pH Meter, Gloss Meter, Rub Tester & Caliper

In offset lithographic printing, the fountain solution (also called dampening solution or fount) is the water-based liquid that wets the non-image areas of the printing plate, preventing them from accepting ink. The chemical properties of the fountain solution, its pH and its electrical conductivity, are the most important variables affecting offset print quality after ink density. A fountain solution that is outside its correct pH and conductivity range causes scumming, tinting, slow ink drying, ink-water imbalance, and colour shift, defects that cannot be corrected at the press until the root cause (fountain solution chemistry) is addressed.

pH measures the acidity or alkalinity of the fountain solution. Conductivity measures the total dissolved solids in the solution, primarily the fountain concentrate, isopropanol, and any contamination absorbed from the paper, ink, or environment. Both measurements are made using a combined pH/conductivity meter, which costs approximately ₹3,000–₹15,000 for a quality unit suitable for press room use. Every offset press room should have one. Measuring fountain solution pH and conductivity takes under 2 minutes per press and is the single most effective preventive quality action available to an offset press room.

pH targets

Conductivity targets

Conductivity measures the total dissolved solids in the fountain solution, expressed in mS/cm (millisiemens per centimetre) or µS/cm (microsiemens per centimetre). The fresh-mixed fountain solution has a specific conductivity that depends on the concentrate, IPA, and water used. During the press run, conductivity increases as paper fibres, coating particles, ink residues, and other contaminants accumulate in the solution. High conductivity indicates a contaminated fountain solution that should be refreshed.

Measurement procedure

• Calibrate before use, calibrate the pH electrode using two buffer solutions (pH 4.0 and pH 7.0 buffers are standard). Calibrate at the start of each shift. A pH meter that has not been calibrated today is producing unreliable readings.
• Rinse the electrode, rinse the electrode with deionised water and blot (do not rub) with a soft tissue between each measurement to prevent cross-contamination between solutions.
• Allow stabilisation, insert the electrode in the fountain solution sample and wait for the reading to stabilise (typically 15–30 seconds) before recording. Do not measure from a sample taken directly from a running fountain, allow the sample to reach room temperature first.
• Measure temperature too, pH readings are temperature-dependent. Most modern meters have automatic temperature compensation (ATC), verify this feature is active. If not, record the temperature alongside the pH reading.
• Frequency, measure pH and conductivity at the start of each press run and every 2 hours during the run for multi-shift or long-run production.

Electrode care

The pH electrode is the most maintenance-sensitive part of the instrument. Keep the electrode bulb constantly moist, store it in the electrode storage solution provided with the instrument, never in distilled water (which leaches ions from the electrode and damages it) and never dry. Replace the electrode when calibration cannot be achieved within ±0.05 pH of the buffer values or when the response time slows significantly, typical electrode life is 12–18 months with correct care.

A gloss meter (also called a reflectometer or glossmeter) measures the specular reflectance of a surface, how much of an incident light beam is reflected in the mirror direction, expressed in Gloss Units (GU). It is the instrument for verifying that lamination and varnish finishes meet their specified gloss level, confirming the gloss-matte contrast of spot UV effects, and detecting process problems in the laminating or coating operation.

The full gloss measurement reference, measurement angles, GU targets by surface type, and the complete substrate-to-GU table, is covered in the Gloss Measurement article in the Quality Testing section. This entry covers the instrument itself: how it works, how to calibrate it, and the practical measurement procedure.

How a gloss meter works

The gloss meter projects a light beam at a defined angle to the surface (20°, 60°, or 85° depending on the measurement geometry selected). A detector positioned at the specular reflection angle measures how much of the incident light is reflected in that direction. This value is compared against a polished black glass reference standard (typically defined as 100 GU at 60°) and expressed as a percentage of that reference, the GU value. A perfectly matte surface reflects nothing in the specular direction (0 GU); a perfectly mirror-like surface reflects everything (approaching 100 GU).

Instrument selection

For most press room applications, a three-angle gloss meter (20°/60°/85° selectable) is the appropriate instrument. Single-angle meters (60° only) are adequate for most applications but cannot measure ultra-high-gloss or ultra-matte surfaces accurately. Major instrument suppliers: BYK-Gardner (micro-TRI-gloss), Rhopoint (IQ meter, which also measures haze and distinctness of image), Elcometer, and PCE Instruments. Cost range: ₹15,000–₹80,000 depending on instrument capability and the number of measurement parameters.

The Sutherland rub tester (TAPPI T830, ISO 11628) applies a defined weight to a rub material and moves it across the printed surface for a defined number of strokes, simulating the abrasive handling a printed surface will experience in its application. It is the instrument for testing whether ink, varnish, or lamination will scuff or transfer during packaging handling, book use, retail display, or product transit. The complete test method, dry vs wet rub, and pass criteria for different applications are covered in the Adhesion and Rub Testing article in the Quality Testing section. This entry covers the instrument and its calibration.

