Scanning vs. single point measurement
Almost all paper machines have scanning measurement for paper grammage and moisture. However, it is very difficult to separate cross direction (CD) variation from machine direction (MD). The following is a simple example of a modern 10 m wide paper machine. If scanning speed is 0.5 m/s and machine speed 20 m/s, during one 20 s long scan paper is produced 400 m.
Frequency of main pressure pulsations before headbox are 10-20 Hz. This means that there are 200-400 grammage peaks in MD during one scan. Therefore it is likely that one scan measures more MD variation than CD variation. Consequently, more scans and time are needed to get a stable CD profile. In practice approx. four scans are needed to separate MD and CD variations. This is approx. 1600 m in paper length.
The other problem of the scanning measurement is that the measured area is very small compared to the real web area. If we imagine that the scanner can measure an area of 4 m2 in one second, so one scan measures 80 m2 which is only 2 % of the total area.
Caliper measurement from parent roll
The roll density analyzers are well known already about 40 years for winding diagnostics. However, they have the limitation being too sensitive to changes in web caliper, making it difficult to determine if fluctuations in the density distribution reflect changes in winding or web caliper, or both. These analyzers measure web caliper inside the roll, and normally the measured caliper is recalculated to density by constant grammage. Density can be calculated from grammage and caliper as:
p = b/c
p = density, kg/m3
b = grammage, kg/m2
c = caliper, m
It is very interesting why this old online measurement principle is not normally used to measure just caliper. Higher bulk and stiffness is a popular trend in papermaking. To get the bulk calculated, caliper must be measured. Today it is possible to get an accurate measurement of parent roll diameter and revolutions. Dividing diameter increase by web layers, it is possible to get web caliper in the parent roll. The average caliper or bulking thickness of each parent roll rotation can be calculated and thus get MD variation of the caliper.
Additional very interesting point is that the final caliper and bulk are not similar before winding and after rewinding parent roll. Bulky products such as tissue papers and nonwovens show plastic compression inside the parent roll. Normally layers close to the reel spool are thinner compared to the outside layers. Winding programs for the parent roll are not optimized for converting but only for the theoretical knowledge of decreasing nip loading.
If the diameter of the parent roll is around 2 m, one revolution is about 6 meters in paper length. This means that we can get three good measurements of the web caliper every second. This can be compared to scanning measurement which requires four scans and total time of 80 seconds. Additionally, this measurement is of 100 % of the total web area.
Even if there is a scanning caliper measurement for CD control, the presented system is a good and low- cost additional tool for analyzing and optimizing MD variations and parent roll structure. Even more important this is for the producers, which cannot use caliper measurements due to soft web surface, dust or stickies.