The main purpose of the transmitter is to convert the sensor measured process variable to an electronic signal and send this electronic signal in a readable form to a system (usually DCS, sometimes subsystems such as ATG system for tanks)
When we open up a transmitter, we obtain some terminals connected with wires. These terminals (most common is ‘+’ and ‘-‘ terminals) and wires are the last connection to the DCS. Careful, when measuring current, you have to take out and measure in series. This attempt will disconnect the wire from the DCS. Some transmitters have a Test point. Connecting either +ve or -ve terminal and hookup the other terminal of the ammeter to this test point would directly measure current without the need of disconnecting the wires. In other words, test terminal is used to measure current without losening ‘signal’ terminal screws.
Some transmitters such as RTDs and Thermocouples, aside from having the ‘+’ and ‘-‘ terminals, have the sensor terminal too. The sensor wire is connected to this terminal. This type of transmitter only exists for transmitter which has a sensor that directly changes the process variable to an electric signal such as RTD’s and thermocouple
Transmitters are very low powered (less than 0.1 watt) to avoid spark creation if a short-circuit occurs. The power needed for a transmitter is usually 4mA times 24V. Power comes from the same source as the signal, i.e. in order to change the mA, the transmitter has to impede the current on a fixed voltage thus changing the power input as well.
If a process value goes beyond the URV. The signal will not clamp at 20mA, instead, the transmitter will give a higher reading. Usually its 21.75mA for upper clamp and 3.75mA for lower clamp. In this case higher peak values are more detectable than lower peak values
Damping is a setting set on the transmitter to average out signals before it is sent to the DCS. A damping of 5 means 5 seconds of the signal will be sampled before it is sent out to the DCS. The normal damping in our plant is usually 2 seconds
The temperature around the transmitter must be less than 85 degrees C to protect the electronic modules. Insulation needs to be install
Smart transmitters can be hooked up with a Highway Addressable Remote Transducer (HART) device to obtain a lot of information about the transmitter and the sensor. The HART device communicate with the transmitter using a small digital FSK (Frequency Shift Keying) type signal, modulated at +0.5mA on the bias 4 – 20 mA main signal. 1200Hz frequency corresponds to a ‘1’ while 2200 Hz frequency corresponds to ‘0’. The HART sensor measures voltage.
FSK shifts between 2 frequencies which is 1200Hz and 2200Hz
HART loops must be in series with a 250ohm resistor. In the field, this resistor is provided by the barrier. However, If a test is to do at workshop a 250ohms resistor must be provided. The resister should be in series with the loop and the hrt communicator parallel on it
Main advantage of Smart Transmitters than non smart are :-
Has the ability to give an alarm if an electronic module fails
Convenience. Calibration parameters are stored in the device
Has a microprocessor, therefore can perform a square root on transmitter, thermocouple calculations and RTD matching.
Has some additional features which are :-
Ability to multi-drop. Several transmitter (2 to 15) will be looped in the same wiring with each transmitters having a unique address (1 to 15). The 4-20mA bias is no longer used. Therefore the signal will only be used for power which is minimal at 4mA.
Burst mode. Hart protocol requires a master to query the device before the device can send a signal. This will take time and will not be fast enough for control if the hart digital signal is being used by the another system such as DCS instead of using the conventional 4-20mA (in this case the bias signal is at minimal 4 mA). A burst mode feature is available where the hart device continuously sends the PV without the need of query. This becomes an advantage if multi-drop is used, otherwise using 4-20mA is just fine
Below is the HART menu tree for a 3051 communicator
Diagnostics and Service
Rerange - Keypad Value, Apply Value
Trim Analog Output - DA Tri, Scaled DA Tri
Lower Sensor Trim
Upper Sensor Tri
Sensor Trim Points
Basic Setup – Tag
Pressure Sensor - Process Variables, Sensor Service, Unit
Signal Condition - Process Variable, Range Values, Unit, Transfer Function, Damping
The trims available for the HART transmitter
Lower Sensor Trim – to trim the lower span point of the sensor input to a desired process value. When this option is selected, one needs to apply a certain process variable with a calibrator and send the data
Upper Sensor Trim - to trim upper span point
Zero Trim – to trim the zero or offset of the sensor input to a desired process value
Sensor Trim Points – To select
DA trim – to trim the offset of the output mA to a desired Output mA
Scaled DA trim – to trim the span of the output mA to a desired Output mA
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