A configuration guide for hydrological monitoring, RTU/PLC integration and SCADA data handling

A radar level sensor can report a technically correct distance while the monitoring platform displays the wrong water level. This is not necessarily a sensor fault. It often happens because distance, depth, stage and elevation have been treated as if they were the same measurement.

For hydrological monitoring, the radar directly observes distance from a manufacturer-defined reference plane to the water surface. Converting it into level requires an installation elevation, zero datum and documented calculation.

Five Terms That Must Not Be Interchanged

Radar distance

Radar distance is the separation between the radar’s measurement reference plane and the detected water surface. That plane is product-specific and may not be the enclosure top or mounting bracket. Confirm it in the applicable product documentation.

Air gap

Air gap, sometimes called ullage, is the open distance to the water surface. It may equal the reported radar distance after internal compensation. As water rises, air gap decreases.

Water depth

Water depth is the vertical distance between the water surface and a defined lower reference, such as a tank bottom or surveyed channel bed. A radar cannot determine water depth from air gap alone unless the position of that lower reference is known.

Stage

Stage is the water-surface height relative to an adopted local datum, commonly the zero of a staff gauge or a project-specific reference. Stage is not automatically the same as water depth. A channel bed can erode or accumulate sediment while the staff-gauge datum remains unchanged.

Water-surface elevation

Water-surface elevation is the surface elevation relative to a surveyed vertical datum, allowing several stations to share one framework.

These quantities may all be valid at the same moment, but they answer different questions. A specification should state both unit and measurand: for example, “air gap in millimetres” or “stage in metres above gauge zero.”

Three Reference Values Engineers Must Define

1. Radar measurement reference plane

This is the manufacturer-defined origin from which the radar reports distance. Do not assume that two models use the same physical reference, even when they fit the same bracket.

2. Installation elevation

This is the surveyed elevation of the radar reference plane in the project’s vertical datum. A bracket or bridge-deck elevation is insufficient unless its separation from the radar reference plane is included.

3. Level zero or datum

This is the reference to which level is reported, such as a tank bottom, staff-gauge zero or channel invert. Record it as a datum rather than an unexplained software offset.

Converting Distance into the Required Level Value

Level above a tank or well bottom

Let:

• H = fixed distance from the radar reference plane to the selected bottom zero;
• D = radar-measured distance to the liquid surface;
• L = liquid level above the bottom zero.

Then:

L=H-D

Tank example

A radar’s reference plane is 8.000 m above the selected tank-bottom zero. The current radar distance is 2.350 m.

L=8.000-2.350=5.650 m

The correct tank level is 5.650 m. The value 2.350 m is still correct, but it is the air gap, not the liquid level.

Water-surface elevation

Let:

• E_s = surveyed elevation of the radar reference plane;
• D = radar-measured distance;
• E_w = water-surface elevation.

Then:

E_w=E_s-D

Stage relative to a staff-gauge zero

If E_0 is the surveyed elevation of the staff-gauge zero, stage S is:

S=(E_s-D)-E_0

River-station example

At a bridge-mounted station:

• radar reference-plane elevation E_s=112.680 m;
• current radar distance D=6.245 m;
• staff-gauge zero elevation E_0=104.500 m.

First calculate the water-surface elevation:

E_w=112.680-6.245=106.435 m

Then calculate stage:

S=106.435-104.500=1.935 m

At the same time, 6.245 m is the air gap, 106.435 m is the water-surface elevation and 1.935 m is the stage above gauge zero. None of these values should be labelled simply as “water level” without qualification.

Apply the Conversion in One Authoritative Place

A monitoring chain may include the radar, RTU, PLC, gateway and SCADA. Several devices may support scaling and offsets, creating a common failure: the same conversion is applied twice.

For example, a radar may output level after subtracting distance from a configured height. If the RTU repeats the subtraction and SCADA adds another datum offset, the result is wrong even though communication is working.

Choose one authoritative conversion point. The radar might output distance, the RTU calculate stage and SCADA display it without another offset. Alternatively, the radar may calculate level and pass it unchanged. Either approach works if documented.

The configuration record should identify:

• the radar output quantity and unit;
• the reference-plane elevation;
• the selected zero-datum elevation;
• every scale factor and offset;
• the device in which each calculation occurs;
• the register address, data type and byte order;
• the displayed quantity and unit in SCADA.

Common Configuration Errors

Reversed sign or direction

Air gap decreases as water rises, while level normally increases. If the displayed value falls during a rising-water test, check whether the system is presenting distance as level or using the wrong subtraction order.

Unit mismatch

A register value representing millimetres may be interpreted as centimetres or metres. Store the raw unit, engineering unit and scaling expression in the commissioning record.

Register interpretation

Incorrect signed/unsigned selection, integer width, floating-point format, byte order or word order can create implausible values. Confirm these items using the exact communication protocol for the selected radar rather than copying settings from another model.

Reference, offset and replacement errors

Surveying a bracket or enclosure instead of the manufacturer-defined reference introduces a fixed bias. The same occurs when an offset is repeated in the radar, RTU and SCADA. A replacement sensor may also have a different reference plane, mounting thickness or output configuration. Review the full signal path after equipment, bracket, gauge or datum changes. Channel-bed change affects depth but does not automatically change stage relative to a fixed gauge datum.

Commissioning a New Monitoring Point

1. Define the required output. Specify whether the project needs distance, air gap, depth, stage or water-surface elevation.
2. Confirm the radar reference plane. Use the applicable product documentation; do not infer its position from the enclosure.
3. Survey the installation. Determine the elevation of that reference plane in the project’s vertical datum.
4. Define the zero. Record the tank bottom, gauge zero, channel invert or other adopted reference and its elevation.
5. Map the data chain. Record what the radar outputs and where scaling, subtraction and offsets will occur.
6. Check digital interpretation. Verify register address, unit, data type, byte order, valid range and exception handling against the communication protocol.
7. Compare independently. Use a staff gauge, surveyed observation or another suitable reference.
8. Verify at more than one level. A single point may hide a sign, scale or range error.
9. Test invalid-data handling. An error value must not become a plausible level.
10. Archive the configuration. Save elevations, equations, offsets, units, settings, photographs and date; repeat checks after movement, repair or replacement.

Conclusion

Radar distance is a direct measurement; water level is a defined engineering result. Reliable monitoring depends on knowing the radar reference plane, surveying its elevation, selecting the correct zero and applying the conversion once. Clear definitions and commissioning records prevent fixed offsets, reversed trends and disagreements between the field sensor and SCADA.

Frequently Asked Questions

1. Does a radar level sensor measure water level directly?

It directly measures distance to the detected surface from a defined reference plane. Some products or host systems can convert that distance into level, but the output mode must be confirmed in the relevant datasheet and protocol.

2. What is the reference point of a radar level sensor?

It is the manufacturer-defined measurement origin used for the reported distance. Its physical location is product-specific and should not be assumed from the housing shape.

3. Why does the displayed level move in the wrong direction?

The system may be displaying air gap as level or subtracting in the wrong direction. Review the measurand, formula and every conversion applied along the data chain.

4. Should the offset be configured in the radar or SCADA?

Either may be appropriate, depending on the system design. The important requirement is to select one authoritative conversion point and prevent the same offset from being applied elsewhere.

5. Must the reference point be checked after replacing a sensor?

Yes. Verify the new sensor’s reference plane, mounting position, output quantity and saved configuration before accepting the measurement.

Contact ZLYRADAR

If you are selecting or integrating a radar level sensor, contact ZLYRADAR. Share the required measurand, range, installation drawing, datum, interface and host-system requirements for product-selection and integration support.