How do you know Hi-Hi level Sensor is not working?

How do you know Hi-Hi level Sensor is not working?

I have a tank with 5 Level sensors, (buoys)

LOW-LOW
LOW
MEDIUM
HI
HI-HI

I know LOW-LOW is not working if water level raises and activates LOW level without PLC receiving the LOW-LOW signal first.
I know LOW is not working if water level raises and activates MEDIUM level without PLC receiving the LOW signal first

This technique is fine for all the other sensors until I reach the Hi-Hi level Sensor.

Question:
How do you know Hi-Hi level Sensor is not working?

Not to be funny, but I guess you'll need a HI HI HI sensor to check the HI HI sensor.

We recently insalled a flood control system so that if a water intake service station flooded the elevator would not get submurged, and drown the passengers. We installed two N/C float switches at the same level. Both signals are on for normal and off for flood. We allow for a short time differential between switches, to check if one switch is off and the other on. If either switch goes off, or if power is lost to the switchs, than the elevator goes on flood control serivce. If only one switch is tripped an aditional alarm light goes on on the panel, and the system will not reset unless the switch is serviced.

Mike.

If the Hi-Hi sensing is very critical then perhaps redundant sensors would be proper. Another would be to establish a typical fill time from the Hi to the Hi-Hi sensor and use an amount above that in a timer as an error indication. Otherwise watch for water blowing out of the top of the tank (joke).

Elevmike

Your idea is good; since I don’t have more space on top of the HI-HI sensor I will place another sensor in the same level as this one.

bernie_carlton

This is municipal runoff water I cannot possibly time it.


Thanks everyone

This is fairly common on pump stations (plonker pumpers).

All you need to do is to expand on the idea you have already thought of...in essence you know when the level is filling or emptying, so then what you need to do is check that the switches correctly CHANGE state in the right sequence.

When the tank stops emptying, say just below LOW, then you will expect the next event to be a covering of the LOW, then the MEDIUM, then the HIGH, then if it is covered in normal operation, the HIHI.

Once you start pumping the level down the opposite occurs in reverse sequence. Look for the transitions in correct order.

The problem with the HIHI is that usually it doesn't get covered in normal operation and so it doesn't get checked.

The only way around this is either to arrange for an operator to allow the tank to fill to the HIHI level as part of a regular check, or to simply have two different kinds of sensor performing this HIHI check. For example, an electrodeless levelswitch AND a mechanical float switch.

A redundant sensor is probably the best solution however a sensor that is active low (Hi level results in an electrical off state) will warn if there is a disconnection in it will show hi even in that case.

Anibal Hernandez said:

Elevmike

Your idea is good; since I don’t have more space on top of the HI-HI sensor I will place another sensor in the same level as this one.

Click to expand...

This will just bring you back with a new topic called "How do you know Hi-Hi-Hi level Sensor is not working?"...

All the suggestions so far are good, which one(s) to use is a matter of just how critical this switch is.

beerchug

-Eric

After the serious answers....

Anibal Hernandez said:

Question:
How do you know Hi-Hi level Sensor is not working?

Click to expand...

When your feet get wet!!

Paul

PLucas said:

When your feet get wet!!

Click to expand...

And as for when the Hi-Hi-Hi level sensor is not working, ask THIS guy-->

beerchug

-Eric

Anibal,

I know LOW-LOW is not working if water level raises and activates LOW level without PLC receiving the LOW-LOW signal first.

Click to expand...

Actually you can only know that one of the sensors is wrong, not which one. The same is true for all the other combinations - but you can off course assume that the LOLO sensor is wrong if all other sensors is ON when LOLO is OFF.

If this is not an existing setup, then in stead of that many level sensors, it would seem to be a better alternative to have one level transmitter (a cheap pressure transmitter will do), backed up with one HIHI sensor for safety.

Anibal,

I would have to say JesperMP's idea would be best. The pressure sensor would provide more accurate level information, and would be more impervious to being fouled up by junk floating around in the retention basin. A single additional level switch could be used to check the proper operation of the pressure transducer.

Simply place the pressure sensor in the bottom of the basin. As the basin fills, the pressure will increase, providing a more accurate level. The pressure measurment could be converted to approximate volume, (gals), or level, (inches or feet) in your program, and displayed on a cheap operator interface. You could even measure rate of rise, like if your basin was filling rapidly you may want to react before it gets full.

This wouldnt be very expensive and provide much more usefull information.

Mike.

Hello,

The pressure sensor would provide more accurate level information

Click to expand...

Please be carefull, when U use an pressuretransmitter (density). If your temperature is constant there is no problem but if your temperature is not constant you might have a problem when the water is hot.
If I was in your position I would install a leveltransmitter, a ultrasonic will do for water, and install an additional overfill (HIHI) protection.

Rudi

Rudi,

It's my impression that the application is a runoff, or retention basin, like for sewage, or storm water overflow. If my presumption is correct, the basin may be subject to debris in the basin, possabley floating by the ultra-sonic sensor and providing a false reading.

If I'm correct about the application, changes in mass due to tempature changes would be very negligable.

However the ultrasonic sensor may be a good idea, (again depending on the application), and Anibal should also give it some consideration.

Mike.

Thanks everyone for the replies. The application is for storm water overflow. I had advised the customer wisely according the many good ideas you guys have written here.

Thanks

Andy

ultrasonic level sensors

We had similar problems with the waste water facility of our plant and eventually moved to ultrasonic level sensors interfacing to a scada pack that is remotely monitored its a costly solution. But even under extremely harsh water conditions they turned out to be extremely reliable. Having to never replace them in 5 years of operation.