AC vs DC Measurement2 Comments May 18, 2015
AC vs DC Measurement
Why Measure Conductivity with AC Instruments not DC Instruments?
- The competition uses Direct Current, (DC) type measurement; D-2 Inc. uses (patent pending) Alternating Current , (AC) measurement technology.
In the graph below the current, which flows through a fuel due to its conductivity, is plotted against time for a DC applied voltage across the fuel. Points to note are as the voltage is first applied there is a steep in-rush current, then, as the electrons pass through; the fuel build up on the electrodes, the current starts to rapidly go back down. Eventually the current stops all together, essentially you have built a battery, not a conductivity sensor.
2. It can been seen from the last graph that the actual conductivity measured is a “function” of when the sample is taken, hence, the need of DC meters to have “3 Seconds and Read” user operation instruction. Note that the rapid change in the value on either side of the “read window” results in large variability of the measured value, simply due to the time of the reading. If you have any question as two of your customers to in their head count to three seconds and put there arm up, when they are back to back and cannot see the other. Hence the fundamental measure is flawed.
Secondly, the shape of the above curve actually varies as a function of fluid movement past the electrodes; hence the value read in a DC meter is also a function of the rate of flow. Flow slows the rate at which electrons can collect at the electrodes. Hence DC Measurement of any precision requires the fluid to be absolutely still, no movement.
Finally, the curve also varies due the “temperature” of the fuel. As we mentioned you are building a battery. Building batteries is an electrochemical process; the chemical aspect is highly dependent on ambient temperature. It is common knowledge that all chemical reaction rates vary is dependent on the temperature at which they are made. All the above factors lead to significant errors in DC type meters, and absolutely prevented their use in flowing of moving samples.
- AC versus DC Measurement. The biggest advantage of AC versus DC measurement is the ability to measure in the product line. Liquid Hydro-Carbons such as fuel build up a DC electrical charge due to work being done on the fuel to move it or filter it. This DC electrical charge affects any DC measurement device such as our competitors whose equipment is all DC measurement. DC devices such as the Emcee equipment require the fuel is relaxed and in a grounded metal beaker. Whereas our handhelds and in line sensor can measure the fuel in any sample container and in the product line in real time because the DC charges do not affect our AC equipment. The Emcee in line sensor is “only” an automated spot sampler than an in line sensor due to it’s use of DC measurement technology.