Understanding Pressure Sensor Accuracy #
Offset Errors and Zero/Span Settings #
Pressure sensors typically output a signal (often 4-20mA) corresponding to the measured pressure. However, due to manufacturing variations, the output signal can be slightly deviated. These are known as offset errors.
Zero and Span Settings #
• Zero refers to the output signal when no pressure is applied (typically 4.00mA).
• Span refers to the output signal range (typically 16.00mA, which is the difference between the full scale 20.00mA and the zero point 4.00mA).
Factory Calibration Tolerances #
During factory calibration, we allow a tolerance of ±0.5% of span for both zero and span settings:
- Zero point (4.00mA): 4.00mA ±0.5% of span = 3.92mA to 4.08mA
- Span (16.00mA): 16.00mA ±0.5% = 15.92mA to 16.08mA
This means a sensor calibrated within these tolerances meets our specifications, even though it may not output exactly 4.00mA at zero or 20.00mA at full scale pressure.
How Customers Accommodate for Offsets #
In practical applications, customers typically handle these offset errors in one of two ways:
- Manual Offset Input: Entering the specific offset values of their sensor into their control system.
- Zero/Tare Function: Performing a “Zero” or “Tare” operation when no pressure is present, which automatically compensates for the zero offset error.
These methods effectively eliminate the impact of offset errors in most applications.
Non-Linearity & Hysteresis (NL&H) #
While offset errors can be easily compensated for, the more critical specification for sensor performance is the Non-Linearity & Hysteresis (NL&H), which describes how accurately the sensor tracks pressure changes throughout its range.
Non-Linearity #
Non-linearity refers to how closely the sensor’s output follows a straight line in relation to the input pressure. In an ideal world, doubling the pressure would double the output signal change. However, actual sensors deviate from this perfect relationship.
We use the Best Fit Straight Line (BFSL) method to quantify this deviation:
1. We plot the sensor’s actual output against various pressure inputs
2. We calculate the straight line that minimizes the deviation from all these points
3. We express the maximum deviation as a percentage of the full scale (%FS)
For example, a non-linearity of ±0.25% FS means that at any point in the pressure range, the output may deviate up to 0.25% of the full scale output from the ideal straight line.
Hysteresis #
Hysteresis refers to the difference in sensor output when measuring the same pressure point while pressure increases versus when pressure decreases. Mechanical memory effects in the sensor materials contribute to this.
A sensor with low hysteresis will give nearly identical readings whether you’re increasing or decreasing pressure, which is desirable for precise measurements.
Combined NL&H (BFSL) #
When we specify NL&H (BFSL), we provide the combined maximum error due to non-linearity and hysteresis, calculated using the Best Fit Straight Line method. This gives you a realistic expectation of the sensor’s accuracy throughout its operating range.
This specification is typically more critical than offset errors because it affects the sensor’s ability to track pressure changes accurately, which offset compensation cannot fix.
How Young’s Modulus Affects Sensor Linearity #
Young’s modulus measures a material’s stiffness—how much it deforms under stress. This property directly impacts pressure sensor linearity.
Most pressure sensors use a diaphragm that deforms under pressure. The relationship between pressure and diaphragm deflection depends on the material’s Young’s modulus. Ideally, this relationship would be perfectly linear, but in reality:
• Materials don’t maintain constant stiffness across their entire deformation range
• Temperature changes can affect Young’s modulus
• Material aging and repeated stress cycles can alter mechanical properties
These factors contribute to the non-linearity observed in pressure sensors. Higher-quality sensors use materials with more stable properties and sophisticated compensation techniques to minimize these effects.
Summary: What Really Matters #
For most practical applications:
1. Offset errors are less critical because they can be compensated easily during system setup.
2. NL&H (BFSL) is the more important specification as it tells you how accurately the sensor will track pressure changes throughout its range—something that cannot be easily compensated for.
When selecting a pressure sensor for your application, pay particular attention to the NL&H specification, especially if you need high accuracy across the entire pressure range or measuring dynamic pressure changes.
For any clarification or comments don’t hesitate to get in touch with the sales team: sales@stork.solutions
The information above primarily refers to pressure products such as the TPTa, TPTLRa, SSPT, and SLS-A.
