Fieldbus & Industrial Networking

Benifits from 'smart' transmitters using Hart protocol

April 2000 Fieldbus & Industrial Networking

Rangeability Rosemount Model 3051 has 100:1 rangeability. This means that, for example, a Range 3 differential pressure transmitter can be spanned from a maximum span of 250 kPa down to a minimum span of 2,5 kPa. This range typically covers 90% of the DP transmitter needs in a plant and can assist in reducing inventory costs. The 1151 Smart has 15:1 rangeability. A conventional analog pressure transmitter has 6:1 rangeability.

Stability, accuracy Stability spec for Rosemount Model 3051 is 0,25% of upper range limit (URL) for five years. Typical stability for an analog pressure transmitter is 0,2% for six months. Increased stability reduces calibration costs by reducing frequency of calibrations. Accuracy for Rosemount Model 3051 and Model 1151 Smart are 0,075% of calibrated span for 3051 and 0,1% for 1151 Smart, compared to 0,2 or 0,25% for a conventional analog pressure transmitter.

Ambient temperature compensation Rosemount Model 3051 provides high-performance under varying ambient temperatures by using a temperature sensor embedded near the pressure sensor module to detect ambient temperature for output signal correction. Rosemount Model 1151 Smart transmitter uses thermistors and resistors to provide temperature compensation of the sensor module signal prior to A/D conversion. A second temperature measurement on the electronics board is converted to a digital signal and used for temperature compensation of the electronics board. The resulting ambient temperature effect performance is much better than conventional transmitters.

Quick start-up Field calibration is not required on start-up because a complete sensor characterisation over the temperature and pressure range of the transmitter is done at the factory. Calibration certificates can be provided to document the factory characterisation. If the 4-20 mA range points need to be changed during start-up, this can be accomplished with a handheld communicator or PC software. No pressure source is required. The handheld communicator or PC software can also be used to conduct loop tests to verify transmission of a test 4–20 mA signal and to test any alarm points (user-selected mA signal), as well as to verify that transmitter terminations are correct per tag number stored in each transmitter.

Reduced maintenance Diagnostics from the control room via a handheld communicator or PC software reduces trips to check out instruments. Increased MTBF offers greater reliability. Reduced number of electronics boards (one in a smart transmitter vs three in a typical analog transmitter) increases reliability and reduces cost of spare boards. Increased stability requires fewer calibrations over time.

Transmitter status indication Rosemount Model 3051 and 1151 Smart pressure transmitters perform continuous self-diagnostics. When a failure is detected in the sensor or electronics, the analog output is driven to the user-selected alarm level (high or low) to alert operators and maintenance personnel. The failure mode alarm output levels differ from the output values that occur when applied pressure is outside the range points. When pressure is outside the range points, the analog output continues to track the input pressure until reaching the saturation value of 3,9 or 20,8 mA. Failure mode alarm analog output values are 3,75 or 22,0 mA for Model 3051 and 3,8 or 21,75 mA for Model 1151. As such, with the analog output you are able to tell the difference between a failed transmitter and one that is just sensing pressure outside of its range points.

Fieldbus compatibility The SP50 specification calls for a communications rate of 10 times per second, requiring a pressure sensor update rate of at least 20 times per second to prevent aliasing. Alpret says that the Rosemount Model 3051 and 1151 Smart are the only transmitters on the market that provide sensor updates 20 times per second. This means that the pressure sensors are already fieldbus capable and only an electronics circuit card change will be necessary to convert the transmitter output to fieldbus communications.

Retrofit 1151 analog to Smart – Rosemount Model 1151 analog transmitter can be easily converted to Smart by replacing the analog circuit boards with a smart circuit board kit and conducting a simple characterisation routine.

HART protocol

Simultaneous communication - HART protocol provides simultaneous communication (4–20 mA analog + digital) which allows the use of other 4–20 mA devices in the loop such as recorders and indicators.

HART Foundation – HART protocol is "open" to anyone who wishes to implement it in their products. The HART Foundation consists of companies that represent over 75% of the field measurement devices manufactured worldwide. The future looks bright for HART protocol. Since it is 4-20 mA based, it will coexist along with fieldbus as the two major options for communications from field devices for years to come.

Universal HART handheld communicator

Polarity insensitive - the Universal HART communicator is polarity insensitive. The user can hook either lead to the positive and negative terminals of the transmitter for communications.

No affect when connecting - there is no affect (ie current spike) on the loop when the communicator is attached to the loop and communication is initiated, therefore no inadvertent alarms are tripped which may cause unnecessary shutdowns.

No affect when disconnecting - transmitter memory is unaffected regardless of the sequence used when disconnecting the communicator leads from the transmitter.

Universal HART communicator - one communicator for all HART vendors means reduced programming hardware. There are over 100 instrument manufacturers registered with the HART Foundation.

AMS performance software

Database record keeping/on-line instrument diagnostics - AMS performance software for the PC facilitates instrument record-keeping and provides interfacing capabilities with various vendors' test equipment to perform automated calibrations. Benefits are reduced errors associated with manual record-keeping and time saved in maintaining current instrument records. AMS has a HART protocol interface which allows automatic population of the instrument database. It can also be used to verify that the correct instrument configurations were received from the vendor. Safety interlocks and loop alarm points can be verified during start-up or routinely by using the loop test function to drive a specified analog output throughout the loop. Functions such as rearranging the transmitter 4-20 mA output or accessing detailed diagnostic information are easily performed. An optional HART communicator interface is available to allow downloading of device configurations.


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