Inside an AWLR and AWVLR station: interfaces, resolution, power, protection and data integrity

If you work with rivers, dams or flood forecasting in India, you will have seen the abbreviation AWLR, an automatic river water level recorder. It is one of the basic building blocks of any river monitoring network. Its close relative, the AWVLR, adds the speed of the water. Most descriptions stop at "a sensor plus a logger plus a modem". This article opens the box: how the parts are connected, what that connection does to accuracy, and what keeps a station trustworthy for years.

What the station does

An AWLR measures the water level at a river, canal, reservoir or drain, records it, and sends it to a central system, automatically and without anyone visiting the site. An AWVLR also records the surface velocity. Level tells you how much water there is. Velocity tells you how quickly it is moving, and so how soon a rise upstream will reach someone downstream, as explained in why surface velocity matters as much as water level.

The anatomy of a station

  mounting structure ── level sensor ──┐
                        velocity sensor ──┤  4-20 mA / SDI-12 / RS-485
                                          ▼
 solar panel ─► charge controller ─► battery ─► DATA LOGGER ─► modem + antenna ─► server
                                          ▲       (clock, memory, alarms)
                  staff gauge / benchmark (the reference you check against)

The Central Water and Power Research Station runs a demonstration facility for surface water level sensors where radar, ultrasonic and shaft-encoder types can be seen side by side.

How the sensor talks to the logger

The interface between sensor and logger is where many stations lose accuracy without anyone noticing. Four are common:

4-20 mA: simple, but it spends your millimetres

A transmitter sends a current between 4 and 20 mA that represents the measuring range. The "live zero" at 4 mA lets the system tell a zero reading from a broken wire, and the NAMUR NE 43 recommendation treats currents below about 3.6 mA or above 21 mA as a fault, with the normal range extended to about 3.8 to 20.5 mA [1]. The logger reads the current through a precision resistor with an analogue-to-digital converter (ADC). The weakness is arithmetic. The 16 mA span is spread across the whole range, so the smaller your measuring resolution, the more it costs:

sensitivity = 16 mA / range     e.g.  0 to 50 m  →  0.32 µA per mm
resolution  = ADC step / sensitivity

And resolution is only the start. Gain error, in the shunt resistor and the converter, scales the whole reading. An error of 0.1 % of span on a 50 m range is 50 mm, and 0.01 % is 5 mm. A sensor specified at ±1 mm gives that only if the entire loop, the sensor's output stage, the cable, the shunt and the ADC, is accurate to about 0.002 % of span and calibrated end to end. Almost no field loop is.

What to do. If the sensor offers a digital output, use it for the level: the value arrives as the sensor computed it, with no conversion. If you must use 4-20 mA, reduce the configured range to what the site needs (a river that never exceeds 12 m does not need a 0 to 50 m span), use a 16-bit input with a precision shunt, and calibrate the whole chain against the staff gauge.

SDI-12: designed for this job

SDI-12 is a serial standard created for environmental sensors and low-power data recorders. It runs at 1,200 baud, with 7 data bits and even parity, on three wires: data, ground and a 12 V line. A bus can carry at least ten sensors, each with about 200 feet (60 m) of cable, and shorter runs allow more sensors on a longer bus [2]. The logger addresses a sensor, asks it to measure, and then reads the result as text with a sign and decimal point. A typical exchange is:

0M!      → 00011        sensor 0 will have 1 value ready in 001 s (format: address, seconds, count)
0D0!     → 0+12.345     the value, as the sensor computed it

The result is a number, not a voltage, so the converter errors of a 4-20 mA loop do not exist. Version 1.4 of the standard also adds commands that return metadata such as the parameter name and units [2].

RS-485 and Modbus RTU: long runs, many devices

RS-485 sends data as a difference between two wires, which rejects noise. The commonly quoted limits are a cable length of up to 1,200 m (at 100 kbit/s), up to 32 devices on a bus, shielded twisted-pair cable of about 120 ohm impedance with a termination resistor at each end [3]. On top of it, Modbus RTU frames a request as an address (1 byte), a function code (1 byte), data, and a 16-bit CRC (2 bytes), and uses a silent gap of at least 3.5 character times to mark the start and end of a frame [4]. Two practical traps: the same register may hold an integer in millimetres or a floating-point value in metres, and 32-bit values can be sent with either word order. Always confirm both against the sensor's register map and test with a known value.

