'Abscisado' is not in the 22nd edition of the spanish language dictionary of the royal spanish academy's website, but the context seems to recommend the english term 'abscissa'. This term usually refers to the horizontal, or x-coordinate on an x,y graph, but in this case I believe it has to do with direction of stress, in this case from up to down.

From what I can gather, 'abscisado' refers to a reference point or direction from which a measurement is made. See the following links for example:
"...el sistema de abscisado actual de las vias nacionales (Puntos de referencia o PR's)..."
http://foros.construaprende.com/sistemas-abscisado-de-vias-v...
"El recorrido del gasoducto empieza en esta locación con un abscisado de K0+000 y a
partir de él se identifican los siguientes cruces especiales, de acuerdo a lo presentado en
la siguiente tabla."
http://www.minambiente.gov.co/documentos/res_1196_040708.pdf

I think the text might be talking about how the level of the material within the silo is measure. I found lots of references to systems that measure the level from top to bottom, e.g.:
"a system where stainless steel cable carries the signal from top to bottom of the silo to give an accurate and repeatable continuos level measurement."
http://www.vegacontrols.co.uk/applications_fp2.asp?caseStudy...