Technical Note No. 98/2 - Hardware Control Options Explained: Conversion of Volts to Pressure, DOBIE Calibration, Sensor Noise

Technical Note No. 98/2 - Hardware Control Options Explained: Conversion of Volts to Pressure, DOBIE Calibration, Sensor Noise

Conversion of Volts to Pressure

DOBIE converts volts to pressure by:

where p is pressure in pounds per square inch, V is volts output by the pressure sensor, G is gain (units of psi/volt) and O is offset (units of psi). The calibration coefficients G and O are preset at the factory, but may be changed by the user at any time by accessing the Hardware Control Options menu.

DOBIE Calibration

To check the calibration, access the Hardware Control Options menu and change G to 1 and O (i.e., offset) to 0 (i.e., zero) (make a note of the original settings first in case you need to reuse them!). Now run either Task 2 or Task 3. Apply a known pressure or series of pressures to the DOBIE. When done, download the data and run the PEDP. Open the datafile that is generated by the PEDP (DOBnnnn.DAT if running Task 2 or DOBIE.DAT if running Task 3). The numbers in column #2 are voltages (because the gain was set at 1 and the offset to 0). You can now plot voltage against your known pressure or series of pressures to verify the calibration coefficients. Return to the Hardware Control Options menu and change the calibration coefficients accordingly.

NOTE: the above notes apply only to the DOBIE A with the ADARA strain-gauge pressure sensor. Calibration coefficients for the DOBIE C, with the Paroscientificâ Digiquartzâ pressure sensor, are not accessible through the DOBIE operating software. Refer to the Digiquartzâ manual.

Power-Supply Voltage

The dimensionless number generated by the TT8 analog-to-digital converter (ADC) is converted by DOBIE to power-supply voltage by:

where Vp is the power-supply voltage, D is the ADC output, Gp is the gain in volts and Op is the offset in volts. The calibration coefficients Gp and Op are preset at the factory, but may be changed at any time by accessing the Hardware Control Options menu. To check the calibration, measure the voltage of the main power supply using a voltmeter and compare to the voltage reported by DOBIE at Task 1.

Sensor Noise

The intrinsic noise level of the pressure sensor is the standard deviation of the signal generated by the pressure sensor when sitting in still water. When calculating wave height, the noise level is subtracted from the measured signal to leave the "real" signal, which is what is then transformed into wave height. The noise level is expressed in units of metres, as follows:

where N is the noise level in metres, SD is standard deviation, V(t) is voltage time series output by the pressure sensor sitting in still water, g is acceleration due to gravity (9.81 m/s2), r is a nominal water density (1025 kg/m3; typical seawater density) and F is a factor (6895 kg/[s2m]/psi) used to convert pressure in pounds per square inch to pressure in kg/(s2m).

The noise level is preset at the factory. To check the noise level, access the Hardware Control Options menu and set the noise level to 0 (i.e., zero.). Now put DOBIE in a bucket of still water and run Task 4/0. When done, download the data and run the PEDP. Open the datafile that is generated by the PEDP (DOBIE.DAT). The standard deviation reported in column #7 is equivalent to N. Return to the Hardware Control Options menu and set the noise level appropriately.

MOG
June, 1998