On automated ships, a wide variety of sensors, actuators, controllers and controls, indicators, and alarms are supplied for use in monitoring and operating the engine system. Electrical signals are used to convey the values that have been measured so that they can be shown in a location that is physically separated from the point of measurement. Because of this, it is necessary for a sensor to transform the physical variable that is being measured (such as pressure or temperature) into an electrical variable (voltage, resistance). Following its processing and transmission, the electrical signal is next evaluated, with the physical significance of the signal as well as the scaling and processing of the sensor being taken into consideration. After then, it is either displayed or made available to a device that controls it.
Processing and transmission of electrical signals each employ their own unique set of guiding principles:
- It is possible to send the electrical signal that was created by the sensor. The display unit is responsible for further processing that takes place.
- The electrical signal that is produced by the sensor is then subjected to local processing before being transmitted in a unified current or voltage range (for example, 4-20 mA), which is then subjected to additional processing in the display unit.
- The data bus is used to transport the digitized version of the electrical signal that was processed locally after it was created by the sensor and then sent to the display unit.
The processing functions as a whole are almost entirely independent of the underlying concept that is applied. The only factor that differentiates the principles is the setting in which each individual function is intended to be applied. In situations in which it is believed that the data will ultimately be shown digitally or will be utilized for control in digital form, the following functionalities are required:
- The transformation of the physical variable into an electrical variable (such as temperature or resistance, for example).
- Preparation of the electrical signal (such as converting resistance to voltage), as well as amplification if it’s required.
- Filtering, if required
- Creating a digital representation of the interpretation
- Any and all forms of transmission
- If it turns out to be essential, the installation of barriers to prevent any potential hazards caused by problems (especially explosion protection, e.g in the case of sensors in fuel tanks).
In many cases, the design of a sensor is made in such a way that it continuously compares a physical variable with a limit value and only outputs the outcome of this comparison as a binary signal. This type of sensor is known as a comparator. This can be in the form of a unified range of current or voltage, or it can represent the condition of a switching contact (limit switch, switch to monitor a pressure to minimum). The correlation of a physical condition with the state on the electrical side is what makes up the characteristic curve of a sensor.
For temperature measurement the Pt100 resistor is the most frequently used onboard vessels and are fitted in standardized casings for practical application.
The video below is self explanatory and you can learn on how these sensors are constructed, their properties and working principle.
The manner in which the measurement resistor is connected to the related evaluation circuit has a significant impact on the overall precision of the measurement. This component is often housed in a cabinet some distance away from the location at which the measurements are taken in ordinary installations.
The measuring result is susceptible to significant alterations brought on by the connecting cabling that exists between the measuring resistor and the monitoring equipment. At a temperature of around 9 ºC, a connection that is 20 meters in length and uses a cable that is 2 * 0.5 mm² in area has a resistance value of 3.6 Ω. Even while this inaccuracy can be readily adjusted for in the monitoring device, it is important to remember that even the resistance of the connecting line is temperature dependant. This is something that should be kept in mind at all times. The resistance goes up by 0.28 Ω, which is equivalent to 0.7ºC, whenever there is a shift of 20 ºC in the conductor’s average temperature. When compared with the limit values that are specified for classes A and B, this constitutes a sizeable disparity between the temperatures that are actually being experienced in the cooling water and fuel systems. As a result, it is vitally necessary to make adjustments to account for the influence that the connecting wiring has.
Because of this, a connection with just two wires is only recommended when the precision requirements are relatively low, the temperature is very high, or the connecting wires are very short. The connection with four wires is both the easiest and the most accurate method. Because the wires for measuring voltage are not charged with current, the voltage at the measurement resistor always precisely corresponds to the value read from those wires. A connection with three wires can achieve virtually the same level of accuracy as a connection with four wires, but the monitoring device needs to have two amplifier inputs in order to accurately measure both voltages.
The fact that the cables of a three-wire connection are typically, always, and invariably twisted in pairs is the primary source of the connection’s namesake issue. A twisted pair must be shared by two measurement resistors in order to achieve optimal cost efficiency. In order to provide the least amount of disturbance, the connections of the two measurement resistors that should be shared by a single twisted pair are determined by the monitoring equipment. It is imperative that any possibility of the three wires of a being separated into separate cables be eliminated at all costs.
A typical application onboard vessels for the thermocouples is the measurement of exhaust gas temperatures. Same can be found on cylinder liner wall temperature monitoring (if you want to know more about it follow this link).
In actual reality, many thermocouples are typically connected in a switch box at a terminal block, and an electric resistance thermometer is used to monitor the temperature of this terminal block (e.g. Pt100). The neutral state of the connecting line between the thermocouple and this connecting block is required at all times.
There is a wide variety of temperature sensors on the market that are based on specific alloys, ceramics, or semiconductors. These sensors have temperature coefficients that are noticeably higher than those of pure metals. However, the precision of these sensors is significantly worse when compared to, for instance, a Pt100 sensor. Because the temperature coefficient is so big, it is possible to evaluate the system using very basic electronics. These sensors are frequently utilized within control cabinets for electrical components or small motors for the purpose of performing straightforward monitoring of the cooling. Changes in resistance of up to 5% per degree Celsius, which is more than ten times the value for metals in their purest form, are typical. For temperature sensors with a positive temperature coefficient (PTC), the resistance goes up as the temperature goes up, whereas for temperature sensors with a negative temperature coefficient (NTC), the resistance goes down (negative). There are additional materials in which the resistance rises at an exponential rate beginning at a certain temperature, creating an effect that is almost identical to that of a switching characteristic.
Here below, I strongly recommend to watch the self explanatory video with regard to temperature transmitter, where you can find and learn how to calibrate them. Enjoy!
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Source and Bibliography:
- Compedium Marine Engineering
- Youtube video training credit – RealPars