Hydrostatic level monitoring is a method to measure liquid levels in tanks, reservoirs, dams, or any other water by a submersible vented gauge level sensor. The readings from the sensor could be obtained by a data logger and transmitted to a central server where the monitored level could be analyzed and visualized. If there is an alarm condition level is getting low/high the level monitoring system generates events, alarms, and notifications.

How does hydrostatic level monitoring work?

Hydrostatics is the science of fluids not in motion. A hydrostatic level sensor is a form of a level probe that is used especially for level monitoring by measuring the hydrostatic pressure in a virtually static liquid at a pre-determined level of submersion. 

A hydrostatic level sensor is a submersible pressure transmitter that has a pressure diaphragm. The inner side of the diaphragm is vented to atmospheric pressure through a vent tube in the cable. The outer side is in contact with the liquid. It is used for measuring the static pressure of the liquid column above the transmitter. The weight of the fluid causes static pressure to the outer side of the pressure diaphragm. The sensor uses that pressure difference to calculate the actual level of the liquid.

Hydrostatic level monitoring in use

In the photo above the hydrostatic level sensor is placed in a casing pipe sunk almost to the bottom of the dam to protect the sensor from the icebreaker during the winter. The level monitoring has been done by a temperature-compensated submersible level sensor with an accuracy 0.01 of FSO and ThingsLog data logger. The level is measured every minute and data is transmitted to the ThingsLog RMAAS (Remote monitoring as a service) level monitoring system through a mobile network over the air.

The simplicity of using hydrostatic level methods makes it the instrument of choice. Wherever level needs to be measured in a vented tank, vessel, lake, river, or reservoir hydrostatic method is the implement and most accurate one. In most cases, it is sunk to the desired level by the cable to the desired level of measurement or just submerged and allowed to sink to the bottom of the reservoir. It does not matter what the structure of the tank, basin, or natural geometry is. It works equally well if there are obstructions, ledges, or complex shapes, a hydrostatic level sensor will always measure the pressure of the liquid unaffected by such disturbances.

As the hydrostatic level sensor commonly is a gauge pressure sensor it automatically compensates for the atmospheric pressure changes, so the pressure measured represents only the pressure of the liquid caused by the liquid’s specific gravity and the vertical distance to the surface. Therefore, the hydrostatic level sensor accurately measures the level of the liquid by calculating the distance from the measuring point to the surface level via the measured pressure. 

Hydrostatic level sensor outputs

The most common output type for hydrostatic level monitoring sensors is a current output of 4-20mA. That allows measurements of level depths of up to 200m without signal regeneration.

The second most common output type is digital serial RS485 outputs for extreme accuracy. Those are used by sensors having (Analog-digital converters) inside and also RS485 output. Typically on top of the RS485 the sensor “speak” protocol such as Modbus or SDL-12.

Hydrostatic level monitoring data loggers

Hydrostatic level monitoring data logger has to have standard input ports – 4-20mA or RS485 and also protected vented cap so that the level sensor be able to compensate for the atmospheric pressure fluctuations.

Conclusion

The hydrostatic level sensor is a specialized sensor for level monitoring, most commonly used in the water and wastewater industry. Its simple use and reliable and accurate measurement technology make it one of the most commonly used level monitoring methods by far and the most appropriate for level monitoring and alerting of dams, tanks, reservoirs, and others.

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