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A New Basis Weight Meter
A NEW BASIS WEIGHT METER
B. Agdur, K. Lindberg and P. Weissglas, Royal Institute of Technology, Stockholm, Sweden
AUTOMATION
PAPER 13.2 SUMMARY A basis weight meter, built on an acoustic principle, has been developed at the M icrowave I nstitute in Stockholm. The method of operation together with laboratory and industrial tests will be presented and discussed. INTRODUCTION An accurate on-line measurement of the basis weight of paper or plastic webs is an important condition for achieving a uniform quality of the product as well as an economic optimum for the production. Radiation type instruments measuring the attenuation of beta-radiation is generally used for basis weight measurements. During many years of development these instruments have been improved and are working satisfactorily in many applications. However, they have certain draw-backs such as: sensitivity to composition of the web, lower accuracy at low basis weights, slow speed of response, demand for a very stable mechanical installation and demand for radiation protection. At the M icrowave I nstitute a basis weight meter has been developed according to an acoustic, and basically, very simple principle. At the present state of development, a laboratory version has been working for more than one year, and tests are now being performed in a paper mill with an instrument especially designed for on-line measurements on paper webs
to vibrate, and il secondary sound wave, of a smaller ampi itude, is transm itted from the back side of the sample. Th is signal is detected with a m icrophone and its ampl itude is determined and used to measure the basis weight of the sample. To make the measurement independent of the ampl itude of the signal from the sound transmitter, and to eliminate influence from variations in pressure and temperature of the surrounding air, a reference channel is applied in which a foil with a constant basis weight is placed . The signals from the two channels are compared in an electronic unit, the out-put of which is a signal, dependent only on the basis weight of the sample .
PRINCIPLE OF OPERATION R t'lf"rf"r'. (f"
~1; c r"Dhunl!'
~llll ., "" j -
-'a~ ·. :...i,
""ti_- __
___
_
_"to I I
Fig.!. Acoustic basis weight meter: Principle of operation
Fig. 2. Acoustic basis weight meter for installation in a paper machine
The principle of the measurement is described in fig.!. A sound wave of a certain frequency causes the sample
493
When measuring the basis weight in a paper machine, the moving hot paper web introduces some problems. Streaming hot air may cause different conditions in the measuring channel, and the reference channel and dust will contaminate parts of the measuring head. After experiments with the equipment in paper machines several momodifications have been made. I n the present construction the measuring head is provided with an air blowing system which seems to have solved most of the problems caused by hot air and contamination. The experiments going on now are looking very promising and an accuracy of about 0.5 010 or better is expected.
The main advantages of the instrument are: It is non-radioactive, it is quite independent of what sort of material that is to be measured. Foils of paper, plastic or metal with the same basis weight will give the same instrument reading. Finally, the instrument is rather insensitive to the mechanical assembly . The basis weight range of the present prototype is 30200 grams per m 2 , but the range can be extended far beyond this limit. At higher basis weights the measuring range is limited by the available signal level and by acoustic noise, but the upper limit may be increased at least ten times. At lower basis weights the measuring range is primarily limited by the porosity of the sample. The material must not be too permeable to air. MEASUREMENT The basis weight meter has been thoroughly tested under laboratory conditions and has shown very good function with an accuracy within a few tenths of a percent. Fig. 2 shows the measuring head mounted on a U-frame for installation in a paper machine.
494
B. Agdur, K. Lindberg and P. Weissglas, Royal Institute of Technology, Stockholm, Sweden
AUTOMATION
PAPER 13.2 SUMMARY A basis weight meter, built on an acoustic principle, has been developed at the M icrowave I nstitute in Stockholm. The method of operation together with laboratory and industrial tests will be presented and discussed. INTRODUCTION An accurate on-line measurement of the basis weight of paper or plastic webs is an important condition for achieving a uniform quality of the product as well as an economic optimum for the production. Radiation type instruments measuring the attenuation of beta-radiation is generally used for basis weight measurements. During many years of development these instruments have been improved and are working satisfactorily in many applications. However, they have certain draw-backs such as: sensitivity to composition of the web, lower accuracy at low basis weights, slow speed of response, demand for a very stable mechanical installation and demand for radiation protection. At the M icrowave I nstitute a basis weight meter has been developed according to an acoustic, and basically, very simple principle. At the present state of development, a laboratory version has been working for more than one year, and tests are now being performed in a paper mill with an instrument especially designed for on-line measurements on paper webs
to vibrate, and il secondary sound wave, of a smaller ampi itude, is transm itted from the back side of the sample. Th is signal is detected with a m icrophone and its ampl itude is determined and used to measure the basis weight of the sample. To make the measurement independent of the ampl itude of the signal from the sound transmitter, and to eliminate influence from variations in pressure and temperature of the surrounding air, a reference channel is applied in which a foil with a constant basis weight is placed . The signals from the two channels are compared in an electronic unit, the out-put of which is a signal, dependent only on the basis weight of the sample .
PRINCIPLE OF OPERATION R t'lf"rf"r'. (f"
~1; c r"Dhunl!'
~llll ., "" j -
-'a~ ·. :...i,
""ti_- __
___
_
_"to I I
Fig.!. Acoustic basis weight meter: Principle of operation
Fig. 2. Acoustic basis weight meter for installation in a paper machine
The principle of the measurement is described in fig.!. A sound wave of a certain frequency causes the sample
493
When measuring the basis weight in a paper machine, the moving hot paper web introduces some problems. Streaming hot air may cause different conditions in the measuring channel, and the reference channel and dust will contaminate parts of the measuring head. After experiments with the equipment in paper machines several momodifications have been made. I n the present construction the measuring head is provided with an air blowing system which seems to have solved most of the problems caused by hot air and contamination. The experiments going on now are looking very promising and an accuracy of about 0.5 010 or better is expected.
The main advantages of the instrument are: It is non-radioactive, it is quite independent of what sort of material that is to be measured. Foils of paper, plastic or metal with the same basis weight will give the same instrument reading. Finally, the instrument is rather insensitive to the mechanical assembly . The basis weight range of the present prototype is 30200 grams per m 2 , but the range can be extended far beyond this limit. At higher basis weights the measuring range is limited by the available signal level and by acoustic noise, but the upper limit may be increased at least ten times. At lower basis weights the measuring range is primarily limited by the porosity of the sample. The material must not be too permeable to air. MEASUREMENT The basis weight meter has been thoroughly tested under laboratory conditions and has shown very good function with an accuracy within a few tenths of a percent. Fig. 2 shows the measuring head mounted on a U-frame for installation in a paper machine.
494