How to Calculate Differential Pressure Level Transmitter Calibration Range for Remote Diaphragm Seals and Wet Leg.

DP (Differential Pressure) Transmitter can be used to measure some process variables e.g. flow rate and level. In this article, I will show you the application of the DP transmitter that is used as a level transmitter.

The points that we want to reach are what LRV (Lower Range Value) that the transmitter sends lower signal 4mA and what URV (Upper Range Value) that the transmitter sends upper signal 20mA. To reach these we will calculate the LRV and URV.

The value of LRV and URV depends on the installation arrangement, measured fluid’s specific gravity, and fill fluid’s specific gravity, the span of level (the distance of lower and upper sensor tap).

I will make a case, this case comes from my experience while installing a DP level transmitter on water storage tank. Since the storage tank uses a breather valve with setting 0.7 PSIG then this is calculated using closed tank principle. Even people commonly call this an atmospheric tank because it has a breather valve, but the setting 0.7 PSIG will influence the measurement.

Scope of the case: the storage tank has zero elevation from the foundation and the transmitter is installed at 1000 mm of elevation, the fluid measured is water with specific gravity =1 and the fill fluid is silicon 200 with specific gravity = 0.94. Figure 1. below shows you how it is.

Figure DP Level Transmitter Calibration
DP Level Transmitter Calibration

The HP (Higher Pressure) port of the transmitter is connected to the lower flange tap and the LP ( Lower Pressure) is connected to the upper flange tap.

Determine the LRV. Bear in mind that the minimum level (LRV) is always never below the lower flange tap because the sensor cannot detect this. So, in this case, the minimum level we use is the lower flange tap level (you may sometime pick a minimum level above the lower flange tap).

In the condition of the minimum level (lower flange tap level), the transmitter does not get any pressure from the fluid inside the tank. So, the transmitter only gets pressure from the fill fluid inside the capillary from the LP side (it means vacuum). Figure 2.

DP Level Transmitter Minimum LRV Condition
Minimum Level Condition

LRV = SGw x H – SGf x H

= 1 x 0 – 0.94 x 2500

= – 2350 mmH2O

When the condition of the maximum level (upper flange tap), the transmitter gets pressure from the fluid (water) inside the tank from the HP side and also the opposite from the fill fluid (silicon 200, the LP side).

URV = SGw x H – SGf x H

= 1 x 2500 – 0.94 x 2500

= 150 mmH2O

From the result above, it means we need to calibrate the transmitter for -2350 mmH2O to 4 mA (zero/ min level) and 150 mmH2O to 20 mA (max level).

So how if the level is going above the upper flange tap? It will undetected because the pressure by the fluid above the upper flange tap will push to both sensors (LP and HP), it will not give DP to the transmitter. To avoid that condition, place an Alarm High High below the upper flange tap to the DCS/PLC/SCADA system so the operator will get informed if the fluid level is going up.

This article categorized in: Engineering, Instrumentation