Data processing: measuring – calibrating – or testing – Measurement system – Dimensional determination
Reexamination Certificate
1998-02-12
2001-06-05
Assouad, Patrick (Department: 2857)
Data processing: measuring, calibrating, or testing
Measurement system
Dimensional determination
C702S087000, C702S097000
Reexamination Certificate
active
06243661
ABSTRACT:
The present invention relates to coating thickness gauges.
DESCRIPTION OF THE PRIOR ART
A wide variety of coating thickness gauges are available for the measurement of coating thicknesses on substrates. Such coating thickness gauges include a probe which, in use, is placed onto a coated substrate. The probe, in conjunction with the gauge is arranged to produce an electrical signal which is indicative of the distance between the probe tip and the substrate, and hence the coating thickness. The electrical signal is interpreted by the gauge which produces a numerical thickness value. A variety of different probe types are employed, depending on the type of coating and associated substrate being measured, for example ferrous or non-ferrous metals. Measured coating thickness values may be stored in a memory for later analysis.
In order to provide meaningful results, coating thickness gauges must be calibrated and, ideally, recalibrated during their use. Where the coating being measured is applied to a smooth substrate the so-called two point calibration method is generally employed. The probe is placed directly onto an uncoated portion of the substrate and a reading taken from the gauge. The gauge can then be adjusted to give a zero thickness reading for this value. When a zero value has been established a foil of known thickness, to represent a coating, is placed onto the uncoated surface and the probe placed on the foil and a reading taken from the gauge. The gauge is then adjusted so that the thickness measured through the foil corresponds to the thickness of the foil. By providing two known values this enables the gauge to be calibrated over a range of values usually between zero and the foil thickness measured. It is desirable to take a number of readings at each reference point and to calculate mean values.
The two point method is simple and reliable where the coated substrate is smooth. Difficulties arise where the substrate is rough, often the case with shot blasted materials. Rough surfaces are widely used as they provide a good key on which to apply a coating. On a microscopic scale rough surfaces include a series of peaks and troughs. The height of the peaks typically varies between 10 and 80 microns. The existence of peaks and troughs makes the measurement of coating thickness difficult especially where coating thicknesses are measured over the peaks on rough surfaces. This leads to difficulty in the calibrating of coating thickness gauges.
A number of different techniques are available for the calibration of coating thickness gauges when used on rough substrates. One method involves the measurement of the thickness of two foils of different but known thickness, similar to the two-point method used for calibration when measuring a coating thickness as applied to a smooth substrate. Another method of calibration for rough surfaces is the zero offset method. When using this method a thickness gauge is first calibrated using a smooth substrate. Readings are then taken of coating thickness applied to a rough substrate and a constant or “zero offset” value subtracted from the measured result. The zero offset value is indicative of the degree of roughness of the surface. The zero offset value is usually of the order of half the mean peak height of the rough surface. The roughness of a particular surface is usually a known quantity often being a result of grit or shot blasting the surface.
Once a gauge has been calibrated, regardless of the method of calibration used, it is desirable that the calibrated state is maintained until such time as re-calibration becomes necessary. If calibration is lost, for instance as a result of accidental reprogramming of a gauge then time is wasted in re-calibrating the gauge.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a coating thickness gauge to enable the convenient use of the zero offset method when measuring coating thickness on rough substrates. It is a further object to provide a coating thickness gauge which reduces the risk of accidental reprogramming or loss of calibration.
According to a first aspect of the present invention there is provided a coating thickness gauge comprising a means for measuring coating thickness to determine a coating thickness value, a means for storing one or more constants and a means to subtract one of said constants from the coating thickness value to produce a modified coating thickness value.
The means for storing one or more constants preferably comprises an electronic memory “M.” The gauge preferably includes a means to enable a plurality of coating thickness values to be stored, said means may comprise an electronic memory M which may be the same electronic memory used to store the constants.
The constants are preferably zero offset values, these values may be predetermined although it is preferable that a means is provided to enable a user of the gauge to input any desired value. A plurality of zero offset values may be stored from which a user may select a particular zero offset value which it is desired to subtract from measured coating thickness values. The stored zero offset values may be referred to by descriptive terms, for example fine, medium and coarse. In one such embodiment suitable values are 10, 25 and 40 microns respectively.
The means to subtract a chosen constant, or zero offset value, is preferably a microprocessor “P.” The microprocessor P may include a memory and may be programmable.
The means to enable a user any desired constant value may comprise a key or keys. In addition, the gauge may also be provided with a means to et the constant value from a computer. The gauge may also include a display
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to display measured values. In addition, the gauge may include a printer and/or a means to enable coating thickness readings to be transmitted to a computer. The constant values may be pre-set in such a manner that the operator may not change them.
The present invention provides a particularly convenient way for coating thickness readings to be taken from rough surfaces using the zero offset method. A chosen zero offset value may be selected and is then stored by the gauge, preventing possible loss or imperfect recollection of the value. In use the gauge subtracts the chosen zero offset value from the measured coating thickness values to give an accurate indication of a coating thickness as applied to a rough surface. As the subtraction is performed by the gauge this gives an instant result saving time and the need for tedious calculations to be made, whilst ensuring accuracy.
Where a gauge is employed to take coating thickness readings from a number of different surfaces appropriate zero offset values may be stored in the memory and selected as appropriate. Where both coating thickness values and zero offset values are stored in the memory the gauge may enable the choice of zero offset value to be retrospectively changed.
According to a second aspect of the present invention there is provided a coating thickness gauge comprising a means for storing an operational parameter and at least one key operable by a user, to enable the alteration of said parameter wherein said parameter may only be altered when said key is operated and held in an operated state for a predetermined period of time.
The means for storing preferably comprises an electronic memory. The operational parameter may relate to any function of the gauge although the parameter is preferably a value relating to the calibration of the gauge. The parameter may also comprise one or more zero offset constants. Preferably the gauge comprises a means to store a plurality of operational parameters which may be altered by operating an appropriate key or sequence of key operations. Such operational parameters are important for the accurate operation of the gauge, it is therefore preferable that these values may only be altered when the appropriate key has been operated and held for a predetermined period of time. The period of time is preferably at least 1 second, more preferably approximately 5 s
Baldwin Peter I
Williams Brian L
Assouad Patrick
Elcometer Instruments Ltd.
Salter & Michaelson
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