# What is the BSL and TSL accuracy of a pressure sensor

29th December 2007
Although it is not always clearly explained on manufacturer’s technical data sheets there are two widely used ways to define the room temperature accuracy of a pressure sensor. The relevance of each will depend on how you intend to install the pressure sensor and how important it is to achieve the optimum accuracy.

There are three significant components that make up the room temperature accuracy which are Non-Linearity, Hysteresis and Short Term Repeatability.  These are normally expressed as a total error combining all three parameters and on occasions only the first two.  When abbreviated they are shown as NLH, NLHR, LH or LHR. E.g. ±0.1% full scale BSL NLHR.

The manufacturer will check the performance of these parameters typically by performing 3 cycles of 6 rising and 5 falling pressure points.  Since with pressure sensors it is normal to set zero and span when installing them, the achievable accuracy is equivalent to how close all the points are to an imaginary Best Straight Line or BSL. This is determined by using a mathematical least squares fit method to analyse all the calibration data collected to determine an ideal reference line that achieves the best possible accuracy for all the measured points.

The other method for reference calibration data is the Terminal Straight Line or TSL method which ties the line for which all measured points are compared to with the lowest and highest measured reading and not the best straight line. This method is considered more relevant to the practicalities of installing a pressure sensor, since it is much easier to set the zero and span to the lowest and highest output rather than a virtual Best Straight Line that is unlikely to coincide exactly with the zero and full scale pressure reading.

However the TSL method leads to accuracies which are typically 2x greater than those obtained using the best straight line method effectively compromising accuracy for convenience of installation.  Since manufacturers wish to promote the full potential accuracy of their pressure sensors the BSL method is often quoted in preference to the TSL method on product data sheets.  There are other factors to consider when installing pressure sensors such as temperature errors and long term drift but typically it is only the room temperature accuracy that is considered when specifying a pressure sensor.

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Associated Topics

Maintaining the long term accuracy of your pressure instruments

What are the separate error components that make up the accuracy of a pressure sensor

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