush membrane.
Pressure transmitter is leaking at the process interface Check the seal, as it is either too loose or defective (with a new seal, check for media compatibility).
The signal is constant but does not exceed a certain value even when the pressure increases The bore opening is blocked
  • Clean the bore opening.
  • Place a filter in front of it.
  • Use a pressure transmitter with a front-flush membrane.
The medium temperature is too low (below -40 °C / -40° Fahrenheit) The measuring cell of a piezoresistive pressure sensor contains a transfer fluid. This can solidify at temperatures below -40 °C. In this case, a pressure transmitter optimized for low temperatures must be selected, with, for example, the AS100 filling fluid (for temperatures down to -55 °C).
The output signal indicates a high value and remains unchanged The permissible measuring range has been exceeded. If the pressure sensor operates in the overload range, it will not yet fail, but does not display accurate measurement results. The output signal has reached saturation point and cannot exceed this any further. A pressure transmitter suited to the measuring range must be selected.
The output signal is too low and does not exceed this low value despite a pressure increase The inlet pressure is too low The port opening is blocked (see above).
Too high a load for mA signals (the electronics connected to the pressure transmitter take too much current) For mA signals, reduce the load according to the data sheet /operating instructions.
Too low a load for V signals Increase the load according to the data sheet/ operating instructions.
Operating voltage too low Operating voltage must be increased in accordance with the operating instructions.
Too wide a measuring range of the pressure transmitter An instrument corresponding to the measuring range must be selected. Rule of thumb is the measuring range should be ca. 75% of device capability.
Zero offset (the zero point signal is too high) The membrane has been deformed by impermissibly high overpressure
  • The pressure transmitter is defective.
  • A suitable measuring range must be selected and, if necessary, a choke used.
The membrane is deformed or ruptured by pressure peaks
Too high tightening torque on installation (measuring cell damaged) This problem is more likely to occur with instruments of a low pressure measuring range. Pay attention to maximum torque during installation at the process (consult mounting instructions).
The output signal shifts greatly under temperature influence There is a blockage to the relative pressure compensation (mostly in devices with low measuring ranges up to 25 bar) The relative pressure compensation should be checked for contamination. It should also be ensured that the installation was carried out correctly.
Strongly fluctuating output signal (flickering) Loose contact A cable break or a loose plug can be the cause.
Strong vibrations or shock pulses in the process The sensor is resonating. Ideally, the permissible shock load should be checked in the data sheet before selecting a pressure transmitter. Shock-resistant devices are characterized by sealed electronics and do without adjustable potentiometers (such as the ATM.1ST). The problem can be solved later by decoupling the measuring device via a flexible pressure line.
The output signal has interference pulses There is too much EMC interference It must be ensured that the cables are shielded.  EMC phenomena can be mostly eliminated with careful installation.
Differing potentials between measuring instrument and process Check the ground connection of the pressure transmitter.
The output signal fails after some time in operation The electronic components fail due to a too high operating temperature The process medium can be sufficiently cooled via a temperature decoupler, such as upstream cooling fins or a cooling section.A siphon is the best solution for steam applications.

Some of the errors listed here are due to incorrectly selected pressure transmitters. To avoid errors, you should know in advance as precisely as possible the requirements for the measuring instrument with regards pressure measurement range and installation (a short guide to the correct transmitter selection can be found here). A detailed consultation beforehand with the manufacturer can help spare your nerves. But we’d also be glad to assist you further