Saturation current of Rosemount Device

Hello,

Is there a way to set the output of ultrasonic level device to show high level when the sensor drives the analog signal to a saturation current ? According to the vendor, when a sensor becomes submerged it will drive the analog signal to a saturation current. The standard saturation value is 20.8mA. The literature states there is a standard alarm and saturation levels. What is the difference between the two? Why would these standard values be slightly different from Namur NE43 values?

For example, if the device is programmed to output low level=4mA and high level =20mA, I'd like the device to output 20mA upon submergence.

Any advice would be greatly appreciated.

Thanks,
EE

You're bucking failsafe practice and the vendor's tendency to do it their own "proprietary" way

Electronic transmitters can detect faults that render a useful process variable value uncertain or unobtainable; just like PID controllers can fail high (thermocouple burnout) or low, field transmitters can fail high or fail low.

Why fault/failsafe is not 20.0mA

As to why fault/failsafe is not 20.0mA, it's because a flag indicating failure cannot be confused with a valid signal. One has to account for actual over or under range process values, drift or slight out-of-calibration signals or inputs.

Many field transmitters will produce valid process signals just below and above the nominal 4.0mA and 20.0mA. So 3.8mA is generally considered still in the valid range, as is 20.3mA. In the table below, what you call "saturation" is the upper range limit of what I call overrange. Values below the saturation/overrange value might still be considered valid process values. The failsafe value is the one in the table called Fault.

A Fault/failsafe value has to be well above and well below what might be valid values, because failsafe infers that action can be taken as result of a failsafe detection.

The table below shows the failsafe levels for several DP transmitters.




Proprietary first

In the world of field transmitters, each tends to do it their own way as you can see from the table above.

Why different fault values than NAMUR?
NAMUR, who's NAMUR? Where's their refinery?

With regard to NAMUR, NAMUR is a European based organization that publishes standards for process instruments. But NAMUR was not based on the oil & gas industry, which is where the bar stool bragging rights in the process instrumentation world reside. All other industries are mentionable in a whisper, but participation in whatever market is not bragging rights like it is for O&G (check out any instrument brochure targeting management and the brochure cover will have a refinery photo on it). If it doesn't target O&G, well, whatever it is might be considered as an option (and NAMUR failsafe is available as an option).

Get a fault value at 20.0mA?

You're not going to find any of the majors who will do 20.0mA for a loss-of-echo/submergence fault. But there are wannabes out there who might jigger one for you, although I would not consider it good practice. That Holykell Chinese outfit will probably do it for you.

Something was nagging me after I posted above. I realized I was using pressure/temperature transmitter fail-safe conditions, not ultrasonics.

I checked Siemens ultrasonics and found that the Siemens lUT400 for a submergence error, the output goes to minimum or maximum 4-20mA (as set by user). If submergence subsides and a valid echo cannot be obtained then the unit goes to LOE, a user assigned fault/fail-safe value anywhere from 3.5 to 22.8mA.

So you should check your ultrasonic manual to see what they do for submergence or LOE. It might be user selectable.

Thank you so much for the response! You explained very well for me.

Best Regards,
EE