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Holt Systems

A delta t spraying chart can't see the dusk inversion. Your mast can

We install a 10 m weather mast at your paddock. It works out Delta T, flags the evening inversion and texts the people you name.

  • MeasuresDelta T, inversion gap, wind
  • You getA text on inversion or wind
  • ReadingsEvery 5 min, kept for your diary
Get a quote for a spray mast See the SMS alerts or call 07 2150 3666

North paddock gap, 10 m minus 1.25 m

Dashboard view

Delta T
6.4
Inversion gap
+0.4 °C
Wind at 2 m
6 km/h
-0.3 0.5 1.4 Inversion flag 3 pm5 pm7 pm11 pm3 am7 am °C
Illustrative readings, not site data. Default flag +0.1 °C on two readings running, set at commissioning.

up to 30,000 ha

Cotton reported hit by suspected 2,4-D drift on the eastern Darling Downs in late 2022.

Source: ABC News (ABC Rural)

over 30 km

GRDC says a 5 km/h wind can carry a concentrated drift layer that far over a summer night.

Source: GRDC

3 to 4 hours

COtL Mesonet reports this many extra spraying hours a day from hazardous inversion data.

Source: COtL Mesonet Network

The chart says spray. The dusk air says don't

A delta t spraying chart works out Delta T from air temperature and humidity. GRDC says spray at 2 to 8 and not above 12.

A hot afternoon pushes Delta T past 10. By the last load it's back in range, and that's about when the inversion sets in. The chart can't see it.

  • The 2,4-D label bans spraying in hazardous inversions, which it says exist most evenings.
  • The nearest regional tower isn't at your paddock.

One day of readings at 1.25 m

  • Delta T is T minus wet bulb, worked out from T and RH with the Stull (2011) formula.
3.00 pm, 34 °C, 25% RH: Delta T 13.9, gap -0.3 °CAbove 12, avoid
6.30 pm, 26 °C, 55% RH: Delta T 6.4, gap +0.4 °CIn band, inversion flag set
9.30 am, 24 °C, 60% RH: Delta T 5.4, gap 0.0 °CIn band, may have cleared

At 6.30 pm the chart says spray and the inversion flag says don't. At 30 °C and 50% RH our calculation gives Delta T 7.7, close to the GRDC chart's 8.

How a delta t spraying chart plus inversion flag works on a mast

Delta T spraying chart: paddock mast, inversion flag and alerts Side elevation of a 10 m mast at the edge of a Darling Downs fallow paddock at dusk, drawn to a metre scale beside a boom sprayer on stubble. The mast carries the gateway antenna and a Starlink or 4G dish at the top, a shielded temperature sensor at 10 m, a wind sensor at 2 m and a temperature and humidity shield at 1.25 m. A small graph of temperature against height shows a daytime curve that is cooler aloft and an evening curve that is warmer at 10 m, with an arrow marking the 10 m minus 1.25 m gap, called VTD. Below, a 24 hour strip from noon to noon has three stacked bands: the inversion flag, on from dusk to mid-morning; the Delta T bands, above 10 in the afternoon, 2 to 8 in the evening, below 2 overnight and 2 to 8 again in the morning; and four numbered points where an SMS or email alert fires: Delta T rising above 10, an evening wind lull, the inversion flag being set, and the inversion possibly clearing. Two calculation boxes read Delta T = T minus Tw, with Tw the wet bulb found from T and RH, and VTD = T10 minus T1.25, where a positive value means an inversion. 10 m 5 m Gateway antenna and Starlink or 4G dish 10 m temperature sensor in shield 2 m wind sensor 1.25 m temperature and humidity shield Boom sprayer on stubble Temperature by height 10 m 1.25 m VTD Day: cooler aloft Evening: warmer aloft One illustrative day, noon to noon Inversion flag: VTD above threshold Delta T band at 1.25 m (grey is 8 to 10) SMS or email alerts, via gateway and Starlink or 4G off Inversion flag on off above 10 2 to 8 below 2 2 to 8 1 2 3 4 12 pm 6 pm 12 am 6 am 12 pm 1 Delta T rises above 10 2 Evening wind lull 3 Inversion flag set 4 Inversion may be clearing Delta T = T minus Tw Wet bulb Tw from T and RH VTD = T10 minus T1.25 Positive means inversion
Side elevation of a 10 m mast beside a boom sprayer on stubble, the temperature profile by day and evening, and one illustrative day of Delta T bands, inversion flag and SMS alerts.
  1. 1

