Neka,
The derivative of a single point is not defined. Look at the right side of your diagram. The value coming from the String to Numeric conversion is a scalar - one double precision number. You then use a Dynamic data conversion block to convert this to an array of DBL which is then inserted into a Waveform data type and passed to the Derivative function. But there is still only one value in that data!
The Temperature Chart has an internal buffer which accumulates the data so the display shows all the temperature changes, but the derivative function only sees one point.
1. Always connect and check error outputs from VIs and functions. The Derivative function output almost certainly generates some kind of error indicating that you did not supply enough data points.
2. The Derivative function takes arrays as the normal inputs. Converting from array to Waveform and then (internally in the Derivative function) back to array is of no value.
3. The Build array primitive is a better way to convert a scalar into a one-element array than the Dynamic Data converter. Notice the coercion dots at the inputs of the Dynamic Data converter and the Derivative function. They tell you that LabVIEW is converting data types.
Since you appear to be getting the data from a serial port, you will need to add a timestamp to the data. The Derivative function assumes that the datapoints are evenly spaced in time which in general is not true of data received from a serial port. Also you only take one reading from the serial port. Does the instrument send an entire time series of data or do you need a loop?
Lynn
