When measuring the liquid level in a tank that is open to the atmosphere, how does a hydrostatic pressure sensor compensate for changes in barometric (air) pressure?
To accurately measure the liquid level (hydrostatic head) in a vented tank, the most common way with a pressure sensor, is to utilise one that has a gauge pressure reference. This ensures it only measures the pressure exerted by the liquid column, automatically subtracting the ambient atmospheric pressure. If an absolute sensor were used, any change in barometric pressure (e.g., from a passing weather system) would be incorrectly registered as a change in the tank level.
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This atmospheric compensation is achieved by ensuring the “dry” or reference side of the sensing diaphragm is exposed to the ambient air. This is accomplished by integrating a small ventilation tube within the electrical cable. This tube runs the length of the cable, from the sensor body to the cable’s termination point (e.g., inside a control cabinet). This design ensures the reference side of the diaphragm is always at the same atmospheric pressure as the liquid’s surface, so the sensor’s output signal corresponds precisely to the true liquid level.
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