If you are using a pressure transducer (also known as a sensor) to measure liquid, gas or air pressure in a critical environment, OEM application or high-purity manufacturing process, you need to have confidence in the accuracy of your instrument. That confidence will be tested if there is an offset in the output of that instrument at zero and/or span. However, if you understand the reasons behind why offset can occur and what you can do to address it, your confidence will be restored.
Ashcroft, an industry leader in pressure measurement instruments, has consistently demonstrated reliability and excellence in the field of pressure measurement instrumentation. That's why we are often asked to share our expertise and shed light on topics like this one.
Read this article to learn how transducers measure pressure, possible causes of offset in the output of your sensor and what you can do to ensure your sensor is providing you with an accurate pressure measurement. You will also have the opportunity to deepen your knowledge through additional articles and resources available to you.
What you need to know about pressure transducers
In a separate article, we explained how pressure transducers are used to measure pressure in a given application and convert that measurement to an electrical or digital output signal, such as current or voltage. These sensors can accurately measure pressure ranges starting from as low as 0 to 0.1 in. H2O (low pressure) up to 72,000 pounds per square inch (high pressure). The accuracy of these sensors can range anywhere from +/-0.02% to +/-5.0% of span, to meet the accuracy requirements of your application.
Figure 1: Pressure transducer pressure ranges
What is a zero and span offset?
Offsets are errors in the output of the sensor seen when the unit is first installed or after a long period of use in its application:
- Zero offset is the error in the output of the sensor with no pressure is applied.
- Span offset is the error in the output of the sensor at its full-scale measurement.
Depending on the accuracy of the sensor you are using, these errors may fall outside of the allowable error tolerance of the device. This will lead to errors in the output of the sensor and will affect the accuracy and reliability of the device, signaling the need to calibrate or replace your instrument.
The greater the offset, the more significant the inaccuracy of the pressure measurement. Conversely, the better accuracy at zero and span, the more accurate your transducer will be in your application.
Note: Keep in mind that 0 psi may not be the zero point of your transducer. The zero offset is measured at full vacuum on transducers that have compound ranges and may be any value on specially calibrated or draft-ranged transducers.
How zero and span offsets are classified
- Zero offset refers to the output error at the bottom end of the measurement range. Although many people associate zero offsets as 0 PSI, in compound ranges, it may represent full vacuum (-14.7 PSI) in compound ranges.
- Span offset indicates the output error at the full-scale measurement range. This is usually the highest pressure that the transducer is ranged to measure. For example, if the transducer is ranged to measure zero to 3,000 psi, the offset error would be measured at 3,000 psi.
Zero offset and span setting errors are generally expressed as a percentage of span or a percentage of output.
Figure 2: Zero offset and span setting errors
Common causes of zero and span offsets
When determining the cause of zero and span offsets, there are many factors to consider:
Manufacturing tolerances: There could be Inherent imperfections in the sensing elements or electronics of your instrument. This can cause offset at zero and/or span in your device. For instance, there may be limitations in the manufacturing process that prevent the ability to eliminate these minor deviations during the production of the device. Tolerances of the electronic components used in the circuitry of the transducer can also contribute to the offset of the output.
Environmental factors: Changes in temperature and humidity can impact a pressure sensor's performance, which can lead to shifts in the output at zero and span. These environmental changes can cause the materials within the sensor to expand or contract, while temperature changes can cause electronic components to drift, altering the sensor's baseline reading and affecting its overall accuracy.
Electrical noise: Unwanted fluctuations or spikes in electrical signals, radio frequencies, power lines or even lightning, can also impact the performance of your pressure transducer. Whether or not your transducer can handle or limit the interference from external electrical noise depends on the design of the electronic circuitry of the sensor. The Ashcroft® E2 Pressure Transducer series and G2 Pressure Transducers, for example, are designed to limit the effects of electromagnetic interference/radio frequency interference (EMI/RFI). This helps to protect the transducer from unwanted electromagnetic signals that can interfere with the output signal.
