Weigher

No.

Btw, "precession" is the wandering of the earth's or other gyroscopic axis, and does not really apply here.

sorry, meant precision

I concur with Tornado.

Depending on the acceptable margin of uncertainty, you may be able to check the span of the weigher with a known weight that is a fraction of the full span.

The mV generator can test whether the downstream electronics are working, but it cannot tell whether the load cell(s) are sending an accurate signal. Thus, it fails to calibrate the system as a whole.

No it over looks the mechanics of the scale. Which may not apply the full weight of what ever is on the pan on the load cell. A lot of scales still use lever to isolate the load from the load cell.

There are a few loadcell manufacturers that either document or adjust the mV output to allow calibration without test weights.

While the best practice is to calibrate the scale using test weights equaling 50% of the measuring span, there are situations where it is simply impractical to add & properly distribute the required calibration weights. Due to the linearity of the loadcell installation, I will trust a full-span electronic calibration over a poorly performed test weight calibration with insufficient span.

We specify Hardy Instruments C2 loadcells when calibration with test weights is not practical. Click the link for additional information:
http://hardyinstruments.com/process_weighing/c2%AE+electronic+calibration/

Thank you for a good answer, An electronic calibration is the only practical choice we have for our current application.

When a load cell is calibrated (in the lab, or by a vendor) a calibration curve of mV output versus pounds input (at a specific excitation voltage) is produced. The calibration should also include a resistance calibration in it. Within this calibration, precision resistors are placed across the active leg of the load cells wheatstone bridge and the mV outputs are measured. This tells you what the pounds input would then be for each resistor used.

Precision resistors are then applied to the load cell in the field as a means for spanning the amplifier. A two point adjustment can be performed by first applying no weight (to adjust the amplifier zero) and then by applying the resistor across the bridge to produce a known output (and to adjust the amplifier gain). Repeat these two steps until zero and gain are stable. Apply a different resistor across the load cell to verify the gain and zero values.

While it is not possible to do by inducing mVolts, it is possible to calibrate without a load.

You may need some modification to you load cell mountings to provide anchorage points. (Jack up your mountings and take out your load cells before you weld anything. A welding earth passing across a load cell will kill it.)

You can simulate the load by pulling down an empty vessel or pushing up a full one by a known amount. You need jacks with integral load cells for this purpose. In the UK these can be hired, but it is usual to sub-contract the procedure to a specialist supplier.

Accuracy of the result equals directly loading with known weights if the hired equipment comes with calibration certificates, but you have to adjust the accuracy tolerance to take into account that this is a secondary method of calibration. If you are calibrating to comply with a regulatory standard, you need to check with the regulating authority that this method is acceptable.

Hi Eltech

The answer is yes and no, that is depending on the weigh controller you have, I would probably have to say no, Unless you have a weigh controller from New Zealand manufactured by EMC, their 1400 series of controllers, I would have to say are the best small money can by, for instance if you have a silo that weighs 100 ton how do you put test weights on it, rather difficult isn't it, but with these controllers you can programme in the MV ratio and then Tare or Zero the silo.

Of course there are other details that need to be programmed but test weights are not necessary, nor any calibration device.

When ever i get the chance i use these units, I believe that they are the best on the market, they are easy to programme and reliable.

Hope this of some assistance

Best Regards

Joe

exactly that, its for a 200 ton silo.I will have to check if the controller you mention meets the hazardous requirements for that zone.

Eltech, I do not think that you have mentioned this in your post.

If you have a load cell in an hazard area, you need to have an intrinsic barrier between the hazard zone and the safe area.

From now on please be more descriptive with your application.

Cheers

Joe

The problem with applying simulated inputs to the measuring circuits is you still don't know if the load cells are giving accurate outputs so you will always have a degree of uncertainty about the indicated weights. If the silo contents are being used in a manufacturing process the measured quantities way be incorrect and may spoil the end product. If the contents are being purchased I doubt if you will get Trading Standards to apply their seal to the measuring system. Being able to 'tweak' the output readings on any measuring system will surely ring alarm bells in the Quality Control office!!

If the silo is being refilled from road tankers/lorries I would take the weighbridge measurements for the vehicle in and out and cross check the readings you get on the silo. Not perfect, but it will at least give some confidence about the cell outputs.

If you can't use test weights to calibrate the system you could also hold a couple of spare calibrated load cells and set up a program to send them away for refurbishing and recalibration.