Zero elevation and zero suppression is the term where many people get confused when working with tank level measurements.
These adjustments are needed when measuring liquid level, especially if the transmitter can not be placed at the same height as the bottom of the tank.
The basic definitions are given below, but the easiest way to understand zero elevation and suppression is to look at them like a math problem, as explained below.
What is Zero Elevation?
Zero Elevation is used when the transmitter (or LP remote seal) is located above the tank’s zero level.
Since the transmitter is higher, it senses less pressure at the zero level.
We must elevate the zero point in the transmitter calibration to account for this offset.
It ensures 4 mA output corresponds to true 0% level.
Zero Elevation can be expressed in units (e.g., mmH₂O, inches) or as a percentage of the transmitter span.
What is Zero Suppression?
Zero Suppression is when the transmitter (or LP remote seal) is located below the tank’s zero level
It senses extra hydrostatic pressure even when the tank is empty.
We must suppress the zero in the calibration to prevent false positive level readings.
Zero Suppression can also be expressed in units or percentage of span.
Let us take an example
Let us assume:
A vented tank 10 ft high.
A DP transmitter with diaphragm seal mounted 5 ft below the tank’s bottom (i.e., 0 level is above the transmitter).
The full level of the tank = 10 ft.
Zero Elevation Example:
The transmitter is mounted 5 ft above the bottom tap of the tank.
The transmitter sees less pressure when the tank is empty, because it is elevated.
We must apply zero elevation so that the transmitter gives 4 mA at actual 0% level.
Calibration:
When tank is empty → transmitter reads 0 ft
When tank is full → transmitter reads only 5 ft (since it’s 5 ft above bottom tap)
So:
LRV = 0 ft
URV = 5 ft
Span = 5 ft
Transmitter Output:
0 ft → 4 mA (empty tank)
5 ft → 20 mA (full tank)
This is Zero Elevation of 5 ft.
Zero Suppression Example
The transmitter is mounted 5 ft below the bottom of the tank.
The tank level ranges from 0 ft (empty) to 10 ft (full).
Since the transmitter is 5 ft below the zero level, it reads 5 ft of liquid head even when the tank is empty.
Therefore, we must apply zero suppression.
Calibration:
LRV = 5 ft (pressure when tank is empty)
URV = 15 ft (pressure when tank is full)
Span = 10 ft
Transmitter Output:
5 ft → 4 mA (empty tank)
15 ft → 20 mA (full tank)
👉 This is Zero Suppression of 5 ft.
Summary
This article explains the commonly misunderstood concepts of zero elevation and zero suppression in level measurement using transmitters.
These terms are important when the transmitter cannot be mounted at the same level as the bottom (zero point) of the tank. The article highlights why such adjustments are needed and how they affect the measurement.
The easiest way to understand these concepts is by using a mathematical approach, which helps clarify how elevation and suppression influence the transmitter’s output.
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