Engineering output
The frequency reading on the is expressed both in units of Hertz (Hz) and in units of digits (Hz2/1000). The can be configured to calculate an output value that is specified in engineering units of measure or dimensions. Examples of units are kPa, millibar, mmHg, mmH20, in., ft, or m for piezometers, µε (microstrain) or ε (strain) for strain gages, kg, ton, or lb for load cells, and mm or in. for crackmeters. The implements a polynomial that works with most vibrating-wire sensors, and it uses frequency and temperature as inputs.
Some sensor calibration sheets report data in units of digits or B units. The relationship between Hertz and digits or B units is shown in the following formula:
digits or B units = Hz2 / 1000
The sensor manual should include information that describes the process of converting a frequency and temperature value pair into engineering output. Sometimes the temperature value is not used. Constants involved in the calculation are often slightly different for each manufactured sensor, in which case a calibration sheet or calibration record will be provided with each individual sensor. This information will be necessary in configuring the settings of the to obtain the desired result. Determine most of the details for calculating engineering output by using typical sensors in the lab or office before field deployment.
The settings used on the to configure engineering output are found in Device Configuration Utility > Settings Editor > Channel 1, Channel 2, and Channel 3. See Configuring the CRVW3 for more information.
A temperature reading cannot be used as part of the calculation unless it is expressed in units of degrees Celsius; raw resistance in ohms will not work. To use temperature as part of the output calculation, set Thermistor Enabled to True and enter the thermistor coefficient A, B, and C values for Steinhart-Hart conversion. The uses the following equation to convert from resistance to temperature:
Tc = (1 / (A + B × ln (Rs) + C × (ln (Rs))3)) – 273.15
Enable the calculation by setting Calculate Engineering Output to True. Enter a label for the desired output units in the Engineering Units box. This is a string label for the units of the calculated output values (for example, mm, ft, kg, kPa, or µε); it does not affect the calculations.
If your sensor manual uses digits as the frequency input for calculating engineering units, set UseDigits to True. The polynomial used for calculation will be based on digits and temperature; frequency in Hz will not be used. Conversely, when UseDigits is False, the polynomial used will be based on frequency (Hz) and temperature; digits are not used.
The following table shows the engineering units polynomial constants and the related terms used in Device Configuration Utility.
|
Device Configuration Utility settings used to set coefficients |
|
|---|---|
|
Use this Device Configuration Utility setting |
To set this coefficient |
|
Frequency/Digits squared (second-degree) coefficient |
A (first polynomial constant) |
|
Frequency/Digits linear (first-degree) coefficient |
B |
|
Constant term |
C |
|
Temperature squared (second-degree) coefficient |
D |
|
Temperature linear (first-degree) coefficient |
E |
|
Hybrid temperature coefficient (Temp × Freq or Temp × Digits) |
F |
When UseDigits is True, the form of the calculation polynomial is:
A × d² + B × d + C + D × t² + E × t + F × d × t
Where:
d = digits
t = temperature
A, B, C, D, E, F = user-specified coefficients (constants)
When UseDigits is False, the form of the polynomial is:
A × f² + B × f + C + D × t² + E × t + F × f × t
Where:
f = frequency (Hz)
t = temperature (°C)
A, B, C, D, E, F = user-specified coefficients (constants)
For correct calculations, sensor documentation must provide data to discern which coefficients of the polynomial are relevant (A, B, C, D, E, or F); all other coefficients must be set to 0 so they have no impact on the output calculation. Do this by comparing the calculation equations of your sensor manual to the polynomials previously shown. Contact your Campbell Scientific support representative if you require assistance for this step. The polynomial coefficient schemes have already been determined for the most common sensor types, and these can be shared.
Enter your sensor-specific coefficients in Device Configuration Utility. See Device Configuration Utility help for more information.
Once the settings have been applied and the restarts, the calculated output in engineering units will be given in the ChanX_EngOut value of the VW_Data output table, where X represents the channel number, unless “ChanX” is the name you gave to the sensor/channel.