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Thread Tools Display Modes
Old March 16th, 2008, 09:04 AM   #1
robertmee
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Green is Go, Red is stop, High Level Confusion

Like most, I use the standard convention for HMI development. Green is good or go, Red is bad or stop. This standard works well for motors, solenoids, conveyors, valves, etc...But it generates confusion IMHO for high/low level detection. In my designs, low level is always fail safe low, with a '1' indicating level and a '0' indicating no level. For high level, the opposite (for failsafe operation), '1' indicating no level, and a '0' for level (which of course catches loss of power to the sensor).

The confusion comes in as to how to display them pictorially. Green/Red for level/no level works fine for low level. Green is good (material present), red is bad (no material, I need some). However, for high level when Green is good (no material) and Red (high level condition) it seems to confuse the operator because we have two graphics very close together on the same tank with green/red meaning two different things because of the nature of the sensor and its purpose.

In the case of a 'good' condition of a tank, with material between the two sensors, they would both display green. Makes sense to me, but not so much for the casual observer or untrained operator. They think the green means both sensors are covered. It's caused so much confusion that in some instances I've eliminated the point sensors graphics alltogether and developed a psuedo continous level indicator based on the states of the two sensors. Or, as an alternative, have a high level indicator only visible in the case of a high level condition.

I was just wondering what others are doing and if you run into the same confusion.

Last edited by robertmee; March 16th, 2008 at 09:06 AM.
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Old March 16th, 2008, 10:01 AM   #2
spaderkung
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Some confusion comes when assigning "Good" or "Bad" to a _snapshot_ of the state of the process. "I want something" in the tank when its filling, but not when its emptying.

One real example that the operators dislike is a tank with three discrete level sensors indicated by lit (green) / unlit, but the condition for lit depends on if its a high or low level sensor.

I keep it simple and recommend indication of "covered with product" for all discrete sensors, and whether that's "Good" or "Bad" I leave for the observer to decide. I find it risky to mix colors and visibility and on top of that change the meaning from sensor to sensor.

If a customer knows what type of indication he wants then it's the end of discussion.
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Old March 16th, 2008, 10:28 AM   #3
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I typically use 2 indicators on the tank: HiHi and LoLo. Green and Red are not good fits for this situation. Keep it simple for the operator. I stay away from "Full" and "Empty" since they can be misleading depending on the locations of the level probes.
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Old March 16th, 2008, 10:33 AM   #4
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You could just use a different color for the high-level condition. Since you are basically depicting three different level conditions, use a multi-stage lamp icon capable of handling more than two states. Then, for example, "Yellow" is too high, "Green" is just right, and "Red" is too low. The operator does not need to know that you are using two different sensors. All they should care about is the condition of the tank.
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Old March 16th, 2008, 12:49 PM   #5
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Robert,

I think you are putting too much reliance on colors. Before HMI panels were available, a control panel consisted of a bunch of buttons and lights. The choice of colors for the lights and buttons was limited, so color was used to represent something. The operator had to know what it meant, and it had to be consistent.

With modern graphic displays, the color is less important (but still should be consistent), because communication of information can be done with words, position, motion, and color.

I prefer to display words where possible. If Tank #1 High Level switch is on, then I display a message near the graphic for Tank 1 that says "High Level". The color can be red or green or whatever the operator is used to seeing. If the level is low, then the message changes to "Low Level". KISS.
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Old March 16th, 2008, 12:52 PM   #6
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I agree with the previous comments, use an analog indicator for tank level if possible. Show the tank level with something more than just color.

In keeping with your standards, what about changing the analog indicator to red when the operator attempts to drain an empty tank or fill a full one, or when there is an automatic process alarm condition, and use green when everything is kool.

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Old March 16th, 2008, 02:16 PM   #7
diat150
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to me, anything inbetween the hi and lo should be green. makes perfect sense. sounds like the operators need to learn how the system works. below low is red alarm above high is red alarm. anything inbetween is normal operations = green.
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Old March 16th, 2008, 03:40 PM   #8
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Is it possible with your HMI, to change the color of the text, with the condition... This way you can show the state of the sensor with the graphic and the condition with the text.

This way the graphics are consistant with the sensor, and you will still have something red appear, when you have a bad condition.... The other option would be to add another red graphic or alarm banner, that only is visable, during an alarm condition. If their are other screens are available, then it should be an alarm banner to tell them what is going on anyway...
I normally use grey for off conditions and another color for on condition, but you would still run into problems with wanting to show red for Lo-Lo condition.

