Purpose: The instrument is newly designed device for large borehole diameter determination. It has been widely used for large borehole diameter determination in hydrology, railway, and bridge construction departments; Technical parameters 1. Measurement range: Ф600mm～Ф1800mm; If equipped with an extending leg of 770 mm, its measurement range can be extended to 2800 mm; 2. Measurement error: ＜±25mm; 3. Maximum working pressure: 20 MPa; 4. Maximum working temperature: 90 ℃; 5. Weight: 25 kg; 6. Size: Φ450╳2200mm;