Radar and ultrasonic level transmitter mounting position
A radar or ultrasonic level transmitter measures the echo of a cone-shaped beam. If that cone touches the shell before reaching the bottom, the wall sends back a false echo and the measurement loses the lower part of the tank. This page shows the minimum distance from the wall, the sloped bottom and the silo.
When to use
When locating the nozzle of a non-contact level transmitter on a new tank, when checking whether an existing nozzle suits the chosen sensor, and when choosing between a wide-beam and a narrow-beam sensor.
Why the nozzle position matters
Radar and ultrasonic transmitters measure the round-trip time of an echo. The beam leaves as a cone and widens with depth. While the cone stays clear of the wall, the only strong echo is the product surface. Once it touches the shell, the wall returns an echo too — and in the lower part of the tank that false echo can be stronger than the product’s.
Losing the bottom of the range is not cosmetic. The low-level alarm and the pump trip live there, and the suction tank level is part of the static head in any pump sizing: the tank levels lesson shows how that level moves the pump’s operating point, and the NPSH and cavitation lesson why a low suction level eats into the NPSH margin.
The calculation
The check is geometric. The beam radius at depth d is the antenna radius plus d times the tangent of the half angle. In a flat-bottom tank the worst point is the bottom: the minimum distance from the wall is the antenna radius plus the inside height times the tangent of the half angle. The tool runs the same check in every direction around the axis, so it also holds for the corner of a rectangular tank and for the tilted sensor of a silo.
When no nozzle position works — a wide beam in a tall, narrow vessel, for instance — a contact measurement avoids the beam problem altogether. A differential pressure level transmitter has no cone to keep off the wall, but brings its own error budget, expressed in millimetres of level.
Sloped bottom and silo
In a settling tank with a sloped bottom the sensor stays vertical. Away from the low side the beam still clears the wall, but the level below the bottom under the sensor is no longer measured. In a solids silo the sensor is aimed at the centre of the outlet so that the beam reaches the cone all the way down. If the transmitter cannot reach that tilt, a tilted flange is recommended to accommodate the sensor.
In a rectangular silo with the bottom sloped to the edge, the outlet sits at the foot of the low-side wall. Aiming at its centre usually makes the beam touch that wall; the tool aims the sensor at the point of the outlet closest to the wall at which the beam clears it, and the nozzle can be on the low or the high side.
Formulas and fundamentals
r(d) = r_ant + d · tan(α/2) α is the full −3 dB beam angle from the sensor data sheet. The beam starts at the antenna rim, so the antenna radius adds directly.
d_min = r_ant + (L + H_in) · tan(α/2) + m H_in is the inside depth from roof to bottom and L is the neck plus the roof thickness — the antenna sits at the top of the neck (150 mm by default on flanges, 50 mm on threads). m is the safety margin between the beam edge and the wall (50 mm by default, adjustable). Any position further in also complies.
θ = atan[e / (H_cyl + h_cone)] In a conical silo the sensor is aimed at the centre of the outlet; e is the horizontal distance from the nozzle to the silo axis. If the transmitter cannot tilt that far, a tilted flange takes the difference.
h = x · tan φ ; V = W · h² / (2 · tan φ) With a vertical sensor at x from the low side, the level below h is not measured. W is the width and φ the bottom slope.
Standards & methods
- No standard fixes the distance from the wall — geometric criterion on the manufacturer's data-sheet beam angle (−3 dB)
Typical reference values
| Quantity | Typical range | Note |
|---|---|---|
| Beam angle | from the sensor data sheet, at −3 dB | high-frequency radar has a narrow beam; ultrasonic opens wider |
| Mounting in liquids | vertical sensor | — |
| Mounting in solids silos | sensor aimed at the centre of the outlet | — |
Worked example
Flat-bottom cylindrical tank
Inputs
- Tank
- 3,000 × 6,000 mm (Ø × height, outside)
- Wall
- 10 mm
- Beam angle
- 8 ° (−3 dB)
- Antenna
- 100 mm
- Connection
- 4" flange, 150 mm neck
Results
- Minimum distance from the wall
- 539.4 mm (outside face)
- Nozzle at 600 mm — clearance
- 110.6 mm
- Nozzle at 300 mm — clearance
- -189.4 mm (touches the shell)
Any nozzle from 539.4 mm from the outside face complies. The nozzle at 600 mm clears with 110.6 mm; at 300 mm the beam touches the shell before the bottom, and the sensor would measure the wall echo in the lower part of the tank.
Common mistakes
- Measuring from the nozzle radius and forgetting the antenna. The beam starts at the antenna rim; with a 100 mm antenna that is 50 mm more in any tank.
- Using the beam angle of another model or frequency. The minimum distance grows with the tangent of the half angle: in a 6 m tank, going from 8° to 9° moves the nozzle 54 mm away from the wall.
- Placing the nozzle over the high side of a settling tank. The beam clears the wall, but the level below the bottom under the sensor is not measured: in a 12 m settling tank with a 10° bottom, the blind level goes from 86 mm on the low side to more than 2 m on the high side.
- Mounting the sensor vertically in a silo. The beam hits the cone wall instead of the product near the outlet; aiming at the centre of the outlet solves it.
Frequently asked questions
What is the minimum distance of a radar level transmitter from the wall?
It depends on the beam angle, the antenna and the tank height: the antenna radius plus the inside height times the tangent of the half angle. A narrow-beam radar can sit close to the wall; a wide-beam ultrasonic cannot.
Can the radar sit at the centre of the tank?
By beam geometry, the centre is the position with the most clearance. Other manufacturer recommendations — such as avoiding the centre of a domed roof — belong to the sensor installation sheet.
Why is the sensor tilted in a silo?
In a silo the product leaves through the cone and the surface is not horizontal. Aiming at the centre of the outlet lets the beam reach the cone all the way down instead of hitting the cone wall.
Does the nozzle neck disturb the measurement?
Inside the neck, no: it lies within the sensor dead band. But since the antenna sits at the top of the neck, the beam enters the tank already open, and the minimum distance from the wall grows with the neck length.
Is there a standard for this?
No standard fixes the distance. The check is geometric, on the beam angle the manufacturer publishes in the data sheet.
Glossary
- Beam angle
- Full angle of the measuring cone, taken at −3 dB of the maximum intensity.
- Shell
- The vertical wall of the tank.
- Blind level
- The lower part of the tank the sensor cannot see because of its position.