    Two matched sensors

    Same-model temperature sensors at 1.25 m and 10 m. We compare them side by side before they go up.

  2. 2

    Delta T and the gap

    Delta T is air temperature minus wet bulb. The gap is T10 minus T1.25. Above zero means an inversion.

  3. 3

    Radio then Starlink or 4G

    Readings go by LoRaWAN radio to the gateway, then out by Starlink or 4G. Wind is logged at 2 m.

What we fit on the mast

  • 1.25 m temperature and humidity shield

    Gives the Delta T inputs and the lower half of the inversion gap.

    Reads every 5 min

  • 10 m temperature sensor

    Same model as the lower one, so their errors match.

    Matched pair

  • 2 m wind sensor

    Logs speed for the label range and your spray record.

    Alert under 3 or over 20 km/h

  • Gateway at the mast top

    Picks up the nodes by radio and sends the alerts.

    Silent alert at 30 min

  • Starlink or 4G backhaul

    Chosen after a coverage test at your paddock.

    Starlink or 4G

What lands on your phone at the end of the day

Paddock mast alerts

  1. 3:00 pm

    North paddock mast: Delta T 13.9 at 34 °C and 25% RH. Above 12, avoid spraying.

  2. 5:40 pm

    North paddock mast: 2 m wind under 11 km/h for 30 min near sunset. Inversion likely.

  3. 6:30 pm

    North paddock mast: 10 m is 0.4 °C warmer than 1.25 m. Inversion present, treat as hazardous.

  4. 9:30 am

    North paddock mast: gap 0.0 °C, wind over 7 km/h for 45 min. Inversion may have cleared. Confirm.

Messages go to the people you nominate, by SMS to any phone.

Messages go to the people you nominate, by SMS to any phone and by email, with the reading and the time in every alert.

Today versus with a paddock mast

TopicTodayWith monitoring
Delta TA chart or a handheld meter at each tank fillCalculated every 5 min, with a text at each band change
InversionVisual clues, or a regional tower down the roadA flag from your own paddock, time stamped
Spray recordWritten up from memory at the end of the jobTemperature, humidity and wind logged, exported daily

On a Darling Downs fallow, or a cane block

Goanna Ag's tower guidance puts the mast at least 10 times the height of the nearest obstruction away. A QBCC-licensed contractor we manage erects it. We do the electrical work under QLD Electrical Contractor Licence 1513082.

Cane growers in Bundaberg, Mackay, the Burdekin and the Wet Tropics spray under the same labels and records rules. We found no published cane guidance, so thresholds follow your label. Vineyards and orchards have their own page.

Add a rain gauge for every block for the rain part of your record. See soil temperature at your paddock for planting, or fire danger at the paddock edge for harvest.

Close view of a cotton crop ready for picking
Illustrative photo. Photo: Pexels

What it costs

We fix the price after a short scoping call or a site visit. It depends on how many sensors, how far the mast is from the gateway, and how it gets out. After handover, ongoing support is available as a plan.

Get a quote for a spray mast

The fixed price covers

  • Matched temperature sensors, wind sensor and shield
  • Gateway and Starlink or 4G backhaul
  • Mast erection by a QBCC-licensed contractor we manage
  • Electrical work under QLD Electrical Contractor Licence 1513082
  • Alert set-up for your label, checked against a reference thermometer
  • Ongoing support plan after handover

What this mast won't do

  • It shows whether an inversion exists, not whether it's hazardous. It isn't a recognised inversion monitoring station.
  • It doesn't replace the label. You still measure wind where you spray and keep to buffer zones.
  • Readings arrive every 5 minutes by LoRaWAN, not live, and stop outside gateway range.
  • One mast measures one point. Low-cost sensors are indicative; audit evidence needs calibrated instruments.

