Single-sensor film probes have non-cylindrical sensors and are used for measurements in liquids at low and medium velocities and in gases. They are considerably more rugged than wire probes and less sensitive to contamination. Sensors are nickel films placed on quartz substrates. The sensor is defined as a line or a ring. Two sputtered silver leads carry the current from the cable, normally attached to the probe body, forward to the sensor.
Film probes (cones, flush-mounting) have cable-equipped probe bodies and connect directly to the probe cable without the need for a probe support. The probe bodies are made of chromium-plated brass and the quartz rods carrying the sensors are glued directly into the probe bodies by means of epoxy resin. The cable extending from the probe body is terminated in a detachable BNC connector.
Flush-mounting film probes
These probes have the sensor (0.75×0.2 mm) placed on the flat end of a quartz cylinder. They are intended for measurement of wall shear stress in both laminar and turbulent boundary layers. They may also be used for determination of points of transition and separation. The working principle is based on the similarity between temperature and velocity profiles in the viscous sub-layer, and the probe is used in practice like any other film probe.
Flush mounted hot-film probe (55R45 and 55R46)
The Flush-mounting probe is available in one straight configuration in versions for either liquid or gas applications.Probe 55R45 has a thin coating (0.5 µm) and probe 55R46 has a heavy coating (2 µm). The picture on the left shows the dimensions for the Flush-mounting probes.
Mounting of the flush-mounting film probe: No probe support. Probe mounts in hole in the wall confining the flow to be investigated, with sensor flush with wall surface.
Measures skin friction (wall shear stress) in both laminar and turbulent boundary layers. Determines the points of transition and separation. Mounts in a hole in the wall confining the flow to be measured with the substrate end plane flush with the wall. The sensor is oriented perpendicular to the flow direction.
|Technical Data For Flush-mounting Film Sensors|
|Thickness of quartz coating||0.5 µm||2 µm|
|Sensor dimensions||0.75 x 0.2 mm|
|Sensor resistance R20 (approx.)||15Ω|
|Temperature coefficient of resistance (TCR) α 20 (approx.)||0.35%/°C|
|Max. sensor temperature||200°C||60°C 1),2)|
|Max. ambient temperature||100°C||100°C|
|Max. ambient pressure||70 bar||70 bar 3)|
|Frequency limit fcpo (CCA mode, 0 m/s)||0.2 Hz||0.5 Hz|
- At atmospheric pressure (1 bar)
- Max. 150°C at elevated pressure. Avoid bubble formation.
- At 20°C, decreasing with increasing temperature, 1 bar at 80°C