Glass Fibre Sheets for Moisture Analyzers: When to Use Them and Why

What the sheet actually changes

A moisture analyzer works by loss-on-drying: an integrated balance weighs the sample continuously while a halogen lamp dries it, and the mass lost as vapour becomes the moisture percentage. The method assumes one thing — that every part of the sample dries at roughly the same rate. A flat aluminium pan delivers that assumption comfortably for flour, spice powders, milk powder or granulate, which spread into an even layer on their own.

Two sample types break the assumption. A liquid pulls itself into a dome instead of spreading, and a sugary or fatty sample hardens on top while the middle stays wet. In both cases the instrument is working perfectly and still gives you the wrong number, because the sample geometry was wrong before the lamp ever switched on.

A&D documents both cases explicitly for its moisture analyzer range: filtration glass fibre sheets for liquid samples with strong surface tension such as sauces and beverages, and glass fibre sheets for heat-sensitive samples that melt or scorch on the surface.

Problem one: liquids that refuse to spread

Surface tension makes a liquid minimise its own surface area, which is why a drop on a flat pan pulls itself into a dome rather than a film. Evaporation happens only at the surface, so the smallest possible surface area is precisely the worst possible geometry for drying something quickly.

It also dries unevenly. The shallow edge of the pool reaches dryness long before the deep centre, so mass loss slows down while a meaningful amount of water is still trapped in the middle. A moisture analyzer set to stop automatically when mass loss falls below a threshold reads that slowdown as "finished" and reports a result that is too low.

The fibre matrix breaks the dome apart. Liquid wicks sideways into the sheet and is held as a thin film across a much larger area, so evaporation happens across the whole sheet at once. Tests finish faster, and — more importantly for QC — they finish at the same point every time, which is what repeatability actually means. A wicked sample also will not slop over the pan edge when the chamber closes.

Problem two: skinning and scorching bias the result in opposite directions

Sugary, fatty and protein-rich samples fail in two distinct ways, and it is worth separating them because they push the answer in opposite directions.

Skinning is when the surface dries and hardens into a crust while the interior is still wet. The crust seals the remaining moisture underneath it, mass loss slows sharply, and the instrument concludes the sample is dry. The reported moisture comes out too low — the water is still in there, behind the crust.

Scorching is the sample beginning to burn. Burnt organic matter is mass leaving the pan that was never water, and the analyzer cannot tell the difference; it only sees the balance reading fall. The reported moisture comes out too high.

This is why simply lowering the temperature is not a complete answer. It does reduce scorching, but it lengthens the test and can make skinning worse on sugary samples, because a long gentle bake is exactly how you set a crust. Spreading the sample thinner attacks the cause rather than the symptom, and that is what the sheet does — it holds the sample in a thin layer and lifts it clear of direct contact with the pan floor, where the temperature gradient is steepest.

Why glass fibre and not filter paper

The sheet sits inside the measurement, so anything it does to the mass reading becomes part of your result. Three properties matter, and glass fibre has all three.

It is chemically inert. Sauces are acidic, dairy is fatty, and cleaning-product samples can be strongly alkaline. A carrier that reacted with any of them would change mass on its own account and corrupt the measurement.

It survives the temperature. A&D's range dries from 30 °C to 200 °C in 1 °C steps, with a 400 W halogen lamp that reaches 200 °C at the pan in under two minutes. Ordinary cellulose filter paper is not rated for the upper end of that range, and any mass the carrier itself loses is counted by the instrument as your sample's moisture.

And it holds very little moisture of its own. Glass fibre picks up some ambient humidity, but far less than paper, and it releases it predictably — which matters for the taring step below.

How to use one correctly

The procedure is short, and one step accounts for most of the mistakes.

Tare with the sheet already in place. If the sheet goes on after the tare, its mass is counted as sample and every result is wrong. This is the single most common error with these consumables, and because it produces a plausible-looking number rather than an obvious fault, it can run undetected for a long time.

In a humid room — which describes most of the year in the UAE — run the pan and sheet through a short empty drying cycle before taring. Glass fibre absorbs some ambient moisture, and anything it releases during the test is added to your sample's moisture figure.

For liquids, apply the sample drop by drop across the whole sheet rather than pouring it in one place; even coverage is the entire point of the exercise. For pastes and semi-solids, spread rather than heap.

Then keep it consistent. A drying method is validated with a specific sample mass, temperature, heating pattern and carrier. Change the carrier — sheet to no sheet, or one sheet type to another — and it is no longer the method you validated. If a product is measured on a sheet, it should be measured on a sheet every time, and that belongs in the written procedure rather than in one operator's memory.

When you do not need one

Free-flowing powders, granules and pellets already form an even layer on the pan. Adding a sheet costs money, adds a tare step, and changes nothing about the result — so it is not good practice, just extra cost. Flour, milk powder, spice powders, sugar crystals and seasoned salt are all normal bare-pan samples.

Plastics are their own case. Low-moisture resins such as PET, ABS and PMMA sit at 1% moisture or below and are handled by A&D's dedicated plastic measurement guide and its PlTemp temperature search rather than by changing the carrier.

And if you already have a validated method that works without a sheet, leave it alone. The reason to introduce one is a specific diagnosed problem — a liquid that will not spread, or a sample coming out of the chamber with a burnt crust — not general improvement.

Which analyzers, and where to get the sheets

A&D's current heat-drying moisture analyzers are the MS-74A and MS-74AT (0.001% moisture readability), MX-53A and MX-53AT (0.01%), MF-53A (0.05%) and ML-53A (0.1%), all drying from 30 °C to 200 °C. They replaced the long-running MS-70, MX-50, MF-50 and ML-50, which map one to one onto the new models and remain in service across the Gulf — the sheets and the technique above apply equally to both generations.

The consumable is A&D part AX-MX-32-1, filtration glass fibre sheets. A&D Gulf holds them in stock at Jebel Ali alongside disposable aluminium sample pans, so a replacement is a delivery rather than an import.

If a particular product is giving you inconsistent moisture figures and you are not sure whether the carrier is the cause, it is usually diagnosable from the drying curve — its shape distinguishes a crust from a genuinely dry sample. Send us the product details and we can work through the method with you.