OverviewofTemperatureMeasurement.pptVIP

Overview of Temperature Measurement Outline Thermocouples overview, reference junction, proper connections, types, special limits of error wire, time constants, sheathing, potential problems, DAQ setup RTDs overview, bridges, calibration, accuracy, response time, potentail problems Thermistors Infrared Thermometry fundamentals, emissivity determination, field of view Other Non-electronic measurement, thin-film heat flux gauge Temperature Controllers How to Choose Standards, cost, accuracy, stability, sensitivity, size, contact/non-contact, temperature range, fluid type Thermocouples Seebeck effect If two wires of dissimilar metals are joined at both ends and one end is heated, current will flow. If the circuit is broken, there will be an open circuit voltage across the wires. Voltage is a function of temperature and metal types. For small DT’s, the relationship with temperature is linear For larger DT’s, non-linearities may occur. Measuring the Thermocouple Voltage If you attach the thermocouple directly to a voltmeter, you will have problems. You have just created another junction! Your displayed voltage will be proportional to the difference between J1 and J2 (and hence T1 and T2). Note that this is “Type T” thermocouple. External Reference Junction A solution is to put J2 in an ice-bath; then you know T2, and your output voltage will be proportional to T1-T2. Other types of thermocouples Many thermocouples don’t have one copper wire. Shown below is a “Type J” thermocouple. If the two terminals aren’t at the same temperature, this also creates an error. Isothermal Block The block is an electrical insulator but good heat conductor. This way the voltages for J3 and J4 cancel out. Thermocouple data acquisition set-ups include these isothermal blocks. If we eliminate the ice-bath, then the isothermal block temperature is our reference temperature Software Compensation How can we find the temperature of the block? Use a thermister or RTD