From distance to stage: the datum

A radar level sensor reports the distance to the water. The station's job is to convert that into stage relative to a defined zero:

stage = H_mount − d

where H_mount is the height of the sensor reference above the gauge zero (or a national datum), surveyed to the millimetre. For example, with a surveyed mounting height of 14.327 m and a measured distance of 3.852 m, the stage is 10.475 m. A mistake in H_mount shifts every reading by the same amount, and no amount of sensor accuracy can fix it. Record the mounting height, the datum and its source, the date of the survey and each later check, in the station's metadata, and keep the history. A level reading without its datum is a number without a meaning.

Protection: moisture and lightning

Two causes account for many field failures:

• Moisture and condensation. Practitioners' guidance identifies moisture as the largest single cause of data-logger failure, because most loggers are not designed for direct exposure to water or condensation [5]. Use a sealed, weatherproof enclosure, cable glands that are tightened and pointing down, a desiccant, and a drip loop in every cable.
• Lightning and surges. Sensors, cables and antennas can all be hit by a direct strike or by voltage induced in the cables. Protection means giving a strike a low-resistance path that bypasses the electronics, typically a lightning rod above all sensors and antennas, and a good local earth connected in a star pattern, with surge protection at each cable entry [6]. Hilltops, bridges and masts attract strikes, which is where river stations often are.

An AWVLR adds a second sensor, not just a second number

A velocity sensor adds power draw, a second interface, and a second set of calibrations. A Pravaah unit, for example, pairs the level radar with a 24 GHz Doppler velocity radar, outputs 4-20 mA, RS-485, RS-232 and MQTT, and speaks Modbus RTU and ASCII, with average consumption under 2.5 W, powered by a 40 to 60 W solar panel and a battery, with over-voltage, under-voltage and short-circuit protection. The velocity reading should travel with a quality flag, because radar velocity data is noisier than level data, as discussed in surface velocity radar for rivers.

Specifying an AWLR or AWVLR: a checklist

1. Range and accuracy. Does the sensor cover the full range between low flow and the highest flood you expect, and is the installed accuracy right for the use? Our radar sensors measure 0 to 50 m (customisable) with ±1 mm accuracy.
2. Interface. Choose a digital output where you can. If you use 4-20 mA, use a narrow span and a 16-bit input.
3. Datum and metadata. Survey the mounting height, record the datum, and keep a reference gauge.
4. Telemetry. Is there mobile coverage at the site, or does it need LoRa or satellite?
5. Power. A power budget, not just the words "solar powered".
6. Environment. The temperature range and a weatherproof enclosure. Ours run from -40 °C to +70 °C.
7. Protection. Earthing, surge protection and moisture control.
8. Mounting. A rigid structure above the highest flood.
9. Maintenance. Who will visit it, how often, and what happens during a flood when nobody can?

How ClimMaTech fits

• Tarang is a 61 GHz radar level sensor, the sensor part of an AWLR, with 4-20 mA, RS-485, RS-232, SDI-12, LoRa and MQTT outputs.
• Pravaah adds a 24 GHz velocity radar, making it the sensor part of an AWVLR.
• A data logger with SDI-12, RS-485 and 4-20 mA inputs and GSM telemetry is coming soon, to complete the station. Until then, our sensors connect to the logger you already use.
• Readings can also go straight to the ClimMaTech cloud and to DeviceCloud.

To see how these stations combine into a full river network, read about our river monitoring solution, and RTDAS explained.

References

1. NAMUR NE 43 fault signal levels for 4-20 mA transmitters, as described in 4-20 mA signal lost threshold configuration guide. industrialmonitordirect.com.
2. SDI-12 Support Group, SDI-12: A Serial-Digital Interface Standard for Microprocessor-Based Sensors, version 1.4. sdi-12.org, and the overview at Wikipedia.
3. TIA/EIA-485 limits (cable length, data rate, unit loads), as discussed in the Texas Instruments E2E interface forum. e2e.ti.com.
4. Modbus Organization, Modbus over serial line specification and implementation guide, RTU framing (see also Kollmorgen's summary).
5. Data Logger Inc., Data logger deployment guidance. dataloggerinc.com.
6. How to cope with lightning (protection of monitoring systems). eprints.dkit.ie.

Worked values (current-loop sensitivity, ADC resolution, gain-error examples and the stage conversion) are computed from the stated relations. They illustrate the size of each effect and are not a specification of any product.

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