Drift: Pressure transducers can experience a gradual change in their output signal. This is known as drift. Drift can be caused by many factors, including mechanical stress, the number of pressure cycles, temperature or humidity. For instance, as the diaphragm of a pressure transducer undergoes repeated pressure cycles, the materials may begin to wear down or deform slightly, leading to a shift in the baseline measurement of the sensor which causes a slight change or drift in the output signal
Ways to address zero and span offset
1. Use a transducer with zero and span adjustments
Zero and span adjustability helps keep the pressure sensor’s accuracy within the tolerance of a transducer’s specifications. Some transducers feature zero and span potentiometers that allow users to fine-tune or recalibrate the output signal of the device. This allows the user to recalibrate the output of the transducer, minimizing zero and span offset that may have been caused by drift. The Ashcroft® E2 series, CXLdp, and DXLdp Pressure Transducers all offer zero and span adjustment options. The E2 series also offers external zero and span adjustments through the use of a magnet, allowing for the unique ability to make zero and span adjustments on hazardous location-approved versions of the product.
2. Choose a pressure sensor that has been temperature compensated by the manufacturer.
Zero and span offsets can be influenced by the operating and ambient temperature of an application. To reduce the effects of temperature some manufacturers perform temperature compensation on their transducers as part of their standard calibration process. Temperature compensation improves the accuracy and reliability of the transducer through the temperature range for which it has been compensated.
Most manufacturers will indicate that the transducer has been temperature compensated as well as provide the range of temperature over which it has been compensated on their datasheet. For example, the Ashcroft® G2 Pressure Transducer datasheet shows that this sensor has been temperature compensated over a -40 to 257 °F (-40 to 125 °C) temperature range, minimizing the effect of offset errors between -40 to 257 °F (-40 to 125 °C).
Figure 3: Ashcroft® G2 Pressure Transducer datasheet temperature limits specifications
Zero and span offset are factors that contribute to the overall accuracy of your pressure instruments. These factors should be included in a manufacturer’s accuracy statement. To learn more about how accuracy is defined and all the elements that are covered for transducers and transmitters, download our guide.
Different methods of calculating accuracy
When it comes to providing accuracy statements for pressure transducers, there are no standards or regulations for manufacturers to follow. Therefore, it is left up to the manufacturer to decide how they state the accuracy of their transducer and how much accuracy information they want to provide on their datasheet or website.
Ideally, an accuracy statement would include a summation of all the error sources of the transducer including nonlinearity, hysteresis, non-repeatability as well as zero offset and span - all of which contribute to how well the sensor performs. But whether all this information is available to users is left up to the manufacturer to decide.
Many manufacturers provide accuracy statements that use concepts like root sum squared (RSS) or best fit straight line (BFSL) to express the accuracy of their sensor. These methods typically use statistically derived accuracy specifications that make the device appear to be more accurate than the actual performance you experience once the instrument is installed.
Ashcroft typically states the accuracy of our transducers using a terminal point statement of accuracy. A terminal point accuracy statement includes the errors of linearity, hysteresis, and repeatability along with the zero and span offsets as part of this accuracy statement. We call this our TruAccuracy™ standard as it incorporates all of the errors of the transducer that allow for plug-and-play functionality in most applications.
Why accurate zero and span calibration matters
Correcting zero offset and span setting errors will affect the overall performance of the transducer. Corrections or changes made to the zero offset will cause a linear shift of the output trend line, while changes to the span will affect the slope of the output line.
If zero and span setting errors are corrected, the accuracy of the transducer should be reevaluated. The bottom line is accurate calibration ensures that transducers provide precise readings across their entire operating range. The better the accuracy at both zero and span, the more reliable the transducer will be in its application.
Ready to learn more?
Now that you understand how zero and span offsets can occur and what you can do to address the issue, you are in a better position to make an informed decision about choosing the best pressure sensor for your needs. Of course, the right choice will depend on the application’s accuracy requirements and the operational conditions of your application. By making informed selections, you can ensure consistent performance and be more confident in your decisions.
For your reference, here are a few more articles that can help deepen your knowledge about zero and span and pressure transducers:
- What are the Benefits of Zero and Span Adjustability?
- What are the Advantages of Ashcroft Pressure Transducers?
- Choosing a Pressure Transducer for Critical Environments
- Choosing the Right Pressure Sensor: 5 Factors to Consider
For personalized assistance, feel free to reach out directly to one of our experts.
To learn about common mistakes you can avoid when selecting your pressure sensor, download our guide.