MONTANA STATE UNIVERSITY   
DEPARTMENT OF MECHANICAL ENGINEERING                                   

ETME 360 Spring 2014  – Measurements and Instrumentation Laboratory

EXERCISE 6

PRESSURE MEASUREMENT APPLICATIONS:
Strain Gage Pressure Transducer

      R. Larson

Description:
In this laboratory exercise, students fabricate and calibrate an electronic pressure transducer. The device will be built using a single electrical resistance strain gage along with components from a commercially-available Bourdon-tube type pressure gage. Electrical response will be measured using a PC-based Computer-controlled Data Acquisition System to acquire the voltage output of the fabricated transducer as it is subjected to changes in air pressure.

Equipment:
Dual pressure-gage apparatus
Strain gage, cyano-acrylate (superglue) adhesive, soldering iron, solder, wire, connectors
PC with LabVIEW program
Data Acquisition System
Power supply, Decade  resistors
Safety Glasses

Discussion:
Bourdon-tube type pressure gages are the most widely used method pressure measurement device. Each gage design requires individual calibration to validate the geometries used in construction. Accuracy varies with device, but pressure values can typically be found to within about +/- 2 PSI.

Analysis of Bourdon tube dimensional changes in response to changes in internal pressure show a relatively linear deflection –vs- pressure relationship. This characteristic makes the Bourdon tube a good candidate for modification into an electrical strain transducer to permit strain to be monitored with computerized data acquisition equipment.

Laboratory Exercise Procedure:
1. Fabrication
  Remove gage housing, and bond a single strain gage to the Bourdon tube pressure gage  as demonstrated by the instructor. Surface preparation and careful handling is critical to effective bonding of the gage. Carefully solder test leads to gage leads, and create a Wheatstone bridge circuit using the gage as the active sensor and three fixed value decade resistors.  Note that initial (no-load) bridge balance requires all bridge resistances to be equal.

2.  Program Create (or modify) a LabVIEW data acquisition program to record Wheatstone bridge circuit output voltage. Your program architecture should address the following considerations:

*Your program should permit manual (keyboard) entry of set pressure values for each test point.
*Appropriate system gain settings should be selected for the application
*Include provisions to account for initial circuit imbalance @ zero gage pressure (i.e. account for an offset voltage.)
*Best sample accuracy is attained by storing the average (mean) value of a large number of samples.
*A single data point should be saved to a common test data file each time the program is run.  

 3. Test.  When all groups have completed setup and are ready to proceed, the instructor will increase regulated gage pressure incrementally while groups each record data.

 Results:

1. Plot output voltage versus pressure. Determine sensitivity of the pressure transducer created, complete with correct units. Thoroughly discuss any uncertainties in the experiment and results.

2. Include a printout of your program front panel and diagram in your written report.

Safety Notes: During this laboratory you will encounter high temperatures (soldering iron), high pressures (upper test limits of pressure test) and chemical hazards (super glue, solder smoke). The laboratory environment can be congested. Use safety glasses while soldering and while pressure testing, and do not rush through the laboratory steps. BE CAREFUL!