How to Correctly Size a Pressure Transmitter for Low-Pressure Tank Vents?

Low-pressure storage tanks don’t operate with large pressure margins. In fact, many atmospheric tanks only fluctuate within a few millibars above or below atmospheric pressure. That’s exactly why sizing a Pressure transmitter correctly for low-pressure Tank vents is so important.

If the transmitter range is too wide, you lose sensitivity. If it’s too narrow, you risk over-ranging during normal operation. Getting the sizing right ensures accurate readings, stable blanketing control, and early detection of vent issues.

Let’s walk through what actually matters when selecting one for this type of application.

First, understand the operating pressure of the tank. Low-pressure storage tanks typically operate just slightly above atmospheric pressure, especially when nitrogen blanketing is used. For example, a tank may normally sit at +5 to +20 mbar. During filling or thermal expansion, it might briefly rise higher before Tank vents relieve excess pressure. During emptying, it may dip slightly below atmospheric pressure before vacuum protection activates.

Those numbers are small. That means you cannot use a transmitter with a large pressure span such as 0–10 bar and expect accurate results. The signal changes would be too small relative to the total range, reducing resolution and accuracy.

The next step is identifying the maximum and minimum expected pressures. Look at the tank design pressure and the setpoints of the pressure-vacuum vents. If the vent opens at +30 mbar and vacuum protection activates at –10 mbar, your transmitter must comfortably cover that full span. A suitable range might be –50 mbar to +100 mbar, allowing safety margin without sacrificing precision.

Choosing a narrow, application-specific range improves measurement sensitivity. A properly sized Pressure transmitter for low-pressure Tank vents should be able to detect even small pressure shifts. That’s especially important when monitoring nitrogen blanketing systems where minor changes indicate valve behaviour or vent restriction.

Accuracy specification also matters more in low-pressure applications. When dealing with small pressure values, even minor percentage errors become noticeable. For example, a 0.5% error on a 100 mbar span is very different from the same percentage on a 10 bar span. Selecting a transmitter with appropriate accuracy for low-range measurement ensures stable control and reliable alarm settings.

Another important consideration is response time. Low-pressure tanks may experience gradual pressure changes, but sudden filling or rapid temperature shifts can create quick spikes. The transmitter should respond quickly enough to capture these fluctuations so operators can see pressure trends clearly.

Installation plays a role too. Mount the transmitter where it can measure vapour space pressure directly without long impulse lines that may introduce lag or condensation. In low-pressure systems, even small line restrictions can distort readings.

Temperature stability is another factor. Because pressure values are small, thermal effects on the sensing element can influence accuracy. Choosing a transmitter with good temperature compensation improves long-term performance.

It’s also wise to align the transmitter range with the relief characteristics of the Tank vents. If vents open at a specific pressure, the transmitter should provide clear resolution leading up to that setpoint. This allows operators to identify whether the vent is opening frequently, partially restricted, or not responding at the correct threshold.

Signal output is typically 4–20mA, but ensure scaling in the control system matches the actual pressure span selected. Incorrect scaling can make low-pressure variations appear insignificant or exaggerated.

Finally, avoid oversizing “just to be safe.” In low-pressure tank applications, oversizing reduces the very sensitivity you need. A correctly sized Pressure transmitter provides meaningful data, supports blanketing control, and works alongside Tank vents to maintain safe operating conditions.

Low-pressure systems demand precision. When the transmitter range matches real operating conditions, you gain accurate monitoring, early warning of vent issues, and smoother overall tank operation without unnecessary false alarms.