Experiment Investigation On Calibration Of Hot Wire Anemometer On Low Air Speed

In the field of Bio pharmaceutical or chip fabrication micro,air speed less than 0.2m/s has to be measured.Usually the hot wire anemometer(HWA)is employed due to its favorable specification in low speed range.However the issue of anemometer traceability is not easy to be solved due to small Reynolds number flow in low speed.In this paper at NIM the 80 m linear guide trial that is built in the 8.3m deep tunnel underground for the baseline length calibration.On the base of existing facility with interferometer and stable temperature gradient in underground space.The low air speed reference facility is established with sliding carriage on the linear guide trial.The HWA is calibrated in this facility.During the calibration process the influence of natural convection,angle-mounted and the secondary flow have to be considered before calibration.(1)For natural convection the HWA is tested at the different positions on the trial to achieve zero shift that is observed to be 6mm/s.As signal noise is included in this zero shift the influence of convection is less than 6mm/s.(2)The problem of angle-mounted is difficult to be avoided.The test is conducted under different angles to evaluate its influence.(3)The secondary flow is considered to be caused by the movement of carriage in the round trip.In calibration trip the probe leaves the influence behind itself.For the influence by return trip of speed of 0.2m/s it has to wait for a period of time for the confirmation that the air is stable enough.For this reason the different waiting-time for each test point is temped and the HWA outputs are compared.(4)The HWA uncertainty of calibration results are analyzed and evaluated.Meanwhile,this HWA is also calibrated by LDV in wind tunnel by considering the test procedure to void sensor probe pollution of scattering particles.The probe pollution is a problem if LDV is taken as reference.The experimental procedures are designed to solve this problem.As air in low speed of(0.1~1)m/s flows in open-channel wind tunnel in which blockage plate inserted in the diffuser in downstream.(1)In this process the determination of LDV measurement position is based on considering the flow around the probe.(2)the speed deviation at different positions in axis flow is corrected.(3)The probe is protected by a sealing cover to wait for air speed(particles)being measured by LDV at the beginning.The cover will not be removed until all the particles in wind tunnel flow out of the tunnel.(4)Calibration of HWA to evaluate the uncertaintyAt the speed of(0.1~1)m/s HWA is calibrated by the low speed reference facility that is built in the tunnel underground at NIM.The expanded uncertainty of reference air speed evaluated to be 5.4 mm/s,and the stability of it is about 3mm/s.It is matched well between HWA theory curve and calibration result curve with the maximum deviation of 6 mm/s at point of 1m/s.This HWA is also calibrated by LDV in wind tunnel.The calibration results suggest the difference between the 2 methods is less than 30mm/s as air temperature is corrected for HWA.