G1 Gauge

Technical specifications

How to install the G1 crack monitor?

Two mounting methods are available:

• Adhesive bonding: quick and easy mounting with the double-sided adhesive strips supplied. The surface must be smooth, dry and clean so that the adhesive is in contact over its entire area.
  If necessary, especially on rough surfaces (e.g. render), bonding can be reinforced with a fast-curing twin-syringe epoxy glue.
• Mechanical fixing: G1-family gauges are supplied with 2 pre-drilled Ø 4 mm holes. These holes make mechanical fixing with screws easy. A 4 mm hole can be drilled very easily in any substrate.
  On difficult substrates (crumbly, dusty, rough or damp surfaces), we recommend screw fixing to prevent any detachment. Packs of 50 screws and wall plugs are available, as well as a ready-to-use mechanical mounting kit including a 4 mm drill bit, a screwdriver and 40 wall plugs and screws.
  

Installation tips

To measure crack movement as effectively as possible, the gauge is usually fixed where the crack is widest (often in the middle of the crack). The gauge can be fixed whatever the direction of the crack (vertical, horizontal, diagonal), straddling the crack, to measure changes in its width. The G1 gauge is made of a material flexible enough to adapt to irregular, convex or concave surfaces.

Ideally, position the vernier triangle between 10 and 11, with the 2 speckled bands visible on the left and right. The aim is to centre the sliding arm. Do not try to position the vernier triangle on 0: it is unnecessary and very difficult to achieve. The principle is to measure the difference between 2 readings: whatever the value of the first reading, it becomes the zero point.

If the crack is narrow and you want to maximise the opening travel, you can position the vernier closer to zero.

Note: the G1 gauge does not measure movement along the Y axis (sliding along the crack). For this, use a G6 gauge, which measures movement on 2 axes in the plane.

How to measure a corner crack?

New versions of the G1 gauge (from mid-2025) are manufactured with a groove marking a fold line on the sliding arm. This groove makes it easy to fold the sliding arm for corner mounting without any additional accessory.

Earlier versions of G1-family gauges were supplied with a foldable strip for corner mounting. This part has a thin section designed to guide and ease folding.

An aluminium angle bar (sold separately) can also be used.

The internal corner forms a 90° angle: G1 gauge installed by folding the sliding arm at the grooved area.

The internal corner is irregular: G1 gauge installed with an intermediate plate (sold separately).

G1 gauge installed in a vertical internal corner with an aluminium angle bar fixed with impact anchors.

G1 gauge installed on a crack at an external corner with an aluminium angle bar.

How to protect the G1 crack monitor?

Gauges installed in public or high-traffic areas (schools, shops, streets…) may be damaged (torn off, graffiti…).

To protect the gauge, we offer a UV-resistant protective cover. The cover is fixed with the wall plugs and screws supplied with it.

In high-traffic areas where the risk of damage is high, the removable E1 gauge with transparent platines can be used instead. After each reading, only the platines remain on the surface, which prevents any damage.

How to read the G1 crack monitor to 0.1 mm?

The upper scale is graduated in mm from 0 to 30: this is the main scale.

The lower scale is movable: this is the vernier, reading to 0.1 mm (10 vernier divisions correspond to 9 mm on the main scale).

Taking a reading

a) Reading the millimetres:
The vernier pointer lies between two graduations of the main scale, for example between 13 and 14. The number of mm is the graduation to the left of the vernier pointer : 13 mm in this example.

Download the Saugnac.app application to track your readings

The Saugnac web application, completely free and unlimited, is available on PC or smartphone at https://saugnac.app. It lets you:

• identify each gauge and its readings with its unique QR code;
• automatically read G1, G1.1 and G1.2 gauges;
• save readings in your account;
• retrieve the temperature and humidity via geolocation;
• calculate readings corrected for thermal expansion based on temperature;
• collaborate with several users on the same gauge;
• manage alert thresholds;
• sort gauges by site and locate them on a map;
• download data in Excel format;
• display graphs automatically;
• share data with other people without an account;
• access the application from your PC or smartphone;
• add readings offline.

Find all the explanations on how the application works.

Download the Excel monitoring file

You can also download the Excel file for tracking G1 readings .

This Excel file is very easy to use: simply duplicate a sheet for each site or project monitored, with each sheet tracking up to 10 gauges.
The file takes into account the expansion coefficient of the gauge body and the sliding arm, adjusting each reading according to the difference between the temperature at the time of the reading and that of the first reading. If you did not record the temperature, simply leave it blank: thermal expansion will then not be taken into account.

Note: the effect of thermal expansion on G1 readings is small (approx. 9 µm per degree).

Frequently asked questions about the G1 crack monitor

What is the G1 gauge used for?

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.

What is the difference between the G1 gauge and a plaster crack monitor?

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.

What is a tell-tale crack monitor, and how does it compare with the G1 gauge?

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.

How do I know if a crack is still moving?

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).

How often should the gauge be read, and for how long?

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.

Does temperature need to be taken into account?

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.

Does the G1 gauge measure sliding along the crack?

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.

Can the G1 gauge be installed outdoors?

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.

Can the G1 gauge be read with a smartphone?

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.

What should I do if the crack exceeds the gauge’s range?

The measuring range of the G1 gauge is approx. 20 mm. If the vernier approaches its end stop, install a new gauge and continue monitoring using its first reading as the new reference, while keeping the history of the first gauge.