Online Laser Caliper for Paper Webs

What is the Online Laser Caliper for Paper Webs?

An online laser caliper measures sheet thickness continuously on the paper machine using non-contact triangulation heads on both sides of the web. It provides a thickness profile for process control without touching or marking the sheet.

Overview

Two laser displacement sensors are mounted opposite each other, one above and one below the sheet, and the difference between the measured distances gives the thickness at the measurement point. Because the measurement is non-contact and applies no pressure, the value corresponds to an unloaded thickness and is therefore higher than a laboratory caliper reading on the same sheet, so a correlation must be established if the online value is to be used against a specification. Scanning heads traverse the web to produce a cross-direction profile, and moisture and basis weight signals are often combined to separate the effects of mass and moisture on the observed thickness. Air stabilisers, edge detection and automatic head protection are common because the environment above a moving web is dusty and hot.

How does it work?

Because paper is compressible, thickness depends strongly on the pressure applied, the area of the measuring foot and the dwell time. Standards therefore define the contact pressure, usually 100 kPa for paper, the foot diameter, and the time between lowering the foot and reading the value, so results from different laboratories are comparable. A dead-weight instrument applies the load through calibrated masses on a fixed-area foot and reads displacement with a dial gauge or digital sensor, while motorised instruments lower the foot at a controlled speed and hold the specified pressure for a set interval. Board and tissue thickness use larger pressures or different foot geometries, and corrugated board requires a foot large enough to bridge the flutes and a comparator plate, so the correct geometry is defined by the product standard rather than chosen freely by the laboratory.

What are the key features?

• Dual non-contact laser triangulation heads
• Traversing scan for cross direction profiles
• No contact pressure and no sheet marking
• Combination with basis weight and moisture signals
• Automatic head protection and edge detection
• Signal output for process control systems

What are the technical specifications?

A typical online laser caliper for paper webs configuration offers measurement principle of Dual laser triangulation, non-contact, thickness range of 0.02–5 mm depending on heads, resolution of 0.1 µm, accuracy after correlation of ±2 µm typical, measurement spots of One per scan position, multiple heads optional and scan speed of Up to 20 m/min traverse. Values are indicative of the product class; the exact figures for each configuration are confirmed on the quotation and the calibration documents supplied with the instrument.

Which applications is it used for?

The Online Laser Caliper for Paper Webs is used in continuous caliper control on the paper machine, cross direction thickness profile improvement, reel-by-reel thickness documentation and detection of thickness streaks and profile faults. Across the wider Thickness Testing class, laboratories also deploy it for thickness and apparent density of paper and board, corrugated board caliper verification and tissue and lightweight paper thickness testing.

Which standards and certifications apply?

Relevant standards and methods include ISO 534, ISO 3034, ISO 12625-3, TAPPI T411, TAPPI T551, ASTM D645 and IEC 61010-1. Certification candidates for this model are CE and ISO 9001 manufacturing; each is to be confirmed for the specific configuration before shipment, and certificates are supplied only where they have been issued.

Condition specimens to the standard atmosphere before testing, because thickness changes with moisture and the change is not fully reversible within a short time. Clean the foot and anvil, since dust or fibre on the surfaces is added to the reading, and check the foot diameter and parallelism at intervals. Lower the foot gently at the defined speed, take the reading at the specified time and lift the foot between readings, and avoid taking measurements on the same spot twice. Verify the instrument with certified shims or gauge blocks at several points in the measuring range, and keep the calibration certificate with the instrument record.

How do I select the right Online Laser Caliper for Paper Webs?

Specify an online laser caliper when thickness must be monitored continuously or its cross-direction profile controlled, and accept that the value is an unloaded thickness. Confirm the head standoffs, environmental protection and the correlation procedure with laboratory measurements, and check the signal interfaces against your control and quality systems.

How does it compare with other Thickness Testing variants?

Select the instrument from the product standard you test against. Flat paper and board are measured with a 100 kPa dead-weight or motorised instrument with a defined foot area, tissue papers require the smaller pressure and foot of their own method, and corrugated or heavy board needs a dedicated gauge with the correct foot diameter and a rigid comparator plate. Choose a motorised instrument when many readings are needed or when operator technique must be removed, and a dead-weight instrument when simplicity and direct traceability of the applied load matter. An online caliper is specified only when continuous reel or sheet thickness data must be recorded during production, since it is measured without contact pressure and therefore correlates with, rather than reproduces, laboratory values.