First we should differentiate between eddy current equipment for inspecting heat exchanger tubes and eddy current equipment for surface crack detection.
In heat exchanger inspections, usually equipment sits in one place for a minimum half day to inspect a small heat exchanger or possibly many days to inspect a large heat exchanger. Obviously, they must be reasonably portable and they must be sufficiently large to house the electronics for the several eddy current channels and data storage hardware devices that are necessary.
On the other hand, surface crack inspection equipment might be used to inspect for surface cracks on a large item, such as an aircraft hull. This requires changing from inspection site to inspection site every few minutes, such that a single handheld unit weighing just a few pounds and running on batteries so that power cords need not be strung is what is required.
Obviously the mainframe requirements for surface crack inspection applications and heat exchanger tube inspection applications is entirely different. This article will consider only heat exchanger tube inspection equipment.
In the past, the obvious choice was an oscilloscope main frame because it provided power supplies and a CRT as a display device. This is no longer done. The three mainframes of choice today are a laptop computer, a lunch box computer, or a Manufacturer's proprietary package. Each has its advantages and disadvantages.
LAPTOP COMPUTER MAINFRAME
A laptop computer offers an easy Operator interface by means of its keyboard and CRT. It probably has a large internal disk drive for bulk data storage. It could have an internal drive with a removable medium, such as a CD burner. These are all advantages.
When a laptop computer is employed, a separate box for the electronics will be necessary. This requires two boxes to be carried to the field, which is inconvenient when the application calls for moving from heat exchanger to heat exchanger from time to time.
Another disadvantage is that a laptop computer and items such as a DVD burner are really built for the office environment and may not stand up well in the field.
Further, should a failure occur to a laptop, it probably has to be returned to the Manufacturer's service facility due to the fact that most of the components are proprietary. Certainly this would mean that an Operator would no longer have access to eddy current data and defect data that may have been stored on the hard drive.
MANUFACTURER'S PROPRIETARY MAINFRAME
Certainly these look rugged and may well be, but sometimes a mainframe that was custom designed for the application may not have been tested in the field sufficiently to confirm ruggedness.
Proprietary mainframes generally do not have a large internal hard drive, nor do they have provision for an internal removable medium drive, such as a Magneto Optic Drive (Magneto Optic is considered here over a CD burner because Magneto Optics tends to stand up better in the field environment). This results in the use of an external Magneto Optic Drive, once again resulting in the need to carry two boxes to the field.
For Operator interface, these units tend to have a good screen, but the keyboard may be optional, resulting in additional items to be carried to the field.
As these mainframes are 100 per cent Manufacturer proprietary should a failure occur there is very little that is serviceable without returning the entire unit to the eddy current instrument Manufacturer.
LUNCH BOX COMPUTER MAINFRAME
Lunch box mainframes are built much more ruggedly than laptop computers.
Lunch box computers have a good quality screen and keyboard (sometimes waterproof) for a good Operator interface.
Lunchbox computers are mass produced; therefore, they contribute less to the cost of an eddy current system than a custom-built proprietary mainframe or a separate box for the electronics, as in the case for a laptop computer mainframe. The eddy current equipment Manufacturer who chooses a lunchbox mainframe does not have to concern himself with sheet metal work, control panel, or controls, as all of these items are part of the lunchbox computer mainframe.
Lunch box computers have adequate room for ISA slotted eddy current instrument boards and for built-in internal Magneto Optical Drives, resulting in a complete multi-frequency eddy current inspection system in only one package. Only one box has to be carried to the field.
Most components in a lunch box computer are generic or can be replaced with generic products, such that if a failure occurs in the field, replacement parts are readily available.
For example, although the keyboard which folds up and attaches to the main unit is proprietary in its packaging, it has a standard interface connector so that any keyboard could be substituted, should the keyboard fail. Likewise, should the LCD screen fail, a standard VGA monitor can be connected to the VGA connector.
Items like the power supply and disk drives are generic and readily available from any computer shop.
A motherboard failure may present an inconvenience as eddy current plug-in boards are designed to occupy ISA slots. It might not be possible to find a motherboard in stock at most competent computer shops that would have sufficient ISA slots for a multi-frequency eddy current system.
If one of the several eddy current boards fails, then it can be disconnected from the other boards, so that the remaining operational boards can be used to successfully complete an inspection.
CONCLUSION
A lunch box computer is ideal as a multi-frequency eddy current instrument mainframe for heat exchanger tube inspection applications.
