Process Transmitters Voltage and Power Requirements in a Two-Wire 4 – 20mA Current Loop

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In two-wire 4-20mA control loops which are the most popular these days, the 2-wire transmitters convert various process signals representing flow, level, temperature, pressure, etc., to 4-20mA DC current for the purpose of transmitting the signal over some distance with little or no loss of signal.

Relationship between the Components in the 4 – 20mA Control Loop
There are three key components in the 4 – 20mA loop as shown below:

They are:

The Callendar Van Dusen Equation for Platinum Based RTD Thermometers

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The Callendar-Van-Dusen equation describes the relationship between the Ohmic Resistance and Temperature of Platinum based temperature instruments. This equation is commonly used in commercial applications in RTD thermometers and RTD Transmitters. In its original form, the Callendar-Van-Dusen Equation is given by:

Rt = R0 + R0α[t – δ(0.01t – 1)(0.01t) – β(0.01t – 1)(0.01t)3]

Rt = Resistance at Temperature t (°C)
R0  = Resistance at t = 0°C
α  = Sensor specific contant
δ  = Sensor specific constant
β  = Sensor specific constant (0 at t > 0°C, 0.11 at t < 0°C)

The above equation has been used in the EN/IEC 60751 standard in the format below:

How to Use a Solenoid Operated Valve to Implement Emergency Shutdown.

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One of the commonest configurations of Solenoid operated valves in use in valve emergency shutdown system is the 3-way valve. These three-way solenoid valves are typically installed in instrument air signal lines, between an instrument air supply to a control valve that is part of an emergency shutdown system.

When used in these emergency shutdown systems, the solenoid valve is energized during normal operation with an electric current and allows the instrument air signal to pass through to the control valve actuator. When the solenoid is de-energised by a remote hand switch, electrical failure or by an automatic emergency trip, the air signal is vented to atmosphere from the control valve and the air supply is isolated. The control valve then moves to its fail safe position on loss of air pressure.

An example of the use of a solenoid valve for emergency signal air switching, in a furnace firing control loop is illustrated below:

During normal operation, the temperature of the heated oil stream is controlled by temperature controller, TC-1, which controls the pressure of fuel gas to the burners indirectly by cascade control of the fuel gas pressure controller PC-2.

If the hot oil temperature exceeds its safe upper limit denoted by, HH (High High) then the high temperature trip instrument TZA-3 de-energizes the solenoid valve in the signal line between PC and its control valve. The control valve then closes to its minimum stop. When temperature falls below the HH limit, the solenoid valve is re-energized permitting fuel gas into the burners for more heating.