Field telemetry and data logging

What a field logger has to do

A logger reads sensors on a schedule, stamps each reading with an accurate time, stores it and sends it when the link allows. Each of those steps can fail. Clocks drift, so we sync time from GNSS or the network. Links drop, so readings queue in local flash and go out in order when the connection returns. Sensors fail quietly, so the logger reports its own health with every message: battery voltage, enclosure temperature, signal quality and sensor status.

We separate the sample interval from the report interval. A soil probe might be read every 15 minutes and reported hourly in one batch, which costs far less energy than sending each reading as it is taken.

Sensors and interfaces

Most field instruments speak one of a handful of interfaces. Where we can, we log what is already installed rather than replace sensors that work.

Power budgets that survive winter

Battery life is arithmetic. We measure current in each state (sleep, sensor warm-up, sampling and radio transmit), multiply by the time spent in each, and get an average draw. A well-designed LoRa logger sleeps at microamps and wakes for milliseconds to seconds at a time, so sleep current and sensor warm-up often matter more than the radio.

For solar sites, we size the panel and battery for the shortest, cloudiest weeks at the location, using Bureau of Meteorology solar exposure data rather than annual averages. We also allow for days with no charge at all, dust on the panel and battery ageing.

Battery chemistry changes the design:

• LiFePO4 handles heat well and lasts many cycles, but it must not be charged below freezing. Frost-prone sites need a charge controller that checks temperature first.
• Lithium thionyl chloride (Li-SOCl2) primary cells suit sensors that must run for years without solar. Self-discharge is very low, but a cell can show voltage delay after long storage, known as passivation, so we test for it under load.
• Lead-acid is cheap and forgiving, but heavy and short-lived in a hot enclosure.

Built for heat, dust and water

Australian sun can push the inside of a dark enclosure well above the air temperature. We use UV-stable enclosures in light colours, add sun shields and choose components rated for the real range. Enclosures are specified by IP rating to IEC 60529, typically IP66 or IP67, with breathable vents so condensation does not pool on circuit boards. Cable glands face down, long cable runs get surge protection, and entry points are sealed against ants, spiders and rodents.

Hardware is mounted for the person who will service it: standard tools, labelled connectors and a status light that shows a device is alive without plugging in a laptop.

Remote monitoring dashboards

Data is only useful if someone sees it at the right moment. Our dashboards show current readings, history and trends, with device health next to the data so a flat battery is never mistaken for a dry paddock. Alarms go out by email or text message on thresholds you set, with a delay and a reset level so one noisy reading does not wake anyone at 2 am.

Behind the dashboard, readings land in a time-series database with the raw value stored beside any calibrated value. You can export to CSV or connect your own tools through an API. For links and gateways, see long-range comms.

Questions