Principle of the hottest optical fiber pressure se

2022-10-15
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Principle of optical fiber pressure sensor

the important sensing element of pressure sensor is Fabry Perot (FP) optical interferometer. The two mirrors of the interferometer are the inner surface of the film at one end and the tip of the optical fiber at the other end. The applied pressure P causes the displacement of the film, which is directly converted into the change of the cavity length of the FP interferometer

in order to obtain the linear relationship between the film offset and the applied pressure, the shape and material of the sensor have been strictly selected. It can be expressed as:

lcav (P) = l0 + (p-p0s)......................... (1)

where p is the pressure applied to the outer surface of the film (unit psi)

P0 is the pressure in the FP cavity, and actively performs the government's policy of large and small pressure (unit psi)

lcav is the length of the cavity measured by the signal demodulator (unit nm)

l0 is the length of the cavity in the initial state of zero point (unit nm), It is usually defined as P =p0

s is the sensitivity of the sensor (unit: nm)

pressure sensors have three different types: 1) gauge type; 2) Absolute type and 3) differential type. In the case of gauge type sensor seeking design freedom, manufacturing convenience and light weight to exceed the traditional aluminum scheme, P0 is equal to the ambient pressure or atmospheric pressure. Gauge type sensor has a) a vent hole, which makes the cavity under ambient pressure or b) a cavity sealed to atmospheric pressure; In the case of absolute pressure sensor, P0 = 0, its cavity is sealed in vacuum during factory production; In the case of differential sensors, P0 is equal to any pressure. This type of sensor has a vent connection to maintain a given pressure in the cavity

signal demodulation processing

principle

the signal demodulators of all our sensors are made according to white light interference technology. The signal demodulator (fti-100i, fti-10, etc.) converts the optical signal from the sensor into absolute FP cavity length. The reason why this cavity length is called that the global ethylene propylene rubber production and automobile production trends are basically parallel is absolutely because it does correspond to the physical cavity length (relative measurement relative to the cavity length) of the FP interferometer when measuring this optical signal Δ 50. It is then determined as an arbitrary initial value). This difference is important because many optical fiber sensing technologies, especially those based on monochromatic light interferometry (as opposed to white light interferometry), can only measure length changes Δ L。 In all applications requiring long-term static measurement, absolute measurement is essential. The optical signal is converted at the frequency determined by the sampling speed of the signal demodulator. In the case of pressure sensor, the conversion accuracy within the working range of 10000nm is ± 1nm

once the cavity length lcav is measured, the demodulator will calculate p-p0p-p0 = (lcav-l0)/s according to the following formula (2)

then record and display this pressure value on the demodulator (Imperial unit psi or SI unit bar). The initial defined values of the sensitivity coefficient s (determined by the instrument serial number) and l0 provide all the necessary information to convert the measured cavity length into pressure

instrument serial number

(printed near the optical fiber connector of the sensor) the instrument serial number has 7 digits, starting with the number 6 in the case of a pressure sensor. The form of the serial number of the pressure sensor instrument is 6xyzzzzz, and its meaning is as follows:

* the number 6 indicates that the sensor is a pressure sensor (the number 4 indicates a temperature sensor)

* digital X is only used to distinguish those sensors with the same sensitivity in a batch

* Zzzz is the product of sensitivity s and scale factor, that is, Zzzz = s · 10 (n+1), where n=0, 1, 2 or 3

* in recent years, digital y gives the value of scale factor N, that is, y=n

our signal demodulator has a complete set of functions, which allows users to store, eliminate or select different instrument serial numbers in the demodulator memory. These functions can be realized by pressing the control key on the front panel or the remote control command connected by RS-232 sequence. Up to 50 instrument serial numbers can be stored in the memory

0 definition

depending on the type of pressure sensor, the cavity length has different definitions

gauge type: the signal demodulator (fti-10, fti-100i, etc.) allows the user to let the sensor set the value of cavity length l0 (the original value is equal to 0), so set the initial state of measurement. This can be performed by zeroing (zero/null) of the demodulator. When the user selects the zero/null command, the demodulator will immediately record the cavity length of the sensor and set it to l0. This process is usually completed under ambient pressure or atmospheric pressure. The demodulator will retain this defined value in the memory until the next time the user changes (Note: when the memory disappears or the demodulator fails, the user can (apply the offset function of the internal unit) retrieve the previously defined l0 value)

absolute: this type of sensor has a factory calibrated cavity length l0. The user can input this value into the demodulator memory through the offset function of the internal unit

differential type: the steps are the same as those of gauge type

accuracy

the accuracy of the sensor depends on the accuracy (0.1% FS), zero point thermal drift (%fs/℃) and sensitivity thermal drift (% reading/℃). The total error is a function of these three separation errors, and the worst case is the algebraic sum of their maximum or minimum values. The following figure shows the effect of these errors on the response characteristics of the sensor

nominal response of pressure sensor zero thermal drift sensitivity thermal drift

pressure unit

conversion relationship of common pressure units

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