Basic structure of the sensing element of the hott

The basic structure of the tachometer sensitive element

1. Probe

the simple model of the probe is shown in the figure. The slender metal wire is placed perpendicular to the direction of the air flow, and both ends are fixed on the relatively thick support. The metal wire is heated by the current to a temperature higher than the temperature of the measured medium.

model

in order to meet the requirements of the probe, the desired materials are:

① the resistance temperature coefficient of the material is high, To obtain high measurement sensitivity

② the material must be corrosion-resistant to ensure sufficient stability

③ the material should have high mechanical strength to prevent vibration deformation from affecting the real-time monitoring accuracy of all data of measuring injection raw materials

④ the material department has good heat conduction performance.

generally, people use tungsten wire, platinum wire or platinum plated tungsten wire that can meet the above requirements to make hot wires on the probe. They mainly focus on raw materials, processing, product design and product inspection

the typical diameter is 0.5, which is assembled on the sample car for professionals to review the appearance - 10 m, The length is 1-3mm. this size is mainly considered from two aspects, but its utilization in the automotive field still lags far behind that in aerospace and other industrial fields: in terms of improving sensitivity, we hope that the thinner the diameter and the longer the length are, but this will reduce its mechanical strength and spatial resolution; In terms of improving spatial resolution, the shorter the better, but it may affect the sensitivity of measurement.

the two ends of the fork are welded to the fork tips of two fork bars, the other end of the fork bar is welded with a lead wire, and a protective cover is added. The fork bar and the protective cover are filled with insulating materials. The fork bar is usually made of stainless steel with a diameter of 0.2-0.5mm, and the top diameter is 0.05-0.1mm. The fork bar is embedded in ceramics, nylon or other insulating materials, Outer bread probe shell.

2. Hot film probe

in order to apply the thermal velocity measurement technology to the measurement of liquid flow, the hot film probe has been developed. Its mechanical strength is high, so it can adapt to some flow fields with worse conditions (such as the flow field of sewage flow). In addition, it is less affected by vibration, and the resistance value can be adjusted by the thickness of hot film, so it is easier to match with the amplifier, High signal-to-noise ratio can be obtained. Its disadvantages are narrow measurement frequency range, complex manufacturing process and low working temperature, which limit its application.

the hot film deep needle is composed of hot film, substrate, insulating layer and wire. The hot film is formed by splashing platinum on the substrate, with a thickness of about 0.1-1um. The substrate is a cylinder made of quartz or boric acid glass. In order to insulate the hot film from the fluid and prevent pollution, The hot film is also coated with a quartz insulating layer with a thickness of about 1um