The Application of Measurement Assurance Techniques in a High Capacity Automated Flowmeter Calibration Facility

The paper describes a study that forms part of a measurement assurance program (MAP) in place at Ford Motor Company. The purpose of a MAP is to assure on an ongoing basis that the total uncertainty associated with a measurement process meets the requirements of the customer. The gage repeatability and reproducibility study and statistical control chart are two traditional parts of the Ford MAP. This paper presents a new tool, the Youden plot, which combines features of both of these traditional techniques. This paper is divided into two parts: First, the test part, measurement process and traditional MAP program are described. Second, the new technique is described and applied to historical data. 1. TEST PART AND MEASUREMENT PROCESS To meet the requirements set forth by the Clean Air Act for vehicle tail pipe emissions and fuel economy requirements set by the CAFE Standards, Ford Motor Company has chosen a mass air flow sensor (MAFS) based powertrain control system. The sensor and calibration process are described in this section. Test Part The MAFS manufactured at Ford’s North Penn Electronics Facility determines mass flow by using a constant temperature thermal anemometer. The sensor operates in the following manner: The flow of electrical curent through a sensing element, called a hot wire, results in a temperature rise. The resistance of the hot wire varies with temperature and is maintained at a specified value by a control circuit. As air flows across the wire its temperature decreases due to convective cooling. The control circuit then increases the flow of current to increase hot wire temperature. The current required to maintain the hot wire at the specified temperature is related to the mass flowrate. The output of the circuit is a voltage that varies between 0.5 and 4.75 volts. In order to differentiate between changes in mass flowrate and flowing temperature, a second element, called a cold wire, is included in the MAFS. The resistance of the cold wire is used to control the resistance, and therefore the temperature, of the hot wire. Additional details regarding the MAFS are given in Ref. 1. Measurement Process The End Of Line (EOL) flow test is the final step in the manufacturing process. Each production sensor is flow tested on one of thirteen EOL test stands to confirm that the transfer function is within required specifications. Each test stand is based on an array of twelve critical flow venturies (CFVs) that is used to control and measure mass flowrate. The venturies are arranged in a binary sequence which means that each unit flows twice as much air as the next smaller one (eg. 0.21 kg/hr, 0.42 kg/hr, 0.84 kg/hr, etc.). This design allows the test stand to generate 212 different flowrates.