AN0002: Understanding Tiltmeter Count Readings

Many of our tiltmeter and inclination instruments report their readings as “counts” which is an often confusing term. Counts refers to the raw output of the analog to digital converter in the system and is the lowest level form of the measurement. This reading is provided as it allows ultimate versatility in processing because you always begin with data is the purest form. Some instruments, generally our lower resolution MEMS based instruments, will also provide an output in degrees or microradians which uses the stock nominal calibration of the instrument. If you wish to apply an instrument specific calibration or perform other corrections, starting with the raw count reading is best.

Counts are always an integer number and represent the voltage output of the sensor as measured by the analog to digital converter. High resolution converters have a wider range of counts that they can divide the measurement range into. For example a 16-bit ADC can output the numbers 0 - 65,535 while a 24-bit ADC can output 0 - 16,777,216.

Your instrument will have an expected or nominal calibration that is the mathematical ideal calibration for a perfect instrument. For example, a MEMS inclinometer may have an expected calibration of 0.00179 degrees/count. Therefore if the output of the instrument is 16,435 counts we can convert to degrees by multiplying. We obtain a result of 29.41865 degrees.

    Angle = Counts x Calibration
  

If your instrument had a full custom calibration or you have performed a calibration you may have a different number to use other than the nominal. The new calibration should be close to the ideal or there may be a problem with the instrument or calibration method. A MEMS calibrated once installed in its casing and operating at a temperature for performance in a geothermal well may have a calibration of 0.00182 degrees/count instead of the 0.00179 degrees per count nominal for example. It is very likely that the calibration factor is a function of temperature and depending upon the application this may need to be considered.

Using raw counts also allows you to fully document every step of the analysis process - from what calibration was used to any temperature, cross axis, bias, or other corrections are performed on the data. Be sure to check out our website for application notes on those corrections and ways to further improve the performance of your tiltmeter.