Differential pH Probe (Direct 4-20mA Output) This item is currently on backorder but you can still purchase it now and we'll ship as soon as more become available. Differential pH Probe (Direct 4-20mA Output) Specifications pH SENSOR, BULB 1/2" NPT, PPS, DJ, 20FT SL How To Use Contact your Norchem rep for step-by-step instructions. Tweet