Selecting a Moisture Sensor for 300 m/min Silicone Paper

A silicone-paper machine running at about 300 m/min (5 m/s) rules out contact moisture probes. The choice is between a non-contact infrared gauge and a microwave gauge. A gamma gauge measures something different from moisture. Work through the steps below in order; each ends with a check that must pass before the next.

Sheet Composition Check Before Choosing a Principle

Prerequisite: get the furnish and filler data for every grade the gauge will see. The principle choice depends on it.

1. List each grade with its filler, carbon and mineral content.
2. Mark any grade that is heavily loaded with carbon or mineral material.
3. Convert the line speed for the vendor RFQ: 300 m/min = 5 m/s. One cross-direction (CD) traverse of duration T seconds covers 5 x T metres of web in the machine direction (MD), so the scan profile is a zig-zag across the moving sheet, not a straight CD line.

Check: every grade is classed as either "infrared-suitable" or "heavily loaded (carbon/mineral)". A single heavily loaded grade moves the whole selection toward microwave.

Principle Selection: Infrared, Microwave, Gamma

Infrared moisture gauges use two wavelengths, a reference that water does not absorb and a measuring wavelength that it does. Per the evidence, the reference is about 1.8 µm and the measuring band about 1.9 µm. A lamp and filters in the lower measuring head generate both signals. Two detectors in the upper head receive them. A computer converts the pair of signals into moisture.

The ratio of the two signals is the working variable. Sheet thickness, scatter and lamp ageing affect both wavelengths about equally and largely cancel in the ratio. The absorbed-band signal falls as water content rises. The gauge maps the ratio to moisture using a calibration curve fitted to lab reference samples.

The gamma question needs a direct answer. Gamma attenuation follows I = I0 * exp(-mu * rho * x), so the detector sees total areal mass (fibre, filler, coating and water together). It gives no moisture signal on its own. Moisture from a gamma-based system requires a second, water-selective measurement, which is why moisture gauges are normally paired with a basis-weight gauge.

Gamma sources are radioactive, so licensing, source handling, shielding and disposal fall under your national radiation-protection regulator and the gauge manufacturer's documentation. Obtain the applicable standards and permit requirements from those two sources before considering a nuclear gauge; no specific standard is cited here.

Check: each grade is assigned infrared or microwave, and the gamma option is either dropped or scoped as basis weight only.

Scanner Geometry and Basis-Weight Pairing

1. Specify a traversing (scanning) frame so the gauge covers the full sheet width. Moisture gauges of this type are commonly mounted on a traveling frame.
2. Specify the lower head as the source (lamp and filters) and the upper head as the detector pair (two detectors), matching the layout above.
3. Ask for a basis-weight gauge on the same frame. Moisture and basis weight are normally reported together, and the moisture calibration benefits from a concurrent mass reading.
4. Request the sheet-path clearance, head-to-head gap tolerance and flutter tolerance at 5 m/s. Flutter changes the path length through the sheet and the signal level, so read these limits from the vendor datasheet.

Check: the vendor drawing shows a two-head traversing frame with the sheet passing between the heads, and it states the maximum web speed the sensor supports at or above 300 m/min.

Vendor Shortlist and RFQ Content

Gauges in this class are very expensive, so a written RFQ is worth the effort. Send it to each of these suppliers of scanning moisture and basis-weight gauges:

• ABB (Process Industries)
• Honeywell Measurex
• Metso Automation
• Voith Paper

Include in the RFQ:

1. Product: silicone-coated paper, line speed 300 m/min.
2. Grade table with filler, carbon and mineral content from the first step.
3. Request for a written statement of whether infrared is valid for each grade, and which grades need microwave.
4. Request for the calibration method and the number of reference samples they need.
5. Request for a sample-testing offer on your production sheet before purchase.

Check: each vendor replies with a per-grade statement of infrared or microwave suitability. Any vendor that quotes infrared for a heavily loaded grade without a supporting sample test fails the check.

Calibration Against Lab Reference Moisture

Prerequisite: the gauge is installed, aligned and warmed up per the vendor manual.

1. Collect sheet samples from the exact scan position and time the gauge reads, and seal them immediately to stop moisture exchange with room air.
2. Determine moisture with your laboratory reference method (for example, oven-drying) on the same samples.
3. Enter the pairs into the gauge calibration. Do this per grade, or per grade group if the vendor supports grade-specific curves.
4. Repeat across the moisture range the process actually runs, not one operating point.

Check: the gauge reading agrees with the lab value across the range within the tolerance you defined in the RFQ, for every grade in the table.

End-to-End Verification on the Running Line

1. Run each grade at 300 m/min and confirm the gauge holds a valid reading with no signal-loss alarms at full speed.
2. Park the scanner at a fixed CD position on a steady sheet and confirm the reading is stable. Then scan and confirm the CD profile is repeatable across consecutive traverses.
3. Take a fresh set of lab samples at three CD positions per grade and compare them with the gauge readings at those positions.
4. For any grade that failed the infrared suitability check, repeat steps 1 to 3 on the microwave gauge and accept it only if the lab comparison passes.
5. Change the moisture setpoint in a controlled step and confirm the gauge follows the lab-confirmed change in the correct direction and size.