-brian
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Old March 16th, 2008, 03:48 PM   #9
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I use red and green for running/open indication. I use yellow for alarms. Hi/Low level for alarms would be yellow for my screens, greyed out if they are not in alarm. For valves that are in between open and closed feedback switches I use blue for traveling.
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Old March 16th, 2008, 06:10 PM   #10
robertmee
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Thanks for all the great replies. I've augmented the indicator with a visible only text box that appears indicating "Hi Level!". As for comments on color indications, I do understand the limitations of relying on them. The last facility where I did alot of work, the main ops manager was color blind and could not distinguish red from green, so I almost always had directional arrows or text or rotation indicators in addition to color for that reason. But tank level always seems to be a non-conforming application. I have also used the suggestion of psuedo analog indicator relying on states for operators but then get grief from the electricians who don't understand why I don't have point indicators for the probes themselves. Ya can't win for losing sometimes
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Old March 16th, 2008, 07:17 PM   #11
Bob A.
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To summarize, Green = Safe (Stopped) and Red = Operate (Unsafe) and naturally we use a Green PB to leave the Green State and a Red one to Leave the Unsafe for the Safe condition.

I've been struggling with this for nearly 30 years. Now it is what ever the customer's convention requires.
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Old March 16th, 2008, 07:50 PM   #12
daba
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Tech

I would never show different colours to mean the same thing (or conversely, the same colour to mean different things) on a HMI or SCADA screen.

I have seen tanks with 7 or more level probes at various heights. Some are used during the filling processes, and are thus wired fail-safe (ie. a logical "1" = uncovered), and others are used during emptying processes, so are wired conventionally (ie. logical "1" = covered). The highest level is always wired fail-safe.

Think of the utter confusion with a full tank if the input state is displayed - eg. reading from bottom up - green, green, red, green, red, red, green.

From a plant operators viewpoint, he wants to see whether a level probe is "covered" or "uncovered", and the fact that high-level probes or switches are often wired "fail-safe" is totally irrelevant.

I would always convert any fail-safe inputs to a logical "covered", "wet", "product sensed" etc. condition in the PLC anyway, and use it in my logic. It removes any confusion as to the input wiring sense, and makes control code more readable.

If the suitably converted signals are used to indicate the tank level, the probes "light up" (ie. turn green) in order, from bottom to top, as the tank is filled.

I would also blink the alarm state for a probe (yellow is fine), with the active state of the probe (eg. Green/Yellow blinking means the probe is covered, and an alarm is generated by that probe).

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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Old March 17th, 2008, 02:06 AM   #13
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From a slightly different perspective; 8%-9% of men are red-green colour-blind which means that they have some degree of difficulty distiguishing beteen red and green, especially against a grey background when the contrast will be lower.

I'd always assumed that plant operators would have passed the same colour-blindness tests as electricians until I did a SCADA project and found out during commisioning that both shift operators were colour-blind! Ever since then I use colour and words.
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Old March 17th, 2008, 06:22 AM   #14
robertmee
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Quote:
Originally Posted by daba
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.
I agree. The issue of red vs green for High level, however, doesn't really occur because the input state, but from the general policy that red is bad/stop, green is good/go. In the case of high level, that is certainly stop (cease filling) and bad (overflow the tank) and hence the occasion to use green for not covered, red for covered. Regardless of whether you reversed states in the PLC logic.

I've also found, too, that there are variations depending on the target audience. Operators tend to understand covered/not covered. Technicians tend to want safe/unsafe good/bad. Those tend to contradict each other when it comes to visually indicating high level with color.
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Old March 17th, 2008, 07:03 AM   #15
diat150
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Quote:
Originally Posted by robertmee
I agree. The issue of red vs green for High level, however, doesn't really occur because the input state, but from the general policy that red is bad/stop, green is good/go. In the case of high level, that is certainly stop (cease filling) and bad (overflow the tank) and hence the occasion to use green for not covered, red for covered. Regardless of whether you reversed states in the PLC logic.

I've also found, too, that there are variations depending on the target audience. Operators tend to understand covered/not covered. Technicians tend to want safe/unsafe good/bad. Those tend to contradict each other when it comes to visually indicating high level with color.
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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