In both cases a wet leg installation is done with SMART positioners, that is why.
I will try and give a brief explanation how it works and why the readings are negative but it is actually a long explanation.
In the old days before SMART transmitters we only had pneumatic and electronic transmitters to work with. The problem with these were that they could only measure in the positive so to solve the problem of wet leg installations we had to change the LP and HP legs around in order to get a positive reading. This means the LP leg will go to the bottom of the vessel and the HP leg to the top of the vessel. We would then do the calibration in reverse and get a positive 0 to 100% reading from the transmitters.
With the SMART transmitters there is no need to change the legs around and they can be installed as normal since all SMART transmitters can measure in the negative. So to do a wet leg with a SMART transmitter is as simple as installing it as normal and fill the legs with vessel product and see what it reads. This value will always be in the negative since the LP leg is on top and filled therefore pushing the reading into the negative. What ever this value is is not important since you use whatever it reads after the lines are filled as your zero point and just add the calibration values to this negative value.
You have to have a reference leg when using DP to measure level in a pressurized drum. This reference leg is usually on the low side of the transmitter. So, when the drum is empty and the reference leg is full, you have a negative press reading on the transmitter. When the drum is full and equals the ref leg, you have zero differential. If you were to reverence H/L on the transmitter, you would start out positive and as the drum increases level, you would show decreasing pressure, could be confusing. The other option would be to "zero out" the reference leg to show 0" when the drum is empty then increasing pressure as the drum is filled, but I wouldn't recommend it.
Facebook
Google
GitHub
Linkedin
