A submersible probe measures the pressure of the liquid column above it. This pressure depends solely on the height—not on the width, not on the shape, and not on internal fixtures. This makes the method the most robust solution for tanks that do not have a simple geometry.
The Measuring Principle
The probe is suspended to the bottom of the tank and measures the hydrostatic pressure there. A one-meter column of water always generates the same pressure, regardless of whether the tank is narrow or wide. The display then converts the height into volume.
There are no moving parts. This is the second major advantage besides shape independence: nothing can jam, nothing wears out mechanically.
The measuring range extends up to 200 mbar with a resolution of 1 cm and a maximum tank height of 200 cm. The output signal is a 4–20 mA current signal using a two-wire principle—insensitive to long cables because the current remains constant over the entire distance.
The Air Hose in the Cable
A detail that determines the accuracy of the measurement: A thin air hose runs inside the PUR connection cable for rear pressure compensation.
The reason is simple. The probe measures the pressure of the liquid column—but the air pressure is also present on top of this column. Without compensation, every change in weather would appear as a change in level. The hose leads the ambient pressure to the rear of the measuring element, so that both components cancel each other out.
WarningThe cable end with the air hose must remain dry and must not be sealed or squeezed. A closed or damp end cancels out the pressure compensation—the probe then measures the weather as well.
TDS or TDN: The Media Question
The two versions differ in the approved media:
- TDS 200 — additionally approved for diesel, as well as water, wastewater, and sewage.
- TDN 200 — for water, wastewater, and sewage.
Both are not suitable for gasoline, kerosene, and petroleum, nor for explosion-hazard areas. This is not a recommendation, but a limit—here it must be checked which solution is permissible.
The probe housing is made of stainless steel 1.4404, and the submersible cable is made of oil-resistant PUR, 2.5 m long. The operating temperature range is 0–40 °C—the point where one should look more closely for outdoor tanks in cold regions.
Installation
A mounting flange is included in the scope of delivery. The probe is suspended through the tank ceiling until it reaches the bottom—where it remains.
Three points deserve attention:
- The probe should rest steadily. In a place where it is constantly moved by incoming water or withdrawal, the measured value will fluctuate.
- The cable needs strain relief above the feedthrough. The weight of the probe must not permanently hang on the seal.
- The height of the probe above the bottom is the zero point of the measurement. If it rests on a deposit, it measures too little.
Connection to the Display
The operating voltage of 10–30 V DC comes via the measuring line—a separate supply cable is not necessary.
The probe works directly with common tank monitors and in the P-Bus system via the tank interface, which recognizes 4–20 mA as a sensor type. If, however, analog round instruments are present, the universal tank interface converts the current signal into a resistance or voltage signal—then the existing display can be retained.
Typical Errors
- Air hose sealed or squeezed. The probe then also measures changes in air pressure.
- Wrong version for diesel. Only one of the two is approved for it.
- Used in gasoline. Both are explicitly not approved for this.
- Probe hangs freely instead of on the bottom. Then the lower part of the measuring range is missing.
- Cable without strain relief. Otherwise, the weight of the probe hangs permanently on the feedthrough.
