FARMERS WHO haul their own commodities or animals know that when they drive with an overweight load, they are risking a stiff fine. Now, thanks to a growing number of ever-sophisticated onboard scales, those risks can be eliminated.

Onboard scales have been available since the late 1970s, but the earliest models all employed load cells, which are based on “strain gauge” technology invented more than 100 years ago. Load-cell scales were renown for their accuracy, but also their cost — more than $10,000 in some applications.

The onboard-scale market started changing in the early 1980s, with the proliferation of air-ride suspensions on trucks and trailers. Drivers quickly realized that the suspension air-pressure gauge, a normal part of most such systems, could be used as a crude scale: More weight meant more internal pressure. The trick, however, was in determining the pressure at which a tandem's legal weight was reached, then factoring in other variables that could easily alter this relationship.

Although suspension air gauges still occupy space in countless truck dashboards, fewer of them are serving as makeshift scales because truck owners want a more exact, reliable and convenient method of immediate weighing, and they're willing to pay for it. Today they have four distinct choices.

Load cells

Load cells remain the preferred choice of log haulers, mine operators and refuse collectors. The technology is accurate (but usually not certifiable) in all environmental conditions and on all surfaces, with any type of suspension on any type of vehicle. Its chief downside is expense. Outfitting a straight truck (Class 6 to Class 8) with one of these systems could cost $2,500 to $3,500, and about double that amount for a standard tractor-trailer. Complexity is also an issue. Typically, trained professionals must install load-cell units, and the shop time ranges from four to 40 hours per job.

It's all about precision, says Eric Elefson, director of sales and marketing at Kent, WA-based Stress-Tek, a leader in load-cell transportation scales. “We use sensing devices that best fit each application and mount them in a position where we can measure the downward force most accurately,” he says.

Elefson says that Stress-Tek has a number of ag customers, most of them in the feed and fertilizer business. “These are people who need to know how much product they're delivering to each location,” he says. “A lot of them will use our scales for their billable weights. That's acceptable as long as buyers agree to the arrangement.”

The capabilities of load-cell models, however impressive, are probably overkill in most farm operations. Fortunately for cost-conscious consumers, the alternatives are cheaper — much cheaper in some cases. All of these products derive their weight information from the pressure within a truck's (or trailer's) air suspension. They differ by brand, features, pressure-to-pound calculations and installation methods.

Air-based scales

The oldest and most well-known brand in this lineup is Air-Weigh, based in Eugene, OR. Founded nearly 20 years ago, the company was built on the success of its digital displays, the newest version of which is a 2-in. round gauge meant to replace a truck's standard suspension-pressure gauge.

Air-Weigh scales are used in a wide range of farm applications, says Michael Ferguson, the company's southeast regional sales manager. “We have customers in hog, grain, feed and produce businesses,” he says.

“Normal” Air-Weigh units — that is, the AW5800 and AW5802, which are suited for air-suspended tractors and trailers — cost $880 and $645, respectively. Each can be installed in one to two hours. The company also offers a mixed system (air sensor and load cell) for straight trucks with air-suspended rear axles and a leaf spring front axle. That's priced at $2,145.

Air-Weigh guarantees its scales' accuracy to within 300 lbs. of a certified scale, but Ferguson says they're actually much closer than that. “A level surface under the truck is the biggest factor in getting good air-based scale readings,” he says.

This detail was immediately clear to Randy Headrick after his company, Pritchett Trucking in Lake Butler, FL, mounted Air-Weigh scales on several trucks hauling bulk feed-stocks from a brewery to dairies in Florida. “In our initial tests, we discovered that the scales varied 1,000 lbs. between the sloped loading area and a nearby level parking lot,” he says. “Once we knew the difference, we just told the drivers to factor in that amount when they loaded.”

To further ensure that the trucks weren't overloaded, Headrick devised an alarm system. “The product at this facility comes out of an overhead hopper,” he says. “Our drivers can't repeatedly check the scale display because they're supposed to be on the trailers watching the loading process. So we wired each unit to a light on the back of the tractor's cab. These lights flash when the load is within 2,000 lbs. of the legal limit, and they stay on steady when the weight reaches 1,000 lbs. of that limit.”

Wireless units

Headrick's approach to remote, or outboard, monitoring is ingenious, but he could have saved a lot of design time with a competing brand of scale. TruckWeight of Halifax, Nova Scotia, which is the newest player in this market, sells a wireless unit called the Smart Scale that consists of air sensors and a handheld display that easily fits in a shirt or jacket pocket.

Jay Grimstead, a grain hauler from Rocky Ford, CO, purchased a Smart Scale last fall. He says it's perfect for his type of operation. “We do a lot of field loading,” he says, “and I need to be outside the cab directing the combine operator. The TruckWeight system allows me to do that while watching the load weight at the same time.”

Grimstead is an enthusiastic proponent of this new tool. “I think it's the best available,” he says. “It's accurate and easy to install. If necessary, I can move it from truck to truck in just a few minutes. Everyone who's seen this thing is pretty amazed.”

The key to getting proper weight readings is the initial calibration, Grimstead says. “You need to work with it at first and really tune it in,” he says. “The more often you check it against a platform scale, the more accurate it'll be. There are times when my TruckWeight scale is within 20 lbs. of a certified scale.”

Peter Panagapko, president of Truck-Weight, says, “We'd like to think that our scales are exact all the time, but they're just tools. The people who really learn how to use these tools will get the best results.”

TruckWeight systems sell for $1,326 (tractor-trailer) and $888 (tractor alone or straight truck). At present, there are no accessories for measuring leaf spring axles.

Analog scales

The most economical store-bought onboard scales come from Right Weigh, Sherwood, OR. The company sells two tractor models, priced at $105 and $150. The trailer equivalent is $195. Sid Campbell, founder and president of the company, says he's able to keep costs low because he uses analog gauges instead of digital. “It's the cheapest way to go, yet it works just as well,” he says. “I'm sure other companies would offer analog, too, but I own the patent for this type of application.”

Unlike competing brands, Right Weigh units are purely pneumatic and vehicle specific. One of the tractor gauges fits into a 2-in. dashboard hole. The other fits a 3-in. hole. A trailer gauge is exterior mounted and enclosed in a weather-sealed poly-carbon box. Campbell says the pair will work with all types of air suspensions, and both can be installed in a couple of hours.