static private String getAbrvFromUnit(String t, String s){
		int i=0;	
		Converter[] u;

		if (t == "vol")
			u = volUnits;
		else if (t == "pressure")
			u = pressureUnits;
		else // assume weight
			u = weightUnits;

		while (i < u.length
				&& !u[i].unit.equalsIgnoreCase(s)) {
			i++;
		}
		if (i >= u.length)
			return null;
		else 
			return u[i].abrv;
	}