Instrumentation for Sensing Moisture Content of Material Using a Transient Thermal Pulse

This invention relates to instrumentation for measuring the moisture content of material, particularly moving web material by using a transient thermal pulse.

In many industries, such as in the production of paper and textiles, drying of water in a web is part of the process. Due to the large heat of vaporization required for drying of water, large quantities of energy are consumed. Recent studies indicate that this is an area in which significant energy conservation can be achieved if the efficiency of the drying process can be optimized by precise measurement and control.

Currently available commercial instruments have a number of disadvantages which limit their widespread use. Most are too expensive, particularly for small business establishments having limited capital. This is particularly true of some instrumentation techniques, such as infra-red, neutron and microwave moisture sensing. They involve investments of $5000 to $20,000, or more.

Another disadvantage is the lack of an automated readout which would make possible feedback control of the process. The popular dew point instruments provide only temperature readings. Conversion to relative humidity requires an operator to make reference to a psychrometric table. In addition these instruments cannot be used for automated process control because no convenient electrical signal is produced which would be used for the purpose. This fault is also present in the class of moisture sensors which operate on mechanical principles.

Most of the available moisture sensing instruments, such as the hygroscopic chemical-resistance type sensors, do not have a linear response. This disadvantage necessitates the use of sophisticated electronics to compensate for the nonlinearity. Another limitation is slow response. It is not uncommon for moisture sensing systems to require stabilization times on the order of more than 30 seconds to several minutes.

Yet another limitation is the size of the head of available instruments which contain the probe. Large probe heads are inconvenient for installation due to the large size of the instrumentation package.

Instruments requiring air sampling for reference are also inconvenient. The lack of environmental stability is another disadvantage. Hygroscopic chemical and ion-exchange-resin type resistance probes usually deteriorate with time when exposed to contaminants, such as smoke and certain chemicals. This affects calibration which leads to inaccurate control. Calibration is difficult with many instruments.

Periodic recalibration is essential to insure repeatable results. Only a small number of instruments make provision for this, and in most cases the instrument probe must be dismounted and placed in a humidity controlled chamber for comparison with a standard.

The disadvantages are largely responsible for the limited use of moisture sensing systems in processing. Any instrumentation which minimizes or eliminates such disadvantages would therefore be of great interest.

In accordance with the present invention, a sensing element comprised of a thermally and electrically conductive thin ribbon is excited by pulses of constant current. The Joule heating which takes place during the pulse raises the element temperature above the ambient temperature and changes its resistance linearly. Contact with a moist material produces better heat transfer from the element is compared to air, thus limiting its temperature in relation to moisture content, thereby causing a different amount of resistance change of the element during the pulse. By monitoring the ribbon resistance during the application of the pulse, such as by the use of a Wheatstone bridge connected to an oscilloscope, or other means for displaying the element resistance as a function of time, the moisture in the material can be determined. In the case of the material being in the form of a moving web, such as in a paper or textile mill, the element is periodically pulsed and the resistance is continually monitored to determine the moisture content of the web, thus making possible feedback control of the drying process.

The novel features that are considered characteristic of this invention are set forth with particularity in the appended claims. The invention will best be understood from the following description when read in connection with the accompanying drawings.

FIG. 1 is a schematic diagram illustrating the present invention.

FIG. 2 is a diagram of current and voltage pulse waveforms for the transient thermal pulse technique used in the invention.

FIG. 3 is an isometric view of the bottom of the moisture sensing probe illustrated in FIG. 1.

FIG. 4 is an isometric view of a roller-type moisture sensing probe for use in the present invention.

FIG. 5 is a graph of voltage pulse amplitude as a function of moisture content for various web materials.

Referring now to FIG. 1 of the drawings, instrumentation is disclosed for measuring the moisture content of a moving web 10 of paper, textile or the like, using a transient thermal pulse technique. The instrumentation employs a probe 12 having a sensing element 14 which is both thermally and electrically conductive. This element is mounted on the lower face of the probe 12 so that it may be in contact with the web 10 for sensing the moisture content of this web.

The sensing element 14 is periodically excited by a pulse of constant current from a source 16. Such a pulse is shown in waveform A of FIG. 2. The Joule heating by each current pulse of the sensing element in the atmosphere raises its temperature, .DELTA.T, above the ambient temperature and increases its resistance proportionally, as shown by the dashed-line waveform B in FIG. 2. Contact with the moist web 10 produces heat transfer from the sensing element, thereby reducing its temperature rise, as shown by the solid-line waveform B in FIG. 2. That in turn causes a reduced increase of resistance of the sensing element proportional to the change in temperature, .DELTA.T. The change in resistance of the sensing element 14 is then measured as a change in voltage, .DELTA.E, across the element in a Wheatstone bridge 18 comprised of resistors R.sub.1, R.sub.2 and R.sub.3.

