STRAIN GAUGES AND 4-20mA CALIBRATION

Written in conjunction with Henry Wijgerse of Tarka Systems in the Netherlands.

 

Challenge:

 

4-20mA data-acquisition devices are designed to accept sensors like accelerometers, load cells and full bridges (with integrated amplifier).

To measure the strain of a quarter-bridge input, the bridge must be completed with resistors to get a full bridge setup.

Additionally it must be set up for zero position and span.

 

Solution:

 

Our MRF Bridge Completion Modules (BCMs) contain precision Bulk Metal® Foil resistors to convert a single gauge into a full Wheatstone bridge in a  convenient, robust package – all the engineer needs to do is connect four wires from the BCM to the amplifier and three wires to the strain gauge.

To complete the setup to gain a 4-20mA signal, which reflects the strain in the quarter bridge, an additional module (Strain Gauge Amplifier, SGA) was added that can read a bridge signal and converts it into a 4-20mA signal.

 For calibration our Model 1550B calibrator can be substituted for the strain gauge to set the zero and span. In this case zero was set to 12mA (to allow plus and minus measurement) with a span of +/-12,000 microstrain. Simple push-button strain values makes this extremely easy.

 

The customer explains:

 

A ship monitoring system was built using 4-20mA inputs. This required readings in microstrain so it was essential to use a quarter bridge (one active strain gauge) with the bridge completion module providing the resistors necessary for compatibility with the amplifier.

For the test, this strain gauge and MRF-350-130 bridge completion module are preinstalled on a thin plate to check and verify the readings. Eventually the strain gauges and the BCM are installed directly on the hull (inside) of the ship.

The Model 1550B was used for calibration because it can mimic a quarter bridge and give known specific values of strain at the push of a button. With this setup all SGAs are calibrated and checked in-house. After that all strain gauges, BCM and SGA units are installed on the ship. The monitoring system could then be powered on site with confidence that it can start capturing calibrated data.

Layout of complete system: