Can’t figure out differential pressure level measurement? You might be missing this key point!

According to hydrostatic theory: The static pressure at a given point in a stationary liquid is proportional to the height of the liquid above that point. Therefore, the liquid level can be measured by detecting pressure or differential pressure!

1.Measurement Principle

Two pressure guiding pipes are connected from the bottom and top of the container to the positive and negative pressure chambers of the differential pressure transmitter. The upper part of the container contains dry gas with pressure p₂, while the lower part has pressure p₁. Then:

p₁ = ρgH + p₂  ➡️  Δp = p₁ – p₂ = ρgH ✅

Where:

· H — Liquid level height
· ρ — Medium density
· g — Gravitational acceleration

The differential pressure Δp is proportional to the liquid level height H. If p₂ is atmospheric pressure, simply connect the negative pressure chamber to the atmosphere, and the measurement can be performed using a pressure transmitter or a pressure gauge.

2.Zero Drift

Under ideal conditions, when the liquid level H = 0, the transmitter’s input differential pressure signal ΔP = 0, and the output signal is 4 mA. At H = 0, Δp = Hρg = 0, and I₀ = 4 mA. The zero point is perfectly aligned, which is referred to as no migration.

In practical applications, the pressure cannot be introduced in this manner. For instance, if the liquid is corrosive, isolation measures are required.

Δp = pa – pb = (h1ρ2g + Hρ1g + p0) －(h2ρ2g + p0)= Hρ1g – (h2 – h1)ρ2g

H =0:Δp = -(h2- h1)ρ2g 

As shown in the figure, when an isolation tank is used between the transmitter and the container:

The transmitter’s output can only be within the range of 4–20 mA. However, in this case, the transmitter should output a signal below 4 mA, which cannot be represented. Therefore, adjustment is required.

✅ Method:
Add +(h₂ – h₁)ρ₂g to counteract -(h₂ – h₁)ρ₂g.

✅ What is positive migration and negative migration?
If the fixed differential pressure is negative, negative migration is required;
If the fixed differential pressure is positive, positive migration is required.

3.Flange-Mounted Differential Pressure Transmitter for Liquid Level Measurement

When the liquid to be measured contains crystalline particles, is corrosive, highly viscous, or prone to solidification, the pressure guiding lines may become corroded or clogged. In such cases, a flange-mounted differential pressure transmitter with an isolating diaphragm can be used.