During the rainy season, condensation in outdoor equipment control cabinets can interfere with vacuum sensors at multiple levels—including signal acquisition, signal transmission, the sensor body, and pressure-transmitting auxiliary circuits. This poses more than just a risk of short circuits; many failures manifest as reading drift and response inaccuracies. These failures are intermittent, making troubleshooting extremely difficult.

When condensation droplets adhere to the surfaces of the vacuum sensor’s circuit board and signal conditioning components, they alter the surface insulation resistance of the circuit board, creating minute leakage paths. The 4–20 mA standard analog signal carries a very weak current; even a slight leakage current can directly cause signal offset. As a result, the vacuum values received by the control cabinet drift continuously, often resulting in vacuum readings that fluctuate wildly—from high to low. Although the actual negative pressure in the chamber remains largely unchanged, the automatic control system detects these vacuum fluctuations and continuously adjusts the heating and pumping loads. Minor condensation does not directly trigger a short-circuit trip; rather, it causes persistent reading distortion. In the early stages, this is easily misdiagnosed on-site as a vacuum pump failure or a leak in the manhole gasket.

The hazards of condensation in the terminal block area are particularly severe. Moisture causes oxidation of the terminal blocks, resulting in contact resistance. This unstable contact resistance acts as a variable interference superimposed on the signal line, introducing noise into the sensor’s transmitted signal. The PLC frequently detects abnormal transient values, which can easily trigger vacuum fault alarms. In some units, this may trigger interlocked reductions in evaporation power or automatic vacuum break protection, leading to production interruptions. Long-term exposure to moisture and oxidation can also cause terminal corrosion, eventually resulting in a complete signal interruption.

Many vacuum sensors are equipped with external pressure-tapping hoses and connectors. If condensation and water accumulate at the connector locations, moisture slowly seeps into the sensor’s diaphragm chamber through the pressure-tapping lines. The core sensing diaphragm of a vacuum sensor is extremely precise; prolonged contact with accumulated water can cause zero-point drift. Even if the housing is dried later, the sensor’s zero point is unlikely to recover automatically, and routine zero-point calibration will fail again shortly thereafter. Once condensation accumulates in the pressure-transmitting tubing, it can form a liquid seal, blocking the transfer of negative pressure between the evaporation chamber and the sensor. The sensor will then continuously display atmospheric pressure readings, and the system will lose its vacuum monitoring capability.

Temperature fluctuations inside the chamber cause condensation to repeatedly evaporate and condense. The moist air continuously erodes the sensor’s internal precision electronic components, accelerating the aging of internal resistors and capacitors. The greater the day-night temperature difference during the rainy season, the more frequently condensation forms, resulting in a pattern where malfunctions improve during the day but worsen in the early morning and at night. At the same time, the humid environment accelerates the clogging of the sensor housing’s vent valve; moisture inside the sensor cannot be expelled, and water vapor remains trapped within the sensor body for extended periods, leading to increasingly frequent malfunctions.

In addition to the sensor itself, moisture in the PLC analog modules within the electrical control cabinet can amplify interference with the vacuum signal. This can result in a situation where the sensor itself is intact, but the data interpreted by the control cabinet remains consistently abnormal, further complicating troubleshooting. The erratic vacuum readings caused by condensation can also lead to secondary process issues: the system adjusts operating conditions based on incorrect vacuum parameters, causing evaporation temperatures to spiral out of control and simultaneously increasing the likelihood of foam entrainment and scaling on heat exchange surfaces.

Recommended improvements include: installing a constant-temperature dehumidifying heater paired with a humidity controller in the electrical control cabinet to prevent condensation caused by temperature differences; properly sealing the cable entry and exit points of the cabinet to block the continuous inflow of high-humidity air; positioning the vacuum sensor as high as possible within the cabinet, away from areas where water accumulates at the bottom; adding condensate drain fittings to the vacuum tubing to periodically drain accumulated moisture; and replacing standard, economy-class sensors with vacuum transmitters that meet moisture-proof sealing standards.