## 3 A Short Overview of Selected Examples of Measuring and Monitoring Systems on Sea Ships
### Temperature Measurements in the Ship's Hazardous Areas
#### 3.3.1 Accuracy Aspects of Temperature Measurement Line  
Measurement of temperature can be performed in different ways [3; 4], but platinum thermo-resistance Pt-100 is the most popular sensor for that. A predominant role of this kind of sensors results from the particular environmental and constructional conditions related to ship tanks, as well as with usually measured negative temperature range. Because of the long distance between measurement temperature point inside tanks and monitoring system placed in CCR, it is common to use a transmitter which converts resistance of a sensor to a standard current value. In our example,  
Figure 8: The temperature measurement of liquefied gas in tanks, CCR—Cargo Control Room, T\({}_{\text{b}}\), T\({}_{\text{m}}\)—sensor in bottom and middle of tank to measure temperature of liquefied gas, T\({}_{\text{u}}\)—upper sensor to measure temperature of vapors. the two wires 4-20 mA standard is the most convenient to use [26]. The functional diagram of the discussed measurement line is presented in Figure 9.  
In the analyzed case, combined standard uncertainty is based only on the uncertainty of B type and it is described by the formula:  
\[\mathrm{u_{c}}=\Bigg{(}\sqrt{\frac{1}{3}\big{[}\left(\delta_{1}\right)^{2}+ \left(\delta_{2}\right)^{2}+\left(\delta_{3}\right)^{2}+\left(\delta_{4}\right) ^{2}\big{]}}\Bigg{)}(\mathrm{T_{2}-T_{1}}) \tag{2}\]  
where \(\delta_{1}\), \(\delta_{2}\), \(\delta_{3}\), \(\delta_{4}\) are relative errors of Pt-100 sensor, transmitter, Zenner barrier and monitoring system itself, T\({}_{1}\), T\({}_{2}\) are min and max of the temperature span. For the real data \(\delta_{1}=1.6\times 10^{-3}\), \(\delta_{2}=3\times 10^{-3}\), \(\delta_{3}=1\times 10^{-5}\), \(\delta_{4}=2\times 10^{-3}\), T\({}_{1}=-120\)\({}^{\circ}\)C, T\({}_{2}=120\)\({}^{\circ}\)C and related value of uncertainty corresponds to \(\mathrm{u_{c}}=0.57\)\({}^{\circ}\)C. A set of value for liquefied gas of tanks corresponds to \(-90\)\({}^{\circ}\)C. Accuracy of the temperature measurement in tanks illustrated in Figure 8 plays a fundamental role in estimation of the transported cargo volume and finally the transport cost and price of the gas. Therefore, all disturbing circumstances, causing additional errors in the process of temperature measurement, are very important. On top of that, an additional error was observed. This error was caused in the real measurement circuits by own capacities of long distances of sensors, screened cables, grounding and dynamic properties of a transmitter. In reality, all the elements of the measurement line connected together produce an active electrical circuit with positive feedback. The equivalent electrical diagram of such temperature measurement line is presented in Figure 10.  
In order to analyze the above circuit, a small signal model of the measurement line is presented in Figure 11. Dynamic properties of transmitter R/I should be defined in the model. It is worth  
Figure 10: Equivalent electrical diagram of the temperature measurement line, where: R\({}_{\mathrm{s}}\)—resistance of Pt-100 sensor, R/I—transmitter 4–20 mA, R\({}_{\mathrm{o}}\)—equivalent resistance of all resistances in the 4–20 mA current loop I, C\({}_{1,2,3,A}\)—equivalent capacities of Pt-100 sensor and connecting cables, U\({}_{\mathrm{ps}}\)—source of DC voltage. Figure 9: Functional diagram of temperature measurement line, where: R\({}_{\mathrm{s}}\)—resistance of Pt-100 sensor, R/I—transmitter-source of current 4–20 mA sink type, ZB—Zenner barrier as the associated apparatus, R\({}_{\mathrm{on}}\)—resistance of monitoring system, R\({}_{\mathrm{c}}\)—equivalent resistance of cables, U\({}_{\mathrm{ps}}\)—source of DC voltage, \(\delta_{1}\), \(\delta_{2}\), \(\delta_{3}\), \(\delta_{4}\) are relative errors.