What is a Submersible Pressure Transmitter and How Does it Work?

A submersible pressure transmitter is a pressure sensor designed to operate while fully immersed in a liquid, where it measures hydrostatic pressure to determine liquid level or water depth. These instruments are commonly used in wells, tanks, water treatment facilities, and environmental monitoring applications where accurate level measurement is critical.

The Environmental Protection Agency (EPA) explains how groundwater supplies drinking water for half of the U.S. population and supports agriculture, industrial processes, and municipal infrastructure. Monitoring water levels is an important part of protecting and managing this resource. 

As a leader in pressure and temperature instrumentation, Ashcroft understands the challenges associated with water level monitoring. In this article, you will learn how submersible pressure transmitters work, how they measure water depth, common applications where they are used, and important considerations for selecting the right instrument.

What is a submersible pressure transmitter?

These compact devices are used for water level management because they can accurately measure the hydrostatic pressure created by the liquid above them while operating fully submerged.

Designed for continuous underwater operation, these transmitters feature a fully sealed construction, typically rated IP68/NEMA 6P, enabling reliable operation in wells, tanks, reservoirs, and water treatment facilities. The transmitter measures the pressure exerted by the liquid above it and converts that measurement into an electrical signal that can be monitored by operators or control systems.

By measuring hydrostatic pressure, a submersible pressure transmitter provides accurate information about water level, helping operators manage resources, monitor system performance, and detect changing conditions.

Figure 1. Ashcroft® SL17 submersible pressure transmitter

How does this transmitter measure water depth?

Submersible pressure transmitters operate underwater using a sealed sensing element, protective diaphragm, and vented cable assembly that work together to measure liquid level accurately.

Near the nose cone of the transmitter is a waterproof sensing element protected by a diaphragm. As the height of the liquid increases, pressure on the diaphragm increases proportionally. The sensor converts this pressure into an electrical output signal.

Most submersible transmitters used for water level measurement include a vent tube within the cable assembly. This vent tube references the sensor to atmospheric pressure at the cable termination point, allowing the transmitter to measure only the hydrostatic pressure generated by the liquid column above it.

How is the pressure measurement used?

Water depth is determined by comparing the installed depth of the transmitter to the height of the water column measured above it.

The submersible cable is typically marked at regular intervals, allowing installers to determine the exact depth of the transmitter below ground level. Once installed, the transmitter measures the height of the water above the sensing element.

The calculation is straightforward:

Water Depth = Submersible Depth – Measured Water Height

This measurement method provides operators with reliable information about water levels in wells, aquifers, and other water management systems.

Figure 2. How water depth is measured

How are submersible pressure transmitters used in production wells?

In production wells, submersible pressure transmitters are installed above the pump to continuously monitor water depth.

These applications often present space constraints because the transmitter may need to fit inside PVC pipes as small as one inch in diameter. Selecting a transmitter designed for these conditions can simplify installation and improve long-term performance. The following features can help: 

• Narrow-profile. A compact design allows the transmitter to pass through tight well casings and reduces the risk of becoming stuck during installation.
  
  

• Weighted nose cone. Additional weight helps guide the transmitter through narrow pipes while minimizing cable tangling. Depending on the design, the nose cone may be threaded or welded to the transmitter body.
  
  

• Custom-length polyurethane cable. Polyurethane cable reinforced with Kevlar helps resist stretching over time. Excessive cable stretch can affect the installed depth reference and ultimately impact measurement accuracy.
  
  

• Weatherproof termination box. A weatherproof junction or termination box protects the vented cable from moisture ingress that could negatively affect measurement accuracy.

Figure 3. Water depth measurement in a production well

What challenges can affect submersible pressure transmitter performance?

Pumps and control systems can be damaged by lightning-induced surges and electrical noise. Adding surge protection to the termination box can help protect the transmitter from induced electrical events, with some solutions capable of handling energy pulses up to 500 V or 250 A. However, no instrument can withstand a direct lightning strike.

Electromagnetic compatibility (EMC) protection and shielded cable designs can also help reduce electrical noise from pumps and other equipment, improving signal integrity and system reliability. Some submersible transmitters, such as the Ashcroft® SL17, offer built-in EMC protection and shielded cable designs to help prevent electrical noise from affecting the control system.

How are submersible pressure transmitters used in monitoring wells?

Monitoring wells use submersible pressure transmitters to track groundwater levels and monitor changes in the water table over time.

When a production well pump is operating, a drawdown area, often called a cone of depression, forms around the pump. Monitoring this condition helps operators understand aquifer behavior and avoid situations where the well could run dry.

A monitoring well is typically installed near the production well and equipped with a submersible transmitter to continuously track water table depth. Because these measurements are often collected over long periods, transmitter accuracy and long-term stability are especially important.

For long-term groundwater monitoring, Ashcroft recommends a transmitter with 0.25% of span, terminal point accuracy, and 0.25% full-scale stability per year to support reliable long-term measurements.

Figure 4. Water depth measurement in a monitoring well

How are submersible pressure transmitters used for vented tank level measurement?

Submersible pressure transmitters provide reliable level measurement in vented tanks, including installations located in pits or low-lying areas that may be susceptible to flooding.

In these applications, the transmitter can be mounted through a tank flange near the bottom of the vessel using an NPT adapter in place of the standard nose cone. Because the sensing element remains submerged, water that accumulates around the tank does not affect measurement performance.

The water in the pit will not affect the output or condition of the transmitter. Variations in atmospheric pressure also will not interfere with the transmitter's measurements because the vented design automatically compensates for atmospheric changes.

Figure 5. Vented tank in an open pit

Why is a desiccant filter important?

A desiccant filter keeps moisture from entering the vent tube, helping maintain accurate measurements.

The vent tube ends in a weatherproof junction box, where it connects to a desiccant filter. This filter stops moisture from entering the vented cable and affecting the pressure reading. The desiccant is blue when new and turns pink when it needs to be replaced.

Maintaining the desiccant filter is an important part of ensuring accurate long-term level measurement.

How submersible pressure transmitters monitor leachate in landfills

Submersible pressure transmitters help landfill operators monitor leachate levels and detect potential liner failures.

Landfills use protective liners to prevent leachate from contaminating the surrounding soil and groundwater. In these applications, two submersible transmitters are often used:

1. One transmitter monitors leachate levels inside the liner to support pumping and treatment operations.
2. A second transmitter monitors groundwater levels outside the liner. Sudden changes in water level may indicate a liner breach.

Because landfill leachate can contain aggressive contaminants, material compatibility is an important consideration.

What materials are recommended for leachate applications?

Corrosion-resistant materials help improve transmitter reliability in landfill environments.

For many landfill monitoring applications, a transmitter with a 316L stainless steel body and polyurethane cable provides the durability needed to withstand challenging environmental conditions. Material compatibility should always be verified for the specific process media and operating environment.

Figure 6. Leachate monitoring in a landfill.

Ready to learn more?

Now that you know what a submersible pressure transmitter is, how it works, and the common applications where you will find them, you may have more questions. Contact us to speak to a product specialist or review the related content below. In the meantime, feel free to download our guide to find the right pressure and temperature instruments for your water applications.