Green is Go, Red is stop, High Level Confusion

diat150 said:

but the high level sensor being covered is what triggers an alarm state. like the other poster said, you need to make the hmi around the operation of the device, not the state that the plc is seeing. high level = device covered = alarm = red. To me the operator would be more concerned with the high level, and since he has a high level would want to know he is in alarm, hence the red.

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I'm in agreement. My response was to daba who indicated otherwise. However, with that said, I still see where it causes confusion. Take the case of a single low and single hi sensor. When the tank is operating between the two, both indicators would show green. But to the unitiated (or untrained), one could certainly say that both being green must mean both are covered. I see daba's point too in that you have two sensors on the same tank that are detecting level but their colors mean different things and thereby could be confusing. I've taken the approach to augment with a textual alarm of "Hi Level!" but in normal operating state, someone will still be inevitably confused by two green indicators, one meaning covered (Low) and one meaning uncovered (High).

When the tank is operating between the two, both indicators would show green.

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Why would you make both show green? Make them any color you want, as long as it doesn't confuse someone. Use the colors, instead of letting the colors use you! Do not think you have to stick to some "rule" if it leads to false logic.

...but in normal operating state, someone will still be inevitably confused by two green indicators, one meaning covered (Low) and one meaning uncovered (High).

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Then change one of them to some other color, or change both of them! The colors are not fixed in cement. That is the where the flexibility of an HMI is very useful.

I've sometimes used
dark red or green = normal state
bright red or green = actuated state

The color depends on if it's a trouble signal or normal process indication. The trouble signal may flash as well. This transition from dark to bright color minicks a lamp.

you folks obviously deal with a higher caliber of operators than I’m used to ... I’ve never seen ANY operator get caught up in whether the actual background logic signal is “high” or “low” or anything else along those lines ...



basically if the screen indication is “green across the board” then the operator just sits back and relaxes ... if something turns some OTHER color then he/she SHOULD start taking notice ... at that point a text comment would be nice ... specifically, if a “little round thing” starts flashing red near the top of the tank picture, then Bubba shouldn’t have to paw through the manual to find out that the boiler pressure is too high ...



all of that aside, here’s a technique that helps MOST people distinguish between a lamp which is OFF and a lamp which is ON - and still preserves the intended color of the lamp ... basically when the lamp is OFF, you outline it with the ON color - but make the inside of the lamp a neutral gray ...

robertmee said:

Take the case of a single low and single hi sensor. When the tank is operating between the two, both indicators would show green.

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You could try to make both sensors gray when OFF and change color such way:
LOW-sensor goes to green when activated, and the HIGH-sensor goes to red when activated... I think this way might be more understandable by the operators...

jmo

daba said:

In conclusion, the HMI or SCADA screen should not be a mimic of input states, but should be a usable "visual interpretation" of the plant and its sensors.

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Exactly :nodi:

And since the subject is silos, and sensors in silos, this is what I do.
I display the content of the material in the silo exactly as the sensors seems to detect it. It is simple and intuitive. If there is a problem with one or more sensors, so that for example a low sensor says it is not sensing any material, and at the same time a hi sensor do sense material, then I show exactly that on the HMI: There is material at the top but not the bottom of the silo. At the same time an alarm is triggered "Hardware error level sensors in silo xxx".

If you display only the input signal from the sensor, or try to show several states through colors, you will only confuse the operator.
Normally, you CAN show error state with red color, but in the case of level sensors the "error" is typically a higher level "derived" error - such as you cannot fill the silo, or there is an impossible combination. The sensor itself may work perfectly OK, so the red color is misleading.

Have any of you seen this? I was looking at implementing it where I work but haven't got around to it. Not sure if this is the current version.

Cheers
Peter