Criteria for Local or Remote Indication Instrument


Thread Starter


What is the criteria for an instrument to be used as local or remote indication at plant? Is there any standard for that?

Generally the increase in demand for remote indication results in reduced I/O availability of the DCS/PLC. Please guide.
I have never heard of a standard for this. It doesn't mean that there is none, of course.

As a matter of principle, all data that are involved in the control of your machine/process, should be connected to your control system. Having a local display for the same measurement is optional (I have seen both extremes, either absolutely no local display, or everything doubled).

Now, if, for example, you have a pump starting based on a minimum pressure, the cheap solution is to use a pressure switch, connect it to the control system, put in some pump starting logic and a relay output to start the pump. The expensive solution is to have a pressure transmitter, connect this one to the control system, compare the input to the minimum pressure, then have the same pump starting logic and relay output as in the case of the pressure switch.

Of course, most of the time you have the first solution implemented because it is cheaper and because the actual value of the pressure is not that important. But, in case you have a filter, for example, on that line, you might find yourself in the situation where the pump starts, works for a while, stops, then starts again. Without an actual measurement of the pressure it tries to control, you have no clue where you are in terms of pressures with and without the pump. In this case you want at least a local indication for your actual pressure.

As a guideline, if a measurement is not necessary in the control system, don't put it there. "Nice to have" is not an efficient criterion for engineering decisions, it leads to expensive installation, and higher maintenance costs and effort. But, wherever you don't have an information about process data in the control system, if that information might prove useful, install local indicators or, at least, have tapping points for temporary installation of local indicators.

I hope this helps.
A relatively common statement in a spec covering instrument installation and commissioning is

"Local indicating instruments shall be readable from where the related equipment is operated or from where the primary instruments are to be tested or calibrated." So you'll find local indicators where calibrations or periodic testing are required.

Add to those locations sites where a manual station is needed, or where Operations says local indication is required.

As to implementing location indication, a local indicator on a field transmitter is easily accomplished. I'm hard pressed to think of a process transmitter that is NOT available with a local indicator (it is typically optional and costs something). The addition of local indication to a process transmitter has zero effect on I/O availability at the control end.

If the field transmitter is not located where the indication is needed, a loop powered indicator can be added between the transmitter. A loop powered indicator does not limit I/O at the control end.

Most smart positioners have some form of local indication for their setup which is used for displaying demand or position in operation.

The food industry uses non-electronic pressure gauges and bimet thermometers by the truckload and a good number of those magnetic float bypass level indicators that flip colored flags. These typically do not tie into a control system, there are separate instruments that are used for control.

If control is distributed to single loop controllers or to remote PLCs, then yes, the process transmitter signals do not get directly connected to the DCS, but many plants are not on a DCS. For those that are, there's industrial digital communication protocols - they're proficient at collecting field data from remote locations over twisted pair cabling.

In the PLC world, remote I/O racks lack local indication at the rack (beyond blinking LEDs). I don't know what capabilities PLC remote rack communications has, but I would think in the 21st century that comm is bi-directional and I suspect that an HMI panel could be supported if local indication is needed.

I'm not sure why local indication would limit DCS I/O unless you're talking about either
- pressure gauges and bimet thermometers, or
- remote I/O dedicated to PLCs
I agree with David. we always purchase transmitters with indicators.
Operations will dictate which indicators are required to be read by Ops on a regular basis. These transmitters either need to be a standard height or a remote indicator wired to the transmitter needs to be installed if process conditions require the transmitter to be installed low to grade or above grade and too high for the operator to read.

Many companies have guidelines for indicator height to avoid operators from bending over or looking up to much.