Notes from Transfac Labbookp 48:

140129
Using a 249 ohm resistor
	4 - 20 mA (0 - 1000 ppm H2O)
	0.996 - 4.98 V  difference of 3.98 V
	Sensing w/NI6218   -5 to 5 V range
	gives a resolution of 160 uV (5% overrange)
	169 uV /3.98 V x 1000 ppm = 0.04 ppm

Recollections of electircal at the time:

Using the ODC5 relay board and analog signals wired directly into 
USB6218, ODC5 relay board powered of separate 5/12 V power supply. 
Two 500 ohm low TC resisitors in parallel for sense ressitor.