Please e-mail MSCTI questions to mhead@pme.com.
More MSCTI Questions
Q. What is needed to connect a MSCTI to a personal computer?
The MSCTI produces two analog output voltages, one proportional to conductivity and one non-linearly proportional to temperature. These voltages range from -5 Volts to +5 Volts. These can not be directly connected to a personal computer.
Connection to a personal computer requires that the customer supply an analog-to-digital converter. This is usually a card that plugs into the personal computer in some way. since the MSCTI has only 1% accuracy and can output signals only up to about 800 Hz, a fast, high resolution analog-to-digital converter isn't necessary. 12 bit conversion is acceptable. For example, an inexpensive analog-to-digital converter card is the PCI-DAS6023 available from Measurement Computing (www.measurementcomputing.com). There are many other sources of similar converters.
Q. What is the maximum allowed length of cable between the sensor and the power supply?
The MSCTI is shipped with a 10 meter cable between the sensor and box. One end of this cable is glued into the sensor electronics. The other has a connector. PME is able to make a special sensor extension cable if requested. This would have connectors at both ends.
Q. I would like to measure conductivity between rods or fibers in a liquid flow.
Time response for the MSCTI is very fast, roughly 800 Hz. This is limited by the electronics within the instrument. In most experiments, where fluid flow is slower than 1 meter/second time response of the MSCTI is not a limiting factor.
The MSCTI resolves features of approximately 1 mm in size. The actual sensing is described by a complex weighted average of the conductivities within a small volume nearby the sensor, with the weighting function declining to 0 at large distance from the sensor. The "1 mm" distance captures most, but not all, of the sensitive volume.
If your rods or fibers are closer together than a few mm, then the MSCTI sensor will include their presence in its average. If the positions are fixed relative to the sensor, then you can calibrate the whole arrangement and still make useful measurements. To do this you would position the sensor at a fixed point relative to the rods/fibers, then pass different conductivity water through, recording the MSCTI output for each conductivity. If the sensor position relative to the rods/fibers changes, then a new calibration would be required. If the sensor position relative to the rods/fibers cannot be maintained at a fixed point, then the MSCTI output will vary depending upon location of the rods/fibers and also upon the conductivity of the water.
In addition, the MSCTI output will change slightly with time even if water conductivity and rod/fiber position does not. This effect is less than 1% per 8 hours operation and will also depend upon whether the sensor is continuously immersed in water or if it is exposed to air for periods. The presence of oil or other fouling materials in the water will be detrimental to sensor operation.
The sensor is fragile and must not contact the rods/fibers. It can be damaged if contact occurs.
Q. How many times do I have to repeat the T and C sensors calibration? For examples: after 1 hour, after 24 hours, after a week?
MSCTI should hold about 1% accuracy for 8 hours. It will actually do better, more than 24 hours for 1%. However sub-1% errors are random and occur on hourly time scales. It tends to drift more in the first 1/2 hour after being turned on. A convenient calibration interval is daily. More frequent cals probably won't get you much better than 1%. Less frequent cals will sacrifice some accuracy. Experiment a little with calibrations.
Q. What happens if the sensor is operated in air?
Salt won't crystallize on the sensor if it is immersed so you are OK there. However, if the sensor is lifted into the air then crystallization can occur as the water evaporates. Crystals break up the black plating on the sensor electrodes. Nothing really bad happens, but the calibration can shift a little ( < 1%). If you lift the sensor into the air, then take the time to squirt it with distilled water all over to rinse as much salt water away as possible. Also the electronics should be off when the sensor is in air. If the electronics are on as the sensor dries (and therefore doesn't conduct anymore) the electronics can go a little crazy and apply large voltages to the sensor electrodes. Again, nothing really bad happens but the calibration can drift a little.
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