A Tecweigh zero or span that errors, or sits waiting and never ends, is nearly always reporting the belt, the weights or the order of the steps, not a fault inside the processor. The cycle counts whole belt revolutions, so a belt stopped part way can never finish. And if the load count doesn’t rise when the calibration weight comes down, fix the sensing before you calibrate anything.
This guide sits under our WP20 and WP25 fault codes and calibration guide. We’re an independent service company, not a Tecweigh dealer or agent.
What is the processor telling you?
A failed cycle shows as a message in words, not a number. Copy it down before anyone clears it.
- WP20 and WP25: one message each way, auto zero error or auto span error. Both list the same eight causes, so the screen tells you the cycle failed, not why.
- WP50: three messages, each pointing somewhere different (see the table below).
After any of them, on all three processors: clear the message, run a fresh zero, then run the span. Don’t clear it and go straight back to the span.
Why does the cycle sit there and never finish?
Because the belt stopped. The cycle counts whole belt revolutions, so if the belt stops part way it never reaches its count. It doesn’t fail; it just waits. Cancel it, get the belt running and start again. Belts get stopped from control rooms nowhere near the scale, so check the belt first.
A WP50 shows a countdown, so one that has stopped moving is your answer. Typical minimums:
- WP50 zero: at least two minutes, over whole belt revolutions.
- WP50 span: at least two minutes, six preferred.
- WP25 zero and span: at least three minutes each.
Whole revolutions matter: a part revolution weighs one section of belt twice and misses another. Set the revolution counts to reach those times on your belt. A short span on a fast belt will repeat beautifully and still be wrong.
Why does nothing happen when you lower the calibration weight?
On a WP25 and WP50 the span starts when the processor sees the load change as the weights come down, against a span detect setting. If the weights don’t move the reading past it, the cycle never starts and nothing errors. On a WP25, pressing span again once the weights are down starts it by hand.
Then fix the threshold, or it happens every time. Recalculate the detect setting from the difference between the stored zero counts and span counts: roughly three quarters of that difference on a one or two weight scale, and roughly nine tenths on a four weight scale, so every weight is down before the timing starts. See test weights, chains and billets.
The WP20 has no detect threshold: its span starts on a timer once the button is held.
What has to be right before you run it again?
Work on or near a conveyor, or inside an electrical enclosure, needs isolation under your site’s procedures and trained, authorised people. If that’s not you, stop and call us on 1300 101 666.
Physical checks first. They catch most of it:
- nothing jammed under the weigh idler, and no build-up on the frame
- a clean, empty belt
- no skirting touching the weigh idler or the belt near the scale
- load cells undamaged
Then the order. Zero before span, always: a span on a stale zero gives a confident, repeatable, wrong result. And weight up for the zero, down for the span. The wrong way round is a named cause on all three.
Then prove the sensing. With the belt stopped, the live load count should sit steady, never go negative, and rise when the weight comes down. If it doesn’t rise, the load cells or the signal conditioning card are the problem. See our load cells guide.
Then the settings and set-up:
- Belt speed first, against a timed run over a measured length. More than about 2% out needs investigating.
- Warm-up. The WP25 and WP50 documentation asks for at least half an hour of empty running, longer in the cold; the WP20’s asks for about an hour. Give it the hour on a cold morning.
- The stored numbers. A wrong weigh span or calibration weight entry is a named cause. A weigh span or belt length left at zero gives no message at all. See which calibration method suits which scale.
- Belt length means the belt. On a WP50 it’s the belt as if cut and laid flat, not the conveyor length. Enter the conveyor length and every revolution-based timing is wrong.
