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The Hart way

Display unit shows up to four measured values
The Hart way

For a long time the Hart signal was chosen only for commissioning and monitoring the status of devices. The wireless Hart standard took things a step further by using the Hart protocol to transmit decentralised measured values to a central unit. This motivated Endress+Hauser to think about whether the conducted signal could be used even more efficiently.

The Author: Thomas Knapp Product Manager Components, Endress+Hauser

Displaying key measured values in the control room and in the field is an important factor in plant operation. For the operator, an ability to read the most important measured values on site is critical to ensure an accurate knowledge about the condition of the process.
Loop-powered display units represent the simplest alternative since they draw their energy from the current signal circuit and no additional wiring is necessary. Installing a display unit is therefore extremely easy, particularly with intrinsically safe circuits. The voltage drop reduces the cable length, so that display units with a low voltage drop are required. In practice, a drop of 4 V is not a problem in more than 95 % of all cases.
More information from one device
The metrology in process automation is increasingly complex and includes ever more options. One clear trend is for transmitters to supply a whole series of relevant parameters, which cannot all be output via a 4…20 mA circuit. The following are just three typical examples:
  • The Promass Coriolis flowmeter measures not only the flow rate but also density, temperature and optionally viscosity or the Brix concentration.
  • The Hart TMT82 temperature transmitter measures two temperatures and calculates the difference between them.
  • The FMR50 microwave radar measures not only the level signal but also damping, for example in order to detect foam.
Hart display unit
The Hart RIA15 display unit is loop-powered. The configuration determines whether the process values are connected via the 4…20 mA signal or digitally via the Hart signal. If the Hart signal is used, the display unit can be configured as a primary or secondary Hart master that shows either one or up to four measured values alternately. A unique unit or designation can also be displayed for each value to rule out any risk of confusion.
Good legibility is vital and the display can also be backlit if necessary. The backlighting is activated via the wiring.
A voltage drop of <1 V is sufficient for the RIA15 without backlighting because the Hart display has <1.6 V; a voltage drop <3.9 V is required with backlighting.
Since display units must be provided in the field as well as in the control room, the RIA15 is also available as a field device with IP 66 protection or as a panel-mounted unit with IP 65 protection on the front panel.
Commissioning is very easy using three buttons. As Hart Universal Commands are used, no special device drivers are required. The Hart display unit is consequently ideal for all devices that support Hart protocol Version 5 or higher.
The RIA15 offers additional security by evaluating and displaying the sensor’s diagnostic val-ues, namely overrange and underrange of the current signal. Thanks to two housing variants and international approvals for Ex ia, the RIA15 is universally suited. The device can be installed in intrinsically safe circuits in Zone 0 or Zone 1. The GL approval moreover makes it attractive for applications in the shipbuilding industry. It can even be installed without hesitation in SIL plants, since there is no feedback to the SIL loop.
Hart devices at single measuring points
The RN221N active barrier with Hart status monitoring has been available for some time now. It monitors not only the 4…20 mA circuit for compliance with the Namur NE43 guidelines but also the transmitter, either by means of the Hart status bit or with Endress+Hauser’s diagnostic command 231. The relay which is switched in the event of a fault only returns to its initial position when reset via a digital input. Faults that only occur sporadically can be detected quickly and reliably in this way because they are still indicated by the relay long after the transmitter has resumed operation.
The Diagnostic command 231 also offers the option of monitoring various transmitter service levels, with the result that preventive mainte-nance is greatly simplified.
Display units for every situation
Endress+Hauser offers a wide range of display units, including field and panel-mounted types. Since the requirements vary considerably from one application to another, several different versions can be supplied:
  • Loop-powered display units with or without limit monitoring, also for installation in hazardous areas
  • Display units with a power supply and limit monitoring
  • Display units for SIL 2 applications
  • Display units for optimum control of pump wells
All these devices are equipped with a 5-digit display from -19999 to 99999 and can also show the unit/tag or a bargraph. They are backlit for better legibility in bright or dark environments.
The RIA452 was specifically developed for monitoring pump wells; several pumps can be used alternately in order to optimise the monitoring intervals.
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