G2 Gauge – Min-max recording crackmeter

Technical specifications

G2 gauge overview

The G2 gauge is a mechanical crackmeter designed to measure and record changes in the width of an expansion joint, the opening of a crack’s edges and/or, more generally, changes in the gap between 2 elements.

This movement is recorded graphically as a line showing the minimum/maximum amplitude of the movement. A removable cover, ventilated to prevent condensation, protects the device. The cover can be sealed: two security seals are supplied with the gauge to secure the device if required.

It consists of the gauge body and a folding bar, each fixed on either side of the crack. The lug on the folding bar engages in the slotted hole of the movable arm, which carries the measuring scale graduated from 0 to 40 mm.

As it moves, the movable arm drives the lead holder arm. The lead, housed at its tip, is kept under pressure by a polycarbonate disc and traces the movement on the amplitude monitoring scale.

The gauge works by comparative measurement: the first reading is the reference, and subsequent readings are compared with it to determine the change.

After reading, simply erase the trace with a damp cloth or a little alcohol to reuse the gauge.

Why record the min-max of a crack?

A standard crack monitor only shows the crack width at the time of reading. Between two readings, the crack may open and then close again without this movement being visible. The G2 gauge keeps a record of both extremes reached since installation or since the trace was last wiped off.

• Cracks caused by clay shrinkage and swelling: cracks open during dry periods and partly close again when moisture returns. The min-max record shows the actual amplitude of the seasonal cycle. See what causes cracks in buildings.
• Expansion joints: the gap varies with temperature, between day and night as well as between summer and winter. The G2 gauge records the extreme opening and closing of the joint.
• Structures inspected at long intervals: historic monuments, civil engineering structures, car parks, hard-to-access premises. The min-max record reveals movements that occurred between two visits, even when they are far apart.
• Nearby construction work: during work on an adjacent site, the trace keeps the maximum movement reached between two visits. The sealed cover protects the device against tampering.

The reading frequency depends on the context and the risk level of the structure. Reading at least once per season makes it possible to follow seasonal variations; more frequent readings give more detailed monitoring.

Fixing the G2 gauge

• Bonding: with the self-adhesive pads on the gauge body and the folding bar, if the surface is smooth, clean and dry. If the surface is not perfectly flat and is uneven, it is recommended to reinforce the bond with a two-component epoxy adhesive (not supplied).
• Mechanical fixing: on any difficult substrate (crumbling, dusty, damp or uneven), or when the installation temperature cannot be respected, mechanical fixing should be preferred. The G2 gauge is supplied with impact anchors, fitted through the two Ø4 mm holes in the gauge body and the hole in the folding bar.
• In a corner: the folding bar can be bent to the required angle. In cold weather (below 10 °C), warm it in your hands for a few seconds before bending it. For environments subject to high stress, the aluminium angle bar is available as an add-on.

Reading the measurement

Read according to the vernier principle. Note: the graduations increase from right to left.

• The scale graduated in mm from 0 to 40 mm, on the movable arm: this is the measuring scale.
• The fixed graduation with the triangle, on the gauge body: this is the 1/10 mm vernier (10 vernier divisions correspond to 9 mm on the measuring scale).

a) Reading the millimetre

The vernier triangle lies between two graduations of the measuring scale. The number of millimetres corresponds to the graduation to the right of the vernier triangle: 20 mm in the example.

b) Reading the decimal

Look for the vernier line that coincides with a graduation on the measuring scale. In the example, vernier line 7 coincides with graduation 27 of the measuring scale. The decimal value is therefore 0.7 mm.

The reading in the example photo is therefore 20.7 mm.

c) Reading the min-max

The mark left by the lead on the amplitude monitoring scale is read directly in mm, to within approx. 0.5 mm. Its two ends give the minimum and maximum values reached since the trace was last wiped off. For example, a trace running from 15 mm to 28.5 mm indicates an amplitude of 13.5 mm.

Readings can be entered and stored in the Saugnac app.

G2 gauge or another gauge?

To compare the whole range: which gauge for which crack?

G2 gauge FAQ

What is the difference between the G2 gauge and a G1 crack monitor?

The G1 gauge shows the crack width at the time of reading. The G2 gauge also records, with a lead trace, the maximum opening and minimum closing reached between two readings.

Does the G2 gauge need a battery?

No. Recording is entirely mechanical: no battery, no electronics.

Does the G2 gauge transmit measurements remotely?

No. It is read on site by removing the cover. To receive measurements remotely, see the R1 wireless crackmeter.

Can the G2 gauge be used to monitor an expansion joint?

Yes. The G2 gauge measures the width of any gap between 2 elements, within a range of approximately 25 mm between the minimum and the maximum.

How do I reuse the G2 gauge?

After reading, simply erase the trace with a damp cloth or a little alcohol. A new recording then begins.

What are the security seals for?

They seal the protective cover, so you can check at each visit that the device has not been opened. Using them is optional.