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How do I calibrate my data hardware?

Transducers developed by BIOPAC are linear over their expected ranges of operation. Therefore, knowing the system’s response to two physical stimuli is all that is required. In many cases the slope of the input/output relationship is so well defined that only one measurement, an offset correction, is necessary. For many transducers or applications, calibration is not necessary at all either because results are relative or because absolute signal levels are uninformative. Additional information on procedures to follow for calibration may be found in “How do I calibrate my data (Software)?

Sample cases

Biopotentials (ECG, EEG, EMG, etc.)

The precise magnitude of electrical signals recordable on the skin surface will vary across sessions even for an individual subject because this magnitude depends strongly on the exact positioning of the electrodes and other aspects of electrode site preparation. Consequently these types of signals are best used as relative measures and/or for studies of timing more than magnitude. That said, because there is no transduction involved in the production of these signals, the gain setting of the amplifier characterizes the input/output relationship rather well. Hence the signal can be considered calibrated if the input to the MP150 (output of the amplifier) is assumed to be the electrical potential difference at the electrodes multiplied by the gain setting. If, for some reason, there is concern over the performance of the amplifier, or the input/output relationship needs to be known more precisely, a biopotential amplifier may be calibrated with the aid of a CBLCALC.

Respiration/PPG

The respiratory effort (TSD201) and the photoplethysmogram (TSD200) transducers produce signals that are primarily intended for timing (respiration rate and pulse rate). Given that consideration, calibration is unnecessary. If signals from these devices are recorded without high pass filtering (HP filters set on “DC”), then they can provide an absolute signal. However, even under such circumstances, that signal level is rarely tied to any absolute physical quantity. In the case of respiration, attempting to relate the signal to the volume of air in the lungs will always be problematic as subjects have voluntary control over the extent to which air is pulled into the upper or lower sections of the lungs.

Skin conductance

The EDA amplifiers are well-characterized by their gain settings except for a (normally) slight offset. Consequently, a measurement of the amplifier’s output when the input is open circuited (zero conductance) is all that is necessary for calibration. However, more precise calibration may be achieved if a second measurement is made with a resistor completing the circuit from one electrode to the other (in place of the subject).

Airflow transducers

Pneumotach transducers may be calibrated with syringes of known volume. Volume is the integral of flow rate with respect to time, so the injection of a known volume of air through the transducer should produce a signal whose integral over the time of the injection is equal to the injected volume. For more information on the performance of this integration and subsequent scaling, see “How do I calibrate my data (Software)?

Pressure transducers

Pressure transducers may be calibrated with just an offset correction (local atmospheric pressure may be defined as zero), but typically two measurements should be made. The measurements should bracket the expected range of pressures in the signal to be measured. The applied pressure corresponding to the measurements should be assessed with the aid of, for instance, a sphygmomanometer.

Goniometers

The signal produced by a goniometer is proportional to the bend angle. The units and directions are completely arbitrary. Typically a measurement of the output as the transducer is laid flat out is considered zero, but it could be 180 degrees or pi radians. A second measurement (around a given axis) should be made with the aid of a protractor or any right-angled surface. This measurement may be mapped to ±90 degrees or ±pi/2 radians.

Accelerometers

Due to general relativity, it is not possible for a single measuring device to distinguish between constant acceleration and the presence of a gravitational field. Therefore it is a simple matter to rotate the transducer to orient any given axis such that said axis is 1) along the gravitational field vector, 2) pointed directly opposite, or 3) perpendicular. Any two of these three orientations may be used for mapping the input/output relationship. With the transducer oriented such that a given axis points toward the center of the earth, the amplifier output should correspond to 1 g (which may be expressed as such or as 9.81 m/sec2 or 32 feet/sec2). With the transducer flipped over, the output corresponds to -1 g. With the transducer rotated such that the given axis is perpendicular to the gravity vector, the output corresponds to zero.

Noninvasive blood pressure

The scale of the NIBP100D’s analog out signal is nominally a one volt change in output corresponding to a 100 mm Hg change in pressure when the DA100C is properly set up (gain = 1000 and reference voltage = ±1 volt). The relationship is inverted such that larger signals correspond to lower pressures. There is also an offset such that a signal of zero volts corresponds to a pressure of -30 mm Hg. For greater accuracy, the device can be configured to produce a square wave. The values at the top and bottom of the square wave correspond to 50 and 150 mm Hg respectively. To generate this square wave on the unit:

    1. turn the CNAP monitor on and allow it to go through its initialization.
    2. Press the “Setup” button on the unit; this will generate a menu…
    3. From that menu, select “Service.”
    4. There will be a row of numbers across the top that will allow the entry of a four-digit password.
      • The password is “8737.”
      • To enter that password, turn the knob until the cursor highlights the “8” and then press the knob. Turn it again to highlight the “7” and press…
    5. Once all four digits have been entered, turn the knob to select “Ok.”
    6. In the menu that is generated, select “Function Tests…”
    7. In the menu that is generated, select “Check Up IBP…”
    8. Next select “Rectangle” to generate the square wave.

For information on how to extract the values from this square wave and use them for calibration, see “How do I calibrate my data (Software)?”.

Page last modified 23Mar2015

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