Choosing a meter's pathlength
Our C-Star transmissometer and ac-9 absorption and attenuation meters are available in your choice of 25 cm or 10 cm optical pathlengths. The 25 cm models are best for most open ocean and estuarine work where the attenuation is usually under 10 m-1. However, in extremely productive or otherwise turbid waters, a shorter pathlength will be more suitable. Click on the image at right to see a graph showing respective wavelengths.
Since twice the RMS noise is less than 1 mV out of 5 volts, we consider the resolution to be one bit on a twelve bit A/D converter. The percentage of light transmission and the beam attenuation coefficients and resolutions for the 25 cm pathlength and 10 cm pathlength instruments have been computed for various signal levels from 2 bits to 212 bits. The resolutions are the difference between the attenuation coefficients at the signal level plus and minus a half bit. The graph will show which instrument is best, depending upon the range of values you expect to frequently encounter. We suggest you pick a pathlength that gives adequate resolution at the high end of the range you expect and superior resolution at the lower end. For the cleanest waters the resolutions of the 25 cm and 10 cm pathlength instruments is 0.001 and 0.0025 m-1 respectively. The crossover point is at 6.0 m-1.
For clean water measurements, the accuracy of the instruments is an issue. For example, if the repeatability of generating clean water, cleaning the windows and taking the measurement is ±0.10% in transmission, then the accuracy of the 25 and 10 cm pathlengths will be ±0.004 and ±0.010 m-1 respectively.