By measuring the initial resistance of the sensing element, i.e., the voltage at point 1 in FIG. 2 before application of the current pulse, again at the end of each pulse before contact with the web, which is at point 2 in FIG. 2, and finally at the end of each pulse after contact with the web, shown as point 3 in FIG. 2, data is obtained that can be utilized to determine the amount of moisture in the web. This fact is used to provide on-line measurement of moisture content for feedback control of some part of the web drying process.

Most metals exhibit an increase in resistance with temperature which is linear over some useful range. Alloys usually exhibit a smaller increase, i.e., exhibit a smaller temperature coefficient than pure metals, again over some useful range. Change of the physical character of the metal can also cause a change in its temperature coefficient. Consequently, the choice of metal will depend not only on the range of temperature desired, but also any physical change in the character of the metal used for the sensing element that may occur during assembly of the probe.

The resistance variation of the sensing element can be measured in any of the usual ways known to those skilled in the art. For example, the element can be used as just a resistance-thermometer element with separate voltage (E) and current (I) terminals connected to the ends of the elements, but the more common way is with a Wheatstone bridge arrangement shown in FIG. 1. The bridge is balanced by adjusting resistor R.sub.1, R.sub.2 or R.sub.3 for the initial temperature of the element. Any change in the temperature of the element due to energy applied through current terminals unbalances the bridge across the voltage terminals, causing the appearance of a voltage at a measurement display unit 20, such as an oscilloscope or any analog or digital recording device synchronized by the current pulse to display the waveform B of FIG. 2.

It should be noted that if the current pulses are periodic square-wave pulses, the output between the voltage terminals is an alternating current signal of an amplitude which varies as a function of the resistance of the sensing element. Consequently, it would not be necessary to measure the peak change in temperature on a pulse by pulse basis for continually monitoring the moisture of the web 12. Instead the amplitude of the continuous AC output could be compared with a reference for a desired moisture content. Any differences between the output and the reference can then be used as an error feedback to the system for the drying process.

The present invention was tested by fabricating a probe as shown in FIG. 3. The sensing element 14 was made from type 304 stainless steel foil 12.7 .mu.m (0.0005 inch) thick,, 2.54 cm (1 inch) long and 1.14 mm (0.045 inch) wide. This element has a resistance of about one ohm at ambient temperature (23.degree. C.). The element was soldered at its ends to two copper pins 15a, 15 b for making electrical connection, and glued into a shallow groove in a Lucite bar 15c, flush with the surface bar to protect the element against wear by the moving web 10.

A roller-type probe shown in FIG. 4 could be used to further reduce wear. There a cylindrical Lucite block 21 would be prepared with a groove to receive the sensing element 14 flush with the surface, and the ends of the element would be connected to rotary connectors 22a and 22b. As the web moves, the roller-type probe would turn, and while the element 14 is in contact with the web 10, it would be pulsed. The pulse could be automatically triggered by sensing the temperature difference between the web and the ambient.

For testing the arrangement of FIG. 1, a sample of web material was placed on a Lucite plate and the probe was laid upon the sample with the sensor element 14 in contact with the sample surface. A weight was placed on the sensor so that a constant contact pressure could be maintained between the sensor element and the material under test, but it was found that the probe was insensitive to contact pressure.

A square wave having a constant current amplitude of 0.5 A was found to be the minimum workable level, for the probe design under test. A Wheatstone bridge type of transient testing instrument (Model 605, Pasadena Scientific Industries, Inc. developed for bridgewire testing) was used in conjunction with an oscilloscope. The square-wave pulses had a duration of 170 msec, although it was found that pulse duration did not significantly affect results. After the bridge was nulled to cancel any DC offset, the extracted AC signal produced by the change of resistance was displayed on the oscilloscope. Waveform B of FIG. 2 shows a typical trace which was obtained.

Several common materials such as a paper towel, newsprint, cotton cloth and gauze were tested. Each sample was soaked in water and allowed to dry slowly in air. Water content was determined by acurate weighting of the samples before soaking and just prior to testing. The data obtained in the testing are shown in FIG. 6, with the maximum amplitude of the AC signal plotted against the percentage of water. This test demonstrates there is a definite correlation between the probe output and the moisture content of the sample, but in order to achieve repeatable results, calibration of the instrumentation for each web material is required. However, calibration may be easily carried out at the paper or textile mill.

Although particular embodiments of the invention have been described and illustrated herein, it is recognized that modifications and variations may readily occur to those skilled in the art. For example, the sensing element may be made from a material having a negative temperature coefficient of resistance instead of a positive one. Consequently it is intended that the claims be interpreted to cover such modifications and equivalents.