Tell-tale crack monitors or the G1 vernier gauge: which reading principle for which situation?

Monitoring a crack is not a matter of measuring its width once. What matters is knowing whether it is moving, in which direction and how fast. The principle is always the same: a fixed reference on either side of the crack, and readings spaced out over time, each compared with an initial measurement.

Two main families of mechanical instrument have answered that requirement for decades. They are not read in the same way, and that detail determines what you can expect from them.

Two families of instrument, one requirement

• Overlapping-plate crack monitors, commonly known as tell-tale crack monitors: two plates that overlap, a millimetre grid on the lower plate and a crosshair cursor on a transparent upper plate.
• The Saugnac G1 gauge, with a vernier scale: a graduated body and a sliding arm carrying a vernier that reads to a tenth of a millimetre.

The overlapping-plate reading principle

The instrument consists of two plates fixed separately, one on either side of the crack. The lower plate carries a grid graduated in millimetres; the transparent upper plate carries a crosshair cursor. As the crack opens, closes or shears, the cursor moves across the grid and its position is read directly in X and Y.

It is a proven principle, used for decades by surveyors and engineering consultancies across the UK, and it gives two axes of measurement on a single device.

The principle behind the Saugnac G1 gauge

The vernier is a reading device that has been in use since the 17th century. It is the one found on a vernier caliper: a second scale, slightly offset from the first, makes it possible to read a fraction of a division without having to estimate it by eye. On the G1 gauge, ten vernier divisions span nine millimetres of the main scale, which gives a reading to 0.1 mm.

Reading takes two steps: the millimetres are read on the main scale, to the left of the vernier’s zero mark; the tenth corresponds to the vernier line that aligns exactly with a line on the main scale. The method takes a minute to learn and is unambiguous from then on – two different operators read the same value.

In the example shown here, the triangle sits just to the right of the 13 mark on the main scale, so the reading starts at 13 mm; the vernier line that coincides with a line on the main scale is the eighth, which gives a reading of 13.8 mm.

Advantages of the G1

• A reading to a tenth of a millimetre, a scale division ten times finer than a millimetre grid.
• Two fixing points only: two technical adhesive pads supplied, or two screws through the pre-drilled Ø4 mm holes.
• A single-piece instrument, with no relative alignment of two plates to manage during installation, made of a flexible material that follows a slightly convex or concave surface.
• Corner mounting with no additional accessory, thanks to the fold line built into the sliding arm. The procedure is described in our guide How do you install a G1-type crack monitor?
• A unique QR code on every gauge and automatic reading from a smartphone, with the free Saugnac.app application.

Turn on the camera in the app and point it at the gauge: no manual entry, no risk of transcribing a value onto the wrong crack. The app identifies the instrument from its QR code, reads the vernier and records the value, the date, the operator and a photograph of the reading, then corrects the measurement for temperature.

Limitations of the G1

The G1 gauge monitors a single axis: the opening of the crack. To follow opening and shear at the same time, move up to the G6 gauge, which applies the same principle with two vernier scales and keeps the 0.1 mm reading on both axes.

The two principles side by side

Where the G1 has the edge

Slow movement over a long period. A resolution of 0.1 mm makes it possible to detect and follow small changes more closely: a displacement of 0.3 mm over six months is read directly on the vernier, with no need to estimate it between two graduations. On a case where the question is whether the crack is still moving or has stabilised, that fineness changes the nature of the answer.

An irregular substrate, a corner, or a high-level installation. Two fixing points are better than four, and a single-piece gauge follows a slightly convex or concave surface. The fold line built into the sliding arm allows mounting in an internal corner with no accessory.

Monitoring across several sites or with several people involved. Because every gauge carries a unique identifier, readings are attached to the right instrument automatically, whoever takes them.

Structuring the monitoring, not just the measurement

Every Saugnac gauge carries a QR code that identifies it uniquely. From the free Saugnac.app application, you turn on the smartphone camera and point it at the gauge: the G1 is read automatically. The app recognises the instrument, reads the position of the vernier and records the value, the date, the operator and a photograph of the reading.

The application also provides:

• the full history and evolution graph for each crack;
• the option to attach photographs to a reading, for traceability;
• automatic retrieval of temperature and humidity by geolocation, and correction of the measurement for thermal expansion;
• alert thresholds, with notification to several recipients;
• several people working on the same gauge, and data sharing without the need to create an account;
• Excel export of the data.

The point is not presentation – a set of readings kept by hand can be perfectly rigorous. It is that data collection becomes simpler and more secure: the gauge is identified with no risk of confusion, the value is read and recorded on site, and the photograph of the reading stays attached to it.

Beyond the vernier reading: digital and wireless

For a digital reading taken on site, the E1 gauge works on the removable-instrument principle: only the transparent mounting plates stay fixed on either side of the crack, and the digital crackmeter is positioned on them for the time it takes to read, with a digital display to 0.01 mm. It is the answer where vandalism is a concern, since nothing fragile is left on the wall, and where a number of cracks have to be read with a single instrument.

When the reading frequency becomes a constraint – a remote site, an inaccessible crack, a construction phase that needs close attention – manual monitoring reaches its limit: the cost is not the instrument, it is the journey. The R1 gauge then takes the reading automatically and transmits it to the same application.

If you are unsure which instrument suits your situation, contact us.

The key points to remember

Frequently asked questions

What is a tell-tale crack monitor?

A tell-tale crack monitor is a mechanical instrument made of two overlapping plates fixed on either side of a crack. The lower plate carries a grid graduated in millimetres and the transparent upper plate carries a crosshair cursor. As the crack moves, the cursor shifts across the grid and its position is read in two axes. The term is used generically in the English-speaking construction industry for this type of gauge.

What is the difference between a tell-tale crack monitor and a vernier crack gauge?

The difference lies in the reading principle. A tell-tale is read by locating a crosshair on a grid graduated in millimetres, in two axes. A vernier gauge such as the G1 is read on a sliding scale that gives a division of 0.1 mm, on one axis. The vernier also needs two fixing points instead of four, and can be read automatically with a smartphone.

Can a mechanical crack gauge be read automatically?

Yes. Every Saugnac gauge carries a unique QR code. In the free Saugnac.app application, turning on the smartphone camera and pointing it at a G1 gauge is enough for the app to identify the instrument, read the vernier and record the value, the date, the operator and a photograph of the reading.

Which gauge should be used to monitor shear as well as opening?

A single-axis gauge such as the G1 measures the opening of the crack only. To monitor opening and shear at the same time with a vernier reading of 0.1 mm on both axes, use the G6 gauge. For an out-of-plane movement, or differential settlement, a dedicated instrument such as the G3 gauge is required.