Buying a Weighbridge

Generally speaking we would expect a good quality steel weighbridge deck to give anywhere between 15 and 30+ years service depending on traffic volume and environment. An all concrete or composite bridge could potentially go on to provide service for much longer. The primary drawback with using concrete is the weight; a single piece 15m concrete deck can weigh as much as 60 tonnes so the cost of transport and lifting for installation is often prohibitive. This leads to steel being preferred material in most cases.

The driver for this decision is usually space. If you are considering a surface mounted bridge you should plan for ramps with approximately 1:9 gradient; we recommend a minimum 3.75m long ramp for a 0.4m high deck. This is required as the legs on a lot of trailers will otherwise bottom out and cause damage to the deck or ramp.
Additionally we would recommend building a short flat apron (150mm – 300mm min) at the top of each ramp. Traditionally, much longer aprons were required for engineers to calibrate the bridge with roller weights without having to push them up the ramp. The use of block weights and a fork lift truck for this purpose means that an extended apron is no longer required but including a short one will save a significant amount of wear and tear on the weighbridge, reduce service and maintenance requirements and prolong its life. This is because when the vehicle drives off the weighbridge and down the ramp the trailer tips up and the loading moves forward onto the front axle. At the apex of the ramp the loading on the axle can increase by up to 2.5 times and it is much better to load this onto the concrete ramp rather than onto the end of the weighbridge!

Taking these into account, for an ideal installation you need to allow for an additional 8m on top of the length of the bridge, plus enough space for vehicles to line up and drive on and off straight without skewing.

Usually the cost of a surface mounted installation is much lower than that for a pit however, where space is restricted, you may have no alternative but to go for a pit mounted option. If this is the case then your primary concern when designing the pit should be drainage and how you intend to clean it out. Modern loadcells are hermetically sealed but they are not designed to be permanently under water or encased in mud. Build-up of mud under the deck or between the end of the deck and the end wall of the pit can also lead to a false readings so it is very important that a regular cleaning regime is considered when installing the bridge and is operated throughout its life.

To prolong the life of a pit mounted weighbridge we would also recommend that, where possible, you install measures to ensure that vehicles approach the deck slowly and do not brake sharply on the deck itself. Similarly, bollards or barriers to prevent vehicles approaching the deck from the side or skewing on or off the deck will extend its life.

A common misconception is that you need a weighbridge which is the same length as your vehicle. However, as long as all the wheels fit on the deck then you’ll still get accurate readings. Check your wheel base but currently as a rule of thumb if you are running siz wheelers, then a 6m weighbridge will usually surfice; most 8 wheelers will fit onto an 8m bridge and 15m bridge will take a standard artic. Again given the intended lifespan of your bridge you might want to mitigate future costs which you may incur as a result of changes in legislation relating to vehicle length. The following links may help you decide if this is something to consider.

https://www.bbc.co.uk/news/world-europe-27032476

https://en.wikipedia.org/wiki/Longer_Heavier_Vehicle

Regulation 3 of the The Non-automatic Weighing Instrument (NAWI) Regulations gives the following applications to which the NAWI directive should apply:

1. Determination of mass for commercial transactions.
2. Determination of mass for the calculation of a toll, tariff, tax, bonus, penalty, remuneration, indemnity or similar type of payment.
3. Determination of mass for the application of laws or regulations including expert opinions given in court proceedings.
4. Determination of mass in the practice of medicine for weighing patients for the purpose of monitoring, diagnosis and medical treatment.
5. Determination of mass for making up medicines on prescription in a pharmacy and determination of mass in analyses carried out in medical and pharmaceutical laboratories.
6.  Determination of price on the basis of mass for the purposes of direct sales to the public and the making up of pre-packages.

If you know for certain that you will only ever use the weighbridge for your own internal checking and that you won’t ever need it for any of the above applications then a non-trade bridge will be fine. (Taking regulation 3 item 3 in to consideration it is our opinion that in order to comply with the SOLAS regulations you need a verified weighbridge or weighing instrument)

Given the life span of a weighbridge, even if your initial use is for a non regulation 3 application, we would always recommend using equipment that has a relevant type approval so that use for a regulation 3 application is an option for the future. Retro-fitting equipment with a type approval to a non-trade weighbridge is generally significantly more costly than installing equipment that can be trade approved in the first place.

Consider also that if you are checking your inputs and a dispute comes down to an argument in court (see 3.3 above) then readings from your non verified weighbridge won’t count for much.

The most significant costs associated with owning and operating a modern weighbridge are in the transport, installation, testing and maintenance, therefore spending a small amount more on a sufficiently heavy deck plate and steel will give your weighbridge’s life expectancy a significant boost and significantly reduce your cost of ownership. In  our experience, weighbridges engineered down to a low price often compromise on thickness of deck plates and gauge of steel beams, therefore guaranteeing problems and are a false economy. Do also keep in mind flexibility of servicing. We always recommend going with open protocol instrumentation to allow you to ensure that you get competitive rates for servicing and maintenance throughout the life of your product.

The difference between analogue and ‘digital’ cells is that both use the same mechanism to measure changes of load applied. However, a ‘digital’ cell has its own analogue to digital (A to D) converter and the signal transmitted to the indicator is digital, whereas an analogue cell’s signal isn’t converted to digital until it reaches the weight indicator. Therefore a typical six cell ‘digital’ bridge has six A to D boards located in the harsher environment beneath the deck, we remain unconvinced of the benefits to the customer of digital loadcells. Consider also that digital cells are often associated with closed protocol instrumentation (as referred to above) and the limitations on flexibility of servicing that that brings.

Often we find customers who are having problems with their weighbridge due to the foundations having cracked and moved. The accuracy checks for a trade bridge are very stringent and if the ground your bridge is on is moving the tolerances cannot be achieved. The only remedy is often to start over so you might as well get right first time.

90% of weighbridge faults are due to the build up of dirt and rubbish under the decks which, as referred to earlier can lead to false readings from your weighbridge. Trading standards will disqualify your weighbridge on an unannounced inspection if it doesn’t conform to their strict criteria. Once disqualified the only way to get it legal for trade again is to re-verify with test weights and a verifier after the problem has been rectified. This situation can be very disruptive to your operation and is hugely unnecessary cost that can easily be avoided through an effective and regular cleaning regime.

The best way to ensure that your weighbridge remains in good condition and provides accurate data for your business is to have it serviced and calibrated regularly by competent contractor. You should factor-in service visits from an external provider in addition to your regular in-house maintenance schedule.

There are many ways that a modern weighbridge can interface with other data and control systems in your production process. You should consider how this might be the case and investigate the various options with your supplier before making your buying decision as retro-fitting these later can be more costly than including them from the outset.

In terms of capital outlay a modern weighbridge is very much more affordable than its historical counterpart. By taking all of the above into account you can ensure that you minimise your long term cost of ownership and maximise the very real benefits that a weighbridge offers your business. In terms of the savings and cost control that a weighbridge offers we believe that buying the right weighbridge, installing it in the right way and maintaining it correctly could be the best pound for pound investment you can make in your business.