All control modes previously described can return a process variable to a steady value following a disturbance. This characteristic is called “stability.” Stability is the ability of a control loop to return a controlled variable to a steady, non-cyclic value, following a disturbance. Control loops can be either stable or unstable. Instability is caused by [...]
In the last example, the control of the lube oil temperature may initially seem easy. Apparently, the operator need only measure the lube oil temperature, compare the actual temperature to the desired (setpoint), compute the amount of error (if any), and adjust the temperature control valve to correct the error accordingly. However, processes have the [...]
Temperature The hotness or coldness of a piece of plastic, wood, metal, or other material depends upon the molecular activity of the material. Kinetic energy is a measure of the activity of the atoms which make up the molecules of any material. Therefore, temperature is a measure of the kinetic energy of the material in [...]
Thermocouple Operation Thermocouples will cause an electric current to flow in the attached circuit when subjected to changes in temperature. The amount of current that will be produced is dependent on the temperature difference between the measurement and reference junction; the characteristics of the two metals used; and the characteristics of the attached circuit. Figure [...]
Thermocouple Construction A thermocouple is constructed of two dissimilar metal wires joined at one end. When one end of each wire is connected to a measuring instrument, the thermocouple becomes a sensitive and highly accurate measuring device. Thermocouples may be constructed of several different combinations of materials. The performance of a thermocouple material is generally [...]
Functions of Temperature Detectors Although the temperatures that are monitored vary slightly depending on the details of facility design, temperature detectors are used to provide three basic functions: indication, alarm, and control. The temperatures monitored may normally be displayed in a central location, such as a control room, and may have audible and visual alarms [...]
Bridge Circuit Construction Figure 8 shows a basic bridge circuit which consists of three known resistances, R1, R2, and R3 (variable), an unknown variable resistor RX (RTD), a source of voltage, and a sensitive ammeter. Resistors R1 and R2 are the ratio arms of the bridge. They ratio the two variable resistances for current flow [...]
Bridge Circuit Operation The bridge operates by placing Rx in the circuit, as shown in Figure 8, and then adjusting R3 so that all current flows through the arms of the bridge circuit. When this condition exists, there is no current flow through the ammeter, and the bridge is said to be balanced. When the [...]
Temperature Detection Circuit Figure 11 is a block diagram of a typical temperature detection circuit. This represents a balanced bridge temperature detection circuit that has been modified to eliminate the galvanometer. The block consists of a temperature detector (RTD) that measures the temperature. The detector is felt as resistance to the bridge network. The bridge [...]
Bellows-Type Detectors The need for a pressure sensing element that was extremely sensitive to low pressures and provided power for activating recording and indicating mechanisms resulted in the development of the metallic bellows pressure sensing element. The metallic bellows is most accurate when measuring pressures from 0.5 to 75 psig. However, when used in conjunction [...]
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