Experimental Measurement to Determine Fine Dry-Bulb and Wet-Bulb Thermocouple Response Times

摘要

An experimental apparatus was built and used to measure the response time of very fine wet-bulb thermocouples in air. Rapid response solenoid valves (15 m/sec response time) were used to control airflow, through tubing into which wet-bulb thermocouples were placed. Thermocouple wire (type T, 0.005 cm diameter) was used to fabricate thermocouples for testing. The thermocouple tip (bead) was covered with a wick (cotton fiber) to build a wet-bulb. The experiment was performed for air velocities ranging from 1.50 to 2.5 m/sec, measured at the flow meter (0.166-0.478 m/sec) inside the housing and changes in temperature ranging from 12.5 degrees C to 17.39 degrees C wet-bulb. These experimental conditions were selected to simulate human respiratory conditions. The apparatus produced repeatable square waves and allowed change in direction of temperature gradient without difficulty. The experiment was carried out for both cooling (going from hot-to-cold) and heating (going from cold-to-hot) of the thermocouple using two different tubing diameter (1 and 1 5/16 in.). Wet- bulb thermocouple response time of 0.175 sec (1 in. housing) and 0.153 sec (1 5/16 in. housing) were obtained. Conduction along thermocouple wire and radiation effects was shown to negligibly affect response times. The effect of the temperature change for small increments and direction of the gradient were also found to be insignificant. There was a negative correlation between air velocity and the time response of a wet-bulb thermocouple. This technique allows investigators a means of assessing data acquisition system response times in a repeatable fashion.