21 January 2000

Weigh cells take guess out of application rates

Using weigh cells to

measure the application rate

for granular fertiliser can

take some of the guesswork

out of spreading, and is one

aspect of precision farming

which is attracting real

interest says Mike Williams

MEASURING application rates by volume can be a hit-or-miss affair.

It makes no allowance for variations in fertiliser density and it ignores the fact that application rates are measured in kg/ha.

It follows then, that using weigh cells to measure the weight change as fertiliser leaves the hopper means application rates can be controlled with much greater accuracy.

Linked with an electronic control unit, weighing can also maintain a pre-set spread rate, and include automatic compensation for variations in fertiliser density or forward speed changes. Accurate rate control is also important for varying the application through a GPS link.

Counting the number of empty bags provides a basis for checking the application rate, but this method is not available on farms using fertiliser in bulk, says Keith Rennie, managing director of KRM Machinery. His firm distributes the Denmark-built Bogballe spreader range in the UK which are claimed to be the first tractor mounted spreaders equipped with weigh cells.

"Bogballe has used weigh cells for 10 years, and the Calibrator system was added about five years ago," he says. "Sales have grown steadily, particularly last year when there was much more interest."

There are two main reasons for the recent sales upturn says Mr Rennie. Part is due to interest in precision farming for using inputs more cost-effectively, but there have also been extra sales to farmers who aim to cut fertiliser costs by switching from bags to bulk.

"Getting the application rate right with bulk fertiliser can be difficult, but the weigh cells measure the rate very accurately. There are some good bulk fertiliser deals at present, particularly for urea, and the Calibrator system means farmers can benefit from the lower prices without losing accuracy."

GPS facility

The Bogballe weighing unit uses a single weigh cell, working with the Calibrator 2003 electronic control unit on a twin disc spreader. It includes a GPS facility for variable rate application, and can link to most leading GPS field map programs including Massey Fergusons Fieldstar and the Claas, RDS, Soyl and LH Agro systems, automatically varying the application rate to suit soil analyses or yield data entered on field maps. The equipment can also be used without the map function for spreader calibration and checking the spread rate.

Specifying weigh cells and the Calibrator 2003 package adds an extra £2750 to the price of the standard EXW twin disc spreader. Vicons electronic dosing and weigh cell unit (EDW) is an option on RS-XL twin disc spreaders. Like the Bogballe unit, it works with field maps and a GPS link to vary application rates or on its own for calibration and rate control.

Weight data from the EDW unit is averaged from four weigh cells positioned at different points between the hopper and the spreader frame. A fifth cell acts as a reference, ironing out the distortions caused by bounce as the spreader moves across the field. A display unit in the tractor cab shows the actual spread rate and also the weight remaining in the hopper before a refill is needed.

EDW adds £2565 to the cost of a standard 2t capacity twin disc spreader with electronic controls. It can also be used with Vicons recently announced ProFaS mapping package which includes a GPS receiver and software plus two data cards and readers. ProFaS is available for about £2800.

Customers for the Vicon weigh cell system are mainly arable farmers looking for increased precision, says technical manager, Tim Baker.

The Bogballe and Vicon weighing systems are factory-fit options, but the weigh cell for the Lely Centermatic unit is based on an A-frame which is supplied as an add-on for an existing Centerliner Supabowl spreader. It has been available for four years and the price is £3475 for an existing spreader with electronic controls. Accuracy is said to be maintained within 3% of the target setting.

The Lely unit was developed for calibration and rate control, but it will also be compatible with the Lely GPS control unit for variable rate spreading based on map data.

According to service engineer Robert Shepherd, demand for the Centermatic unit started slowly, but sales have increased every year, and last year was the best so far.

"We sold one machine to a vegetable grower who mixes his own compound fertilisers in bulk, and the weighing system is useful for controlling spread rates where the material can be variable," says Mr Shepherd. "We have also sold Centermatic units to contractors, because it means they can offer their customers extra accuracy."

Using the do-it-yourself approach with a weigh cell kit fitted in the farm workshop is not recommended for tractor-mounted spreaders because big structural alterations are needed, but this is not a problem with most trailed spreaders says David Elder of farm weighing equipment specialists, Griffith Elder.

"Most trailed spreaders are the double-chassis type, and fitting weigh cells is quite easy," he insists. "We have supplied kits to a number of farmers and they work very well. You need four weigh cells to get an accurate reading. Price of the kit is £1800 and anyone with the ability to wire up a 13-amp plug could certainly fit the kit in the farm workshop." &#42