Temperature Transmitter 4-20mA , 0-10Vdc, Output, Head mounted hockey puck, Din Rail, Thermocouple, RTD or 0-50mV inputs

Transmitters can be programmed from a Palm PDA or PC

Standards for some transmitters
Pt100, IEC 60751 0.2%
Thermocouple, IEC 60584 0.3%
Electromagnetic Compatibility *EN 50081-2, *EN 50082-2

Microprocessor Based Temperature Transmitters (Thermocouple Inputs J, K, T, E, R, S, E, N, Rtd Pt100)

is the reliable way to transmit a signal to the readout and control system. The transmitter will take the low-level signal from the thermocouples or RTDs and convert it to a strong high-level signal 4-20mA, which can be transmitted over long distances without compromising or weaken the signal. The signal can be sent to the readout and control system with twisted nickel plated
copper wires instead of using high cost thermocouple wires when using a direct wiring system. Direct wiring system can cause problems especially when running over long distances. Signal from the thermocouple or RTD are very weak and can be easily affected by RFI and EMI interferences. Non-isolated transmitter for use with ungrounded probes. Isolated transmitter should be used with grounded probe.

4-20 mA is an analog electrical transmission standard for industrial instrumentation and communication.

The symbol "mA" is standard SI notation for milliampere, or 1/1000 of an ampere. The signal is a current loop where 4 mA represents zero percent signal and 20 mA represents the one hundred percent signal.

The reason zero is at 4 mA and not 0 mA is that this "live zero" allows the receiving instrumentation to differentiate between a zero signal and a broken wire or a dead instrument. This standard was developed in the 1950s and is still widely used in industry today, even though many attempts have been made to replace it with digital forms of communication such as fieldbus and Profibus. Its benefits of being a widely followed standard, low cost, its reliability and immunity to electrical noise keep it in regular use. Current loop is also much easier to understand and debug than more complicated digital fieldbuses. Using fieldbuses and solving related problems usually requires much more education and understanding than required by simple current loop solutions. Additional digital communication to the device can be added to current loop using HART Protocol.

Process-control use
For industrial process control instruments, analog 4-20 mA and 10-50 mA current loops are commonly used for analog signaling, with 4 mA representing the lowest end of the range and 20 mA the highest. The key advantages of the current loop are that the accuracy of the signal is not affected by voltage drop in the interconnecting wiring, and that the loop can supply operating power to the device. Even if there is significant electrical resistance in the line, the current loop transmitter will maintain the proper current, up to its maximum voltage capability. The live-zero represented by 4 mA allows the receiving instrument to detect some failures of the loop, and also allows transmitter devices to be powered by the same current loop (called two-wire transmitters). Such instruments are used to measure pressure, temperature, flow, pH or other process variables. A current loop can also be used to control a valve positioner or other output actuator. An analog current loop can be converted to a voltage input with a precision resistor. Since input terminals of instruments may have one side of the current loop input tied to the chassis ground (earth), analog isolators may be required when connecting several devices in series.

Taking the point of view of the source of current for the loop, devices may be classified as active (supplying power) or passive (relying on loop power). For example, a chart recorder may provide loop power to a transmitter instrument such as a pressure transmitter. The pressure transmitter modulates the current on the loop to send the signal to the strip chart recorder, but does not in itself supply power to the loop and so is passive. Another loop may contain two passive chart recorders, a passive pressure transmitter, and a 24 V battery. (The battery is the active device). Panel mount displays and chart recorders are commonly termed 'indicator devices' or 'process monitors'. Several passive indicator devices may be connected in series, but a loop must have only one transmitter device and only one power source (active device).

The relationship between current value and process variable measurement is set by calibration, which assigns different ranges of engineering units to the span between 4 and 20 mA. Occasionally the mapping between engineering units and current was inverted, so that 4 mA represented the maximum and 20 mA the minimum.