Disparity in process range and control system range

[maxidivine]

I am in the process of ordering a hydrostatic level measurement sensor. This sensor is to be used to measure water level in a sump 4m high. The sensor however can only measure from 10cm from the bottom of the sump because it cannot get right to the bottom. This means an offset of 10cm. The technology is 4-20mA and the system specification allows an uncertainty of 1% in measurement.

The I&C system was programmed with a range of 0.1 -4m (0.1 to take care of the offset mentioned earlier) but then the upper limit should have been 4.1 to cater for a sensor with a range of 4m. Because we do not want to incure additional costs in having to update the I&C system (from 4 to 4.1m) we are planning on living with this disparity.

My question is how do I account for the disparity, knowing that the sensor has a range of 0-4m,  but the I&C system is 0.1 - 4m?

In terms of thresholds, the max water level that triggers the sump pumps is 3.5m. Am I correct in saying that with the I&C, we are okay up till 3.9m (ie 0.1 - 4m)? Or is there some calculation I have to take into account '4-20mA' wise?

Thanks a lot.

[Henrik_Volkers]

How about some basic math?

You sensor will give you out the value it sense, that could or couldn't be the value of interest 😄

What you wanted:

 Actual_water_level_in_meter = 4m/16mA*($measured_current-4mA) +0.1m

What you get:

System_output_waterlevel= 3.9m/16mA*($measured_current-4mA)+0.1m

So all you need is a correction factor of 4/3.9 to get the actual level or the factor 3.9/4 for new limits ...

At least: Do a field calibration: all you need is a 4m tube surrounding the sensor ....