Level Sensor Problem

JIVM1127

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Join Date
May 2018
Location
Mexico
Posts
20
Hi everyone, we have a problem with a steam boiler preheater from BOSCH LOOS, it uses a differential WIKA Model 891.34.2189 pressure transmitter to measure the level in the container. This sensor stop working, stopping also the process in the plant (all requires steam) so we replace it with a E&H LevelFlex m fmp40 just meanwhile the new WIKA arrives (since the transmitter is 5 week delivery).

The problem here is that I thought E&H sensor would be a nice replace, but in the HMI of the PLC its everything backwards, Let me elaborate: When the level measured in the E&H (real level) increase, the HMI of the steam boiler slowly starts to decrease, and when the real level decreases it increases in the steam boiler.

Why? I checked the manual and both are a 2 wire 4-20 mA.
Am I missing something? We try to change to 4 mA-100% and 20 mA 0% but the sensor goes to fail mode.

We cant change the PLC program since it leave us without the BOSCH support and we dont really want to mess something there.

Could you recommend me something? right now we just leave it with a fixed 7 mA to simulate low level, but we have to put also an operator to always control the level.

Thank you all in advance.
 
1. process connections

A differential pressure transmitter has a process high side connection and a process low side connection.

For level sensing, the high side port connects to the lower elevation point and the low side connection connects to the upper elevation point.

If the tank is open to the atmosphere, then the low side port can be left open to atmosphere.

You must confirm that the E&H high and low sides are connected to the appropriate points. I suspect that the connections are backwards.

2. Range.

The WIka uses a zero and a span potentiometer to set the range that the 4-20mA signal represents, for instance 0-100mm H2Or or 0-1000mmH2O or whatever.

Your PLC is configured to interpret the incoming 4-20mA as a particular range.

If there is no documentation stating what the range is, then you need to determine the range.

If it were me, I would substitute a 4-20mA calibrator/source (it could be the E&H in manual/force/loop test mode, whatever they call it) where you read what the PLC 'sees' at 4.0mA and what the PLC 'sees' at 20.0mA. Whatever pressure transmitter you use has to produce 20mA at that level/pressure in order for things to match.

Then you need to configure the E&H for that 20mA range. That might require HART.

But the same situation exists for the replacment Wika - unless you ordered it from the heater vendor who knows its application, then you get it off-the-shelf at whatever the default 4-20mA range is. You'll need to calibrate the Wika so that its 4-20mA range matches what the PLC is setup for.
 
Thank you for the answers guys,

Try reversing the output signal polarity.

Unfortunately following the attachement highlights aint work, just change the contrast :(.

Your PLC is configured to interpret the incoming 4-20mA as a particular range.

yesteday we were able to determine the operative range following your recommendations, its a 4mA-100% to 10mA-0%.

But the same situation exists for the replacment Wika - unless you ordered it from the heater vendor who knows its application, then you get it off-the-shelf at whatever the default 4-20mA range is. You'll need to calibrate the Wika so that its 4-20mA range matches what the PLC is setup for.

I think you are totally right, we dont ordered it from LOOS since it was too expensive compared with different suppliers, we didnt think about that.

Regarding the configuration, do you guys think it can be configure in 4mA-100% and 10mA-0%, in the attached document you can find the overall steps to config the output, but whenever we reach the output only appears 4 to 20mA and we cant select a range, so what we try to do is put a 4mA as 100% and 20mA as 0% (since those values are the only that appears, but as I said just goes to fail saying "output out of range"), as far as I can tell it has a lot more of parameters that I dont know what are for.

Could you guys help me again?

Thanks
 
Presuming that Group 6 (output) 'curr. output mode' parameter is configured as 'standard' I'd try to set it to 'current turn down' and then adjust (I presume) the 'volume' percentages for the 4 mA and 20 mA limits (4 mA at 100% and 20 mA at 0%).
 
It isn't clear whether the output can be reversed: 4mA = 100%; 20mA = 0%

The examples shown for the most likely settings to do that use conventional forward slope / shift in the same forward direction. What you want is to have the blue line have a negative slope, like the backwards slash \ on a keyboard.

But maybe the 'current turndown' settings can be used to reverse the slope. You'll have to try it and see.


Current_output_settings.jpg
 
It isn't clear whether the output can be reversed: 4mA = 100%; 20mA = 0%

Maybe 'turn down' means slope reversal in a 'classic' German-to-English translation...:D...Mid and upper tier instrumentation often provides this type of features.

Nevertheless, a negative slope response would be a breeze to implement on the CPU side Analog Input channel scaling or logic application. Any modern platform should be easily able to do it, especially Siemens S7 which seems to be the case here.
 
Good point on doing a flip/reversal in the PLC.

I thought most all level instruments offer an output for either

- level: low elevation at 0% output, high elevation at 100% output, or

- distance, which for a top mounted device is low elevation at 100% output, high elevation at 0% output.

Whether it's called level/distance or standard/reverse or whatever, one would expect an instrument with the sophistication of a Guided Wave Radar to provide a typical industry customer defined output.
 
That sounds really promising, Tomorrow I'll test, Production guys dont want me to mess the steam right now.

Thanks a lot !!!!
 
It work perfectly guys!!!

Immediately after we change the parameter (current turn down) we could see the level behave as it should, we just have to configure the range but I want to thank you all.

I really appreciated
 

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