Pressure measurement is an important element in fluid power (hydraulics and pneumatics) as well as in the process industry to ensure safe operation and enhanced performance. Besides accuracy and other factors, the selection of the appropriate measuring range of a pressure sensor is an important step. When selecting the measuring range of pressure sensors, two factors come into play:
- Ensure that the sensor is not destroyed by overloading the sensing element by selecting a measuring range that is too small.
- Avoid compromised accuracy or a too low output signal by selecting a measuring range that is way too large.
This article deals with the second point and the effects of selecting a too-large range. When looking at the data sheets of different manufacturers, you will typically find three different values regarding the pressure range:
- Rated operating pressure
- Safe operating pressure
- Burst pressure
Different manufacturers may use slightly different terms (and you should always enquire about a precise definition), but in principle there is a common understanding. It is important to notice that only in the rated pressure range, all specification values as described in the data sheet are applicable/ valid. This is in particular true if it comes to accuracy/ error values. Once you exceed the rated pressure range, the data sheet values are no longer guaranteed and may be exceeded, i.e., the output signal can no longer be used for obtaining accurate measurements.
But what happens if you select a measuring range that is much larger, i.e., you use only a fraction of the rated pressure range – say 250 psi of a 1000psi sensor? The good news: nothing, or at least not very much. The truth is that most accuracy parameters, like linearity, repeatability, hysteresis, etc., are parameters that are in relation to the actual reading/ value, not to the full scale. Their maximum value is defined in the data sheet and expressed as a percentage of the rated pressure range, but the error in service actually “scales down” with usage.
For example, if linearity (or better, non-linearity) is defined as 0.1% of full scale (f.s.), it means that a 1000 psi pressure sensor can experience as much as 1psi deviation from the ideal line and this is expected to happen somewhere around half of the range, i.e. at 500 psi. But when you measure only 250 psi with the same transducer, the linearity is expected to deviate not by 1 psi but only by a fraction of it. In fact, if you used a 1000 psi sensor to measure pressure from 0 to 500 psi only, the expected linearity error between 0 and 500 psi is about 0.25 psi, and is expected to peak somewhere around 250 psi. (More details about errors and accuracy can be found in the article “Pressure Sensor Fundamentals: Interpreting Accuracy and Error”.
Smart Transmitters
So-called smart transmitters rely heavily on this effect, as they are typically available only in a very few (5 or 6) pressure ranges, i.e., they virtually always only use a part of the rated pressure range by design. The only errors that do not scale down are errors related to the zero-point of the sensors: a) zero point accuracy/calibration, and b) temperature effect on zero (TCzero). In the data sheet, both are defined as a percentage of full scale, and their absolute value remains constant for the individual sensor – independent of how much of the range is used.
In our example above of a 1000 psi rated sensor, let’s assume a 0.5% error on zero = 5 psi. Whether you measure 1000 psi or 500 psi or 10 psi, you must always expect an offset in zero of up to 5 psi in a specific sensor. The practical implication is that there’s very little offset, as most sensors are “zeroed” when hooked up to a meter or programmable logic control (PLC) during installation.
But what about resolution? The only other limitation you need to be aware of is the resolution of your receiving electronics, i.e., your analog input card of your PLC or the input of your meter or display. In most cases today, this is no longer a problem as they typically provide resolution of 12 bit and beyond. If we look at our example above again and assume that we use only the sensor up to 250psi, this would still provide us with a 10-bit resolution (in case of a 12-bit A/D) providing 0.1% resolution from 0 to 250 psi (=0.25psi).
Summary
Choosing the right pressure range for an application is important, but it’s not rocket science. Modern pressure sensors provide a huge number of ranges to choose from. Careful consideration of the individual application with regard to overpressure is more important than worrying about losing accuracy or resolution. Contact the pressure experts at WIKA USA for more information.
Further reading:
Back to Basics: Introduction to Pressure Measurement
Pressure Sensor Fundamentals: Interpreting Accuracy and Error