Data AcQuisition And Real-Time Analysis
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The following is from the Daqarta Help system:



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Fcal High

Controls: Options >> Frequency Counter >> Fcal >> High
Macros: FcalHiSet, FcalHiRaw

Sensor Calibration:

By default (without a Calibration Table loaded), Fcal assumes that there is a linear relationship between the frequency seen by the Frequency Counter and the variable to be measured. For example, a typical LM335 temperature sensor with an output of 10 mV per degree Kelvin driving a Voltage-to-Frequency converter with an output of 1000 Hz per volt will have a linear output of 10 Hz per degree Kelvin, which can be calibrated to read in Celsius or Fahrenheit. (See the Temperature To Frequency section under Fcal Circuits for a complete schematic with printed circuit board layouts.)

The Fcal High (and Low) control pairs allow you to both calibrate the sensor and tell Daqarta about the linear relationship. There are separate High Set and High Raw controls; the Set value is what the Frequency Counter readout will show (in the units to be measured, like degrees Celsius) when it sees the Raw value in Hz (or RPM or msec, in those modes).

With the above LM335 sensor plus V-F setup, if the calibrations of the sensor and V-F were both perfect, you could enter 373 for High Set and 3730 for High Raw to get a display of 373.00 degrees Kelvin when the input frequency was 3730 Hz. You could likewise use 273 for Low Set and 2730 for Low Raw.

Or if you wanted to read in degrees Celsius (which are the same size as degrees Kelvin, but with 0 Celsius = 273 Kelvin), you would use 100 for High Set and 3730 for High Raw, and use 0 for Low Set and 2730 for Low Raw.

But the above assumes that everything is perfectly calibrated; in reality that's usually not the case. The LM335 sensor is fairly accurate, for instance, but your V-F might produce something other than 1000 Hz/V. Or you might be using an ordinary diode instead of a precision sensor. (Diodes are accurate over a much wider temperature range, but each needs to be calibrated individually.)

One standard way to calibrate temperature sensors is to use boiling water to supply 100 C, and a bath of crushed ice in water to supply 0 C. (Note that there are a lot of considerations to get the most accurate temperatures, but this is the basic idea.)

To perform the calibration, you insert the probe in the boiling water and enter 100 for Set. With Fcal Off you could read the measured frequency in Hz and enter that value for Raw. However, Daqarta provides an easier way: When you enter the Set value, just conclude the entry with CTRL+Enter and the current frequency will be entered automatically as the Raw value. This will work correctly whether Fcal is On or Off.

Then you insert the probe in the ice bath and enter 0 for Low Set, likewise concluding the entry with CTRL+Enter to set Low Raw.

There is no actual need for boiling water and ice baths; any two known temperatures will work. You could get the known temperatures from a trusted thermometer exposed to the same temperature as the sensor being calibrated.

For best accuracy, you should use High and Low temperature values that encompass the range you are most interested in.

This same general approach is used for any type of variable. You would apply two different pressures to calibrate a pressure sensor, for example. If it senses pressure relative to ambient, you can use 0 for the Low Set value by simply disconnecting the pressure input.

Or, if you just want to read DC voltage directly using a V-F without a sensor, you would apply two different known DC voltages. Note that you can use negative values for Low Set. (Of course, this assumes your V-F has an input offset so that the negative input is still a positive frequency.)

The Temperature To Frequency circuit can easily be used for arbitrary positive-only sensor outputs just by omitting the LM335 sensor and one resistor, and possibly changing one other resistor value.

The Bipolar Voltage To Frequency circuit can be used for sensors with outputs that may include both positive and negative voltages. The given circuit handles +/-5 volt signals, but can easily be modified for other ranges.

The above discussion assumes linear sensors, but you can use nonlinear sensors such as thermocouples with a Calibration Table. The Thermocouple To Frequency circuit may be used for this application. (Note: If you use that circuit, the Calibration section there takes precedence over the information above.)

Frequency Prescaler:

You can use the Frequency Prescaler circuit to enable the Frequency Counter to measure frequencies much higher than your sound card alone can handle. This simple circuit converts the input signal into a logic-level rectangular wave and divides the frequency by 1024 (or another selected value), so the sound card sees a low-frequency square wave that the Frequency Counter can measure.

To allow the display to show the original input frequency, the Fcal High Set value is set to 1024 (or your chosen division factor), and the Raw value is set to 1. The Fcal Low Set and Raw values must both be 0.

Macro Notes:

The Fcal dialog does not need to be open to make changes to either Fcal High control. However, since separate Fcal values are maintained for each Trigger Source channel and valid Frequency Counter mode (Hertz, RPM, and msec), you must make sure the desired source and mode are set in order to change the relevant Fcal High settings.

FcalHiSet=100 sets Fcal High Set to 100 units (degree C, etc). FcalHiSet=>1 increments it, and FcalHiSet=>-1 decrements it.

Putting an A. prefix ahead of the FcalHiSet command causes it to automatically update the High Raw value with the current frequency, just as if a manual entry had been concluded with CTRL+Enter. For example, A.FcalHiSet=100 sets High Set to 100 and updates High Raw.

FcalHiRaw is used just like FcalHiSet, except that an A. prefix does nothing.

See also Fcal Dialog, Frequency Counter


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