This post is English version of the original post in Bahasa Indonesia (Indonesian) based on the request from one of the readers.
Case: Elevated Zero on Level Transmitter with Dual Remote Seals.
In a post of the tekisiinstrument.com blog, a reader faced to the installation problem of differential pressure transmitter with remote seal which is applied for level measurement with elevated zero. If he install the transmitter at the high pressure tapping point level, the remote seal at the low pressure side is not long enough. He came up with the question: Can he install the transmitter above the high pressure tapping point?
Solution:
The picture discussed in this post refers to the previous related posting in this link.
The answer for the above question is: YES. Differential pressure transmitter with two remote seals implemented for the level measurement can be installed above the high pressure tapping point. This configuration is usually called as “elevated zero”.
For example, the transmitter is installed in the middle of tapping points, between the high pressure and low pressure side tapping points. It means that the transmitter is installed 200 inches above high pressure tapping point or 200 inches below the low pressure tapping point.
Let’s say:
-hP = distance between high pressure tapping point and the transmitter (the minus sign is an indication that the transmitter is above the high pressure tapping point, why minus, because the filling liquid in the capillary tube of the remote seal creates the negative hydrostatic pressure relative to the transmitter sensor module).
hL = distance between low pressure tapping point and the transmitter.
Thus:
The pressure at the high pressure side of the transmitter when the tank/vessel is empty:
PHigh = (-hP)(Sf)
PHigh = (-200 in)(1,07 inH2O/in)
PHigh = -214 in.inH2O/in
PHigh = -214 inH2O
The pressure at the low pressure side of the transmitter when the tank/vessel is empty:
PLow = (hL)(Sf)
PLow = (200)(1,07 inH2O/in)
PLow = 214 in.inH2O/in
PLow = 214 inH2O
Thus, we get the zero elevation:
ZE = PHigh-PLow
ZE = -214 inH2O – 214 inH2O
ZE = -428 inH2O
When the tank/vessel is empty, the process liquid hydrostatic pressure is 0 (zero), because the process liquid has no level or H=0 (zero)
When the tank/vessel is full (up to the maximum measurable level), where H=350 inches, the process liquid hydrostatic pressure is:
When the tank/vessel is full (up to the maximum measurable level), where H=350 inches, the process liquid hydrostatic pressure is:
PH = (H)(Sp)
PH = (350 in)(0,9 inH2O/in)
PH = 315 in.inH2O/in
PH = 315 inH2O (PH is the transmitter span)
The LRV (Lower Range Value) and URV (Upper Range Value) can be calculated:
Calibration = “Zero Elevation” to (“Zero Elevation” + “Span”)
LRV = Zero Elevation
LRV = -428 inH2O
URV = Zero Elevation + Span
URV = -428 inH2 + 315 inH2O
URV = -113 inH2O
Conclusion:
Transmitter calibration is:
LRV = -428 inH2O
URV = -113 inH2O
Note:
Correction on the content, typo, grammar and any other aspects is welcome. English is not my native language.