G1 Gauge – Indoor / outdoor crack monitor

The G1 gauge is a vernier crack monitor, also known as a crack gauge, that measures changes in crack width to 0.1 mm. Fixed across the crack, it shows whether the crack is opening, closing or stable, and quantifies that movement through successive readings.

A plaster crack monitor (a plaster patch bridging the crack) only shows that movement has occurred: it cracks or comes loose, without indicating the size or direction of the movement. The G1 gauge gives a numerical value at each reading, for both opening and closing, over a range of approx. 20 mm. For a detailed comparison, see our sheet the G1 crack monitor, an alternative to plaster crack monitors.

A tell-tale crack monitor is made of two overlapping plates, a graduated one and a transparent one with a cross-hair, fixed on either side of the crack; the reading is taken where the cross-hair sits on the printed grid. The G1 gauge is a one-piece gauge with a sliding arm and a vernier, read to 0.1 mm. The two devices are compared point by point in our sheet tell-tale vs vernier crack monitors.

Yes, as long as the crack lies between the gauge’s two fixing points, approx. 15 cm apart: the initial crack width does not affect the choice of gauge. What matters is the expected movement: the G1 gauge follows it over a range of approx. 20 mm. For a wide crack with large movements, see the G1.3 gauge.

By comparing each reading with the first one, which serves as the reference (see how to read the G1 crack monitor). A crack whose width remains stable from one reading to the next is considered stabilised; a change that continues or increases indicates an active crack. Interpreting the readings and making a diagnosis is a matter for a professional (structural engineer, surveyor).

There is no absolute rule: the reading frequency should be agreed with the professional monitoring the building, depending on the context and the level of risk. Some cracks open and close with the seasons, particularly during droughts and with clay shrinkage and swelling. To observe these variations, one reading per season is a bare minimum. More frequent readings, for example monthly, give a more detailed picture of the movement and allow earlier action.

The gauge expands slightly with temperature: approx. 9 µm per degree, i.e. less than 0.1 mm for a 10 °C difference. The effect is therefore small compared with the gauge’s resolution, but the Saugnac application and the Excel file can correct each reading if you note the temperature at the time of the reading.

No: the G1 gauge measures movement on a single axis, perpendicular to the crack (opening and closing). To also monitor sliding along the crack, use the G6 gauge, which measures movement on 2 axes in the plane.

Yes. The gauge is made of PVC with a UV stabiliser, and the adhesive strips supplied remain effective from -40 °C to +90 °C. On crumbly, dusty or damp surfaces, screw fixing is preferable. In high-traffic areas, a protective cover reduces the risk of damage.

According to our specifications and tests, the G1 gauge withstands UV exposure for at least 7 to 10 years.

Yes, for gauges with a QR code: the free Saugnac application automatically reads the measurement with the smartphone camera and saves it in your monitoring account.

This can be due to parallax error: the vernier and the main scale are not in the same plane, and if your eye or the smartphone is not directly facing the gauge, the alignment of the lines can shift by one line, i.e. 0.1 mm, roughly the thickness of a hair. Position yourself directly in front of the vernier; with the Saugnac application, you can also take several readings and average them (see parallax error). To read the tenths without having to look for aligned lines, see the G1+ gauge.

The G1 gauge is sold in boxes of 10. For smaller quantities, the G1.2 gauge is sold in boxes of 5: it is transparent, offers the same performance as the G1 and also has a unique QR code.