**4. Real data example**

The following examples refer to a real data set recorded at a field station with measurements, as presented in **Table 2**. The table's quality measurements include Free Chlorine (Cl measured in ppm), Turbidity (TU measured in NTU), pH, Conductivity (CO measured in mS), and pressure (PRI measured in bars). For each measurement, the algorithm dynamically calculates the minimum and maximum values of the last 48 hours. (see first two rows of **Table 1**).


**Table 2.** Measurements.

Typical minimum and maximum values are shown in **Table 2**. Given these minimum and maximum values, the raw measurements are transformed into normalized measurements, as shown by Eq. (4) of Section 2. The normalized measurements are used to calculate the "Dynamic Noise", as shown in Eq. (5), with a lag difference between the records of 10 timestamps. **Figure 4** shows the distribution of the dynamic noise values.

As can be seen from the histogram, a value which is more than 0.25 is rare (see red arrow). Hence, the threshold for the dynamic noise was set to 0.3. In terms of Section 2 of this chapter, L = 0.3.

The first data analysis step with regard to the dynamic noise algorithm is to estimate the normal conditions, i.e., to observe how a dynamic noise curve behavie in case of a normal data flow. **Figure 5** shows a normal period of time for the four water quality measurements. Note that pH ranges between 7.70 and 7.79; Free Chlorine ranges between 0.37 and 0.48; Conductivity usually has an average of around 520–530 with short drops to 450; and Turbidity ranges between 0.09 and 0.12.

**Figure 6** shows the equivalent dynamic noise for the relevant measurements. As can be seen, the values range between 0.03 and 0.25 at the most.

We will now discuss four different cases, in which the dynamic noise violation threshold is analyzed. Note please that violation of the threshold L triggers an alarm only after a delay time in which the value of the DN is above the level of L. This in order to avoid false alarms caused by short spikes.