What does each WP50 calibration error point at?
| Message | What we check first | When to call us |
|---|---|---|
| Auto zero error, low or negative counts | Shipping stops left in place (new installations and relocated portable conveyors). Then damaged load cells, a blocked or jammed weigh bridge, or damage to the load cell input on the scale card. | If the frame is free and it still errors. Running the cycle again won’t fix any of these. |
| Auto span error, low or negative counts | The weights actually deployed and hanging free, the raw load count rising as they come down, and the calibration weight entered matching what’s stamped on the weights fitted. | If the weights and the entry are right and the count still doesn’t rise enough. |
| Auto span error, span below zero | The steps done the wrong way round: weight lowered for the zero or raised for the span. | If the order was definitely right. What’s left is load cell polarity, or a displacement carriage biased the wrong way. |
On a displacement carriage, the current scale card needs a small positive bias, not the mid-range target the older card used, because it doesn’t read in the negative direction. A sensor aligned to the old target reads happily and never calibrates properly.
Why was a zero or span from the control room refused?
On a WP50, a zero or span commanded from a digital input, the serial port or the fieldbus is refused if the change would be more than about 5%, and shows as out of range. It then has to be done at the keypad, where you can accept it knowingly and run a second zero to confirm. If nobody knows why it moved, troubleshoot instead.
A zero that has moved that far usually has a visible cause: build-up, material under a weigh idler, or a failing load cell. See why a belt weigher zero drifts. Also note that security menu locks only block the keypad; the same commands still work remotely. See locked out of your integrator.
What happens when the cycle does finish?
The WP25 and WP50 show the percentage change from the last calibration and ask you to accept or reject it. A change nobody can explain is a reason to reject and investigate. The WP20 doesn’t ask, so the judgement is yours. See what zero, span and repeatability mean.
The numbers we work to: repeat span runs within about 0.1%, and a routine span that has moved more than about 0.25% needs attention.
On a multi-weight scale, raise the weights one at a time and check the rate falls in proportion. Then watch for three things:
- The weight left down. It gives a false rate, and on a WP25 totalising doesn’t resume until the weight is raised or a minute has passed.
- Chasing a rate that’s meant to wander. Until a span is done the rate drifts. Don’t reach for the deadband (see why a Tecweigh under-reads at low rate).
- The WP20 save step. Write the settings to permanent memory after any change, or the backup is still the old copy.
Record the running rate with the calibration weight applied, so the next visit can confirm in minutes that the calibration has held. We keep those figures in AIMM, our asset management system.
Who adjusts what. On our service visits we adjust the zero and span when the checks show it’s needed. Between visits, site should run a zero about every two weeks and after a big change in the weather, and check the span but record it rather than accepting it. We recommend a service calibration every quarter so the scale keeps up with the seasons.
Need a manual for your equipment?
You may need more than one book: the processor manual, the document for the signal conditioning card at the frame (where the alignment targets live) and, for a new or relocated scale, the frame’s installation guide.
Talk to us about manuals for your equipment. Send the make, model, serial number and software version (a photo of the label is fine) and we’ll help you find the right document.
When should you call us rather than run the cycle again?
- It still errors on a clean, empty, warmed-up belt with the weight correctly positioned.
- The load count doesn’t rise as the weight comes down. That’s the sensing, not the calibration.
- A remote zero has been refused. Something has moved more than about 5%, and it’s worth finding while it’s small.
- Span below zero survives a correctly ordered zero and span.
We work the whole scale, not just the processor. See how we service Tecweigh belt weighers, or call 1300 101 666.
Frequently asked questions
What does auto zero error or auto span error mean on a Tecweigh?
The cycle hit an abnormal condition and didn’t store a result. It’s nearly always physical or procedural, so clear it, run a fresh zero, then the span.
Can a Tecweigh be calibrated with the belt stopped?
The cycle needs the belt running. The WP50 documentation does describe fitting the weights with the belt stopped, then restarting it and driving the span through by hand, where deploying weights on a running conveyor isn’t safe.
Does a failed zero or span leave the scale reading wrong?
Don’t assume the numbers are good either way. Work through to a result you’ve accepted at the prompt. If the tonnes since the last visit matter, say so before anyone re-zeroes, because the as-found figures go once a new zero is stored.