Bolt Tension Monitors

The Dakota NDT ultrasonic bolt tension monitors are instruments dedicated to the measurement of controlled tightening of critical flanged and bolted connections. They use the ultrasonic principle to measure the bolt elongation directly with an accuracy of ±0.0001 mm, and from it calculate time of flight in nanoseconds, applied load, stress and percentage strain. Unlike traditional torque-wrench techniques — affected by friction, lubrication, temperature and thread surface conditions — ultrasonic measurement provides the actual tension value applied, reducing errors from the 30-40% typical of torque to fractions of a percent.

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BT1-DL, BT2-DL and MultiMax comparison

Feature	BT1-DL MiniMax	BT2-DL Max II	MultiMax
Part Number	Z-156-0008	Z-197-0001	—
Form factor	Portable	Benchtop	Fixed installation
Weight	383 g	2.04 kg	383 g
Elongation accuracy	±0.0001 mm
Max fastener length	Standard	Up to 30 metres	Up to 15.2 metres
Measurement channels	1	1	4 multiplexed, stackable modules
Pulser types	1 (Square Wave)	3 (Spike, Square, Tone Burst)	1 (Square Wave)
Voltage	100/150/200 V	100-400 V	100/150/200 V
Gates	1	3 independent	1
Power supply	3 × AA (35 h)	Li-Ion (18 h) / 6 × AA (12 h)	Mains: USB to PC or AC adapter
USB	USB-C	USB-A/B	USB-C
Control	On board	On board	PC only (MultiUI software)
Scenario ideale	Field, periodic monitoring	Certified assembly, long bolts	Continuous unattended monitoring

BT1-DL and BT2-DL are spot-measurement instruments: you take the instrument to the bolt and read the value. The MultiMax belongs to a different category — it is a 4-channel remote monitoring system that stays installed on the plant, with permanently bonded transducers, and returns not a single measurement but the trend of preload over time, with automatic temperature compensation.

How to choose between the models

The ultrasonic method vs the torque wrench

When a bolt is tightened, the pull of the nut increases the applied force, causing an elastic elongation of the bolt itself. This deformation is directly proportional to the actual tension in the threaded assembly. The ultrasonic method measures the time of flight through the whole bolt: the greater the elongation, the greater the time of flight. Knowing the material properties, the instrument calculates all the relevant quantities.

The torque wrench, by contrast, measures the resistance to tightening, which depends on friction, lubrication, temperature and nut condition — factors that introduce errors of 30-40% on the actual tension. On critical bolts (aerospace, nuclear, wind, offshore) this margin is unacceptable. Further reading: Ultrasonic measurement vs torque wrench in bolt tightening.

Typical applications

Common features of the portable models

Standards and certifications

• ASME PCC-1 — Guidelines for Pressure Boundary Bolted Flange Joint Assembly
• EN 1591-1 — Flanges and their joints
• ASTM E1685-20 — ultrasonic measurement of bolt elongation (declared for the MultiMax)
• NIST-traceable factory calibration and MIL-STD-45662A
• CE marking and EMC compliance
• Calibration certificate supplied as standard

Application guides from the blog:

• Bolt tension in wind turbines: the ultrasonic method with Dakota
• ASME PCC-1: the reference standard for controlled flange tightening
• Ultrasonic measurement vs torque wrench
• Data logger vs manual recording

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Do you have critical tightening to manage?We will support you in the application analysis (bolt type/length/material, inspection frequency, required certification) to propose the right model with the optimal probe and accessories. Contact us for dedicated advice or for a demonstration on your critical bolts.