The Sutherland rub tester · instrument description

The standard Sutherland rub tester consists of a flat base plate that holds the test sample, a weighted rub block (2 lb or 4 lb) that holds the rub material, and a motor that drives the rub block back and forth across the test sample at a defined stroke speed (typically 21 strokes per minute for the standard instrument). The instrument counts the number of rub cycles and stops automatically at the preset count.

Instruments are available from Thwing-Albert, Testing Machines Inc. (TMI), and Presto Group (which manufactures the most widely used version in Indian press rooms). The basic Sutherland rub tester costs approximately ₹25,000–₹60,000 in India. It requires no calibration for routine use, the weight is a physical standard and the stroke count is electronically controlled. The only maintenance required is ensuring the rub base plate is clean and flat before each test.

Rub weight selection

• 2 lb (910g) weight: standard test weight for most commercial print and packaging applications. Used for testing under normal handling conditions.
• 4 lb (1,820g) weight: aggressive test weight for applications where heavier contact abrasion is expected, book covers in educational use, packaging in rough transit, industrial labels.
• Always specify the weight used when recording rub test results, "50 rubs" without specifying 2 lb or 4 lb is an incomplete specification.

• Condition the samples: condition test samples at 23°C ±1°C, 50% ±2% RH for a minimum of 24 hours before testing. This is mandatory, rub resistance of ink and lamination depends strongly on moisture content of the substrate.
• Prepare the rub material: for standard dry rub, cut a 100mm × 100mm piece of unprinted 80 GSM white offset paper. For ink-to-ink rub (testing whether a printed surface transfers ink to itself), use a printed sample of the same material. For wet rub, dampen the rub paper with 0.1 ml of distilled water immediately before the test.
• Set up the instrument: clamp the test sample face-up on the base plate. Clamp the rub material face-down on the rub weight. Set the stroke counter to the required number (25, 50, or 100 as specified).
• Run the test: start the instrument and allow it to run to the preset count. Do not touch the instrument during the test.
• Evaluate: after the test, examine: (a) the test sample surface, for visible scuffing, ink removal, or surface marking; (b) the rub paper, for ink transferred from the test surface to the rub paper. Grade visually on a 1–5 scale or compare against reference standards. Record condition, weight, rub count, and result.

Paper and board thickness (caliper) is one of the most important physical properties for packaging production, it determines the structural performance of the finished pack, the correctness of the crease rule depth in die-cutting, and whether the product fits the filling line tooling. A board that is 10% thinner than specified may make packs that fail to erect correctly. A board that is 10% thicker may jam in automatic filling machines.

Two instruments measure thickness in the press room: the micrometer (a hand-operated precision measuring instrument for single sheets and films) and the caliper gauge (a bench-mounted instrument with a defined pressure that measures under standardised conditions as specified by ISO 534 or TAPPI T411).

The paper and board micrometer

A paper micrometer (also called a thickness gauge) is a handheld instrument with two flat measuring faces, an anvil and a spindle, that close against the material being measured. The distance between the faces is the thickness. Most paper micrometers have a ratchet mechanism that applies a standardised measurement pressure (typically 100 kPa as specified in ISO 534) to ensure consistent readings regardless of how hard the operator squeezes. Cost: ₹3,000–₹20,000 depending on accuracy and resolution.

Why measurement pressure matters for paper caliper

Paper is compressible. Under higher measurement pressure, paper appears thinner, the same sheet measured at 10 kPa and at 200 kPa will give measurably different thickness readings. This is why ISO 534 specifies a defined measurement pressure (100 kPa for single-sheet measurement), without a defined pressure, caliper measurements from different instruments or operators are not comparable. Always use a micrometer with a ratchet or other pressure-limiting device when measuring paper caliper to a specified standard.

When to measure caliper in production

• Incoming material verification: measure caliper on every new batch of board before production. Caliper variation between batches from the same supplier can be significant, particularly for board grades at the lower end of the GSM specification range. A board specified as 300 GSM arriving at 265 GSM is not necessarily a supply quality failure, GSM specifies weight, not caliper. But if the carton design was dimensioned for 300 GSM at 0.40mm caliper and the actual board is 0.33mm, the crease rules will be incorrect and the carton may not erect properly.
• Die-cutting setup: the crease rule depth must be set for the actual board caliper, not the nominal caliper. Always measure the actual caliper of the board being run and set the crease rules accordingly. A 0.05mm difference in board caliper can be the difference between a clean crease and a cracked one.
• Quality disputes: caliper measurement provides objective evidence in disputes about whether supplied board meets its specification. Document the measurement instrument used, the measurement conditions, and the number of samples measured.