With you every step

How the five stages go on a spray mast, from choosing the ground to setting alerts for the product you spray.

  1. 1

    Discovery

    We pick open, level ground, check overhead lines and coverage, and confirm your product label limits.

  2. 2

    Design

    You get a written scope: sensors, heights, thresholds, backhaul and the mast work.

  3. 3

    Build and test

    We build the node and gateway, set the alert rules and test the two sensors side by side.

  4. 4

    Install and commission

    We raise the mast, test the link and check a full evening to morning cycle.

  5. 5

    Ongoing support

    Alerts go to your nominated people, readings export daily, and we provide ongoing support.

Why Holt Systems

We design, install and commission every system ourselves, then look after it with an ongoing support plan. Monitoring supplements the site's own control, protection, fire and safety systems; it does not replace them.

Alerts
SMS to any phone, no app
Based in
Brisbane, QLD
Electrical licence
QLD 1513082
After install
Ongoing support plans
Call
07 2150 3666

Delta T and inversion questions

What is the ideal Delta T for spraying?

GRDC advises 2 to 8, with caution below 2 or above 10. At 10 to 12 use very coarse or larger droplets only, and don't spray above 12. The label and nozzle still govern. A good Delta T says nothing about an evening inversion.

Is it illegal to spray 2,4-D at night?

The current model label sets no time-of-day rule. It bans spraying in hazardous inversions, which it says exist most evenings. Business Queensland says it's an offence for a commercial operator to allow drift that might reasonably damage other crops.

What weather records do I have to keep when spraying in Queensland?

Business Queensland says you need a record within 2 days of each application, kept 2 years. It covers temperature, humidity, wind, cloud cover and rain. The mast logs all but cloud cover.

How does a paddock mast compare with the regional inversion network?

The network uses vertical turbulence to tell hazardous inversions from harmless ones. A two-height mast can't. CRDC says it's for cotton and grain farmers only, and its towers aren't at your paddock. The mast adds your own record.

Can the mast forecast an inversion?

No. It reports the air at your paddock every 5 minutes. After a few weeks it shows your typical onset time. That's history, not a forecast. CRDC reported in January 2024 that forecasts are a paid network feature.

Sources

  1. Spray drift damages up to $100 million in cotton, prompting calls for more herbicide controls, ABC News (ABC Rural), 6 January 2023
  2. Practical tips for spraying fact sheet (revised January 2025), GRDC
  3. Hazardous inversion fact sheet (May 2024), GRDC (copy hosted by WeedSmart)
  4. Not all inversions are equal: there's only one sure way to tell (10 January 2024), CRDC
  5. Inversions and spray drift (hazardous inversions, VTD and sigma w), COtL Mesonet Network
  6. Group 9a 2,4-D representative (model) label, APVMA
  7. Making and keeping records of agricultural chemical applications, Business Queensland
  8. Responsible ground distribution of herbicides, Business Queensland
  9. Safeguarding against spray drift and extending safe spray windows (inversion tower siting), Goanna Ag
  10. Wet-Bulb Temperature from Relative Humidity and Air Temperature (Stull, 2011), Journal of Applied Meteorology and Climatology

Get a quote for a spray mast

Tell us what you need to watch and where. We confirm the sensors, gateway location and alert recipients, then give you a fixed price.

Or email hello@holtsystems.com.au

Your details are handled in accordance with our privacy policy.

  1. 1

    Tell us about the site

    A few lines on the assets, distances and what you need to know about.

  2. 2

    A call to scope it

    We confirm the sensors, gateway location and alert recipients, and whether a site visit is needed.

  3. 3

    Fixed price

    You receive a defined scope and a fixed price before any work starts.

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