We recently bought a fiber-optic pressure transducer from Opsens (the machine is called Lifesens). We ar einterested on using it to measure intracranial pressure in vivo. However, we had just tested it once and after taking it out of the skull it started getting highly variable readings by just being in the air, and neither cleaning it or nulling it would fix it. After a while it went down to normal, so we decided to test it by submerging it in water. We had done the null in air, and when we put it a couple of cm inside a 15ml falcon with some water it started reading a varying pressure of about 200 ± 30mmHg. We took the sensor out of the water and cleaned it as per manufacturer's guidelines but it would continue reading 200mmHg even out of the water, and varying a lot even after being nulled.
I've contacted the company but they are taking very long to give an appropiate reply and I need to use this machine this week.
Has anybody have any similar problems? Do you have any suggestions I could try?
Thank you!
Hello, Ester.
There are several types of fiber optics, as different materials and concentric layer are used, but most of them work based on the change in refraction index from one layer to the surrounding one. Refraction can produce a special kind of reflection called "total internal reflection", which happens in a range of incidence angles (of light over the wall). Assuming that the material of fiber optic you have does not absorb any water, changing the refractive index of the surrounding medium form air to water does affect the light conduction of your fiber. First thing to try: You could try to modify the inclination angle of the light source, but likely, even if you could do that, the instrument will lose its calibration, and a re-calibration will be required. Second try: Get a fiber with opaque external layer, designed to work at different surrounding media. May be you can get one short segment of fiber adaptable to your instrument, but again, even if you could do that, you will need to re-calibrate. I wish you will not need buy another instrument, but if that is the case, insure it uses a fiber with opaque and waterproof (perhaps plastic) external layer.
Yet another point to consider, which is about uniformity of pressure/deformation along the fiber. Most likely, pressure must be the same all along the fiber.
It definitely sounds like the fiber outer layer has water intrusion, as Mr. Casillas indicated in his response. I am not familiar with the wavelength at which the "Opsens" works, but as you move more deeply into the infrared, the effects of liquid intrusion become more and more apparent.
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