Test equipment and measurement systems are generally sufficiently complex to require a special engineering group to provide measurement hardware The requirement for test equipment is set by the quality-control engineer. The test — and inspection-equipment engineers, who are basically design oriented, take the quality-control engineer’s require­ments and provide the hardware that will be used by the process-control engineer in implementing the quality — control engineer’s plans The most comprehensive measure­ment expertise in an organization usually is in the test and inspection engineering group. Calibration to ensure correct functioning of test equipment is often a responsibility of this group as well.

Test equipment may be defined as that equipment used to measure or generate any of the various units of measure. This distinguishes between equipment that is part of the manufacturing process (and thus the responsibility of the manufacturing organization) and the test and inspection equipment. It is convenient to consider two categories of test equipment commercial equipment (equipment that is commercially available and may be purchased from a vendor) and special equipment (equipment that is designed, usually by the test and inspection-equipment engineering group, to solve a particular measurement problem) Since both categories involve intimate knowledge of measurement techniques (although no design skills are required in specifying commercial test equipment), both categories of test equipment are usually made the responsibility of the test- and inspection-equipment engineering group The two categories may be further broken into mechanical and electrical equipment

(a) Commercial Test Equipment. The availability of suitable commercial equipment should always be explored before the decision is made to design special equipment The cost of the commercial instrument will almost without exception be less than the cost of designing a piece of special equipment In addition to lower cost, there are a number of other advantages to using commercial equip­ment Commercial equipment is generally flexible, having been designed for the broadest possible market This flexibility reduces the chance of obsolescence when a measurement requirement changes or a new process must be measured An additional, and often major, advantage is that, when a particular commercial instrument breaks down, a substitute is available within the plant or from the vendor

There are many pitfalls to be avoided in purchasing commercial test equipment, and it is wise to let the responsibility rest with a group that specializes m measure­ment problems and test equipment The newest manufac­turer with equipment having the latest innovations is not necessarily the best choice of vendor. Unfortunately, new companies frequently drop a product line or go out of business, leaving the purchaser with an unsolvable mainte­nance and parts problem. The purchaser must also be wary of “specmanship,” where the manufacturer carefully selects his specification wording to make his product appear better than his competitor’s An example of this might be a digital voltmeter that operates within its temperature specification on a hot day but does not meet its accuracy specification unless it is operating at the low end of its line-voltage specification and (in addition) unless it has been calibrated and adjusted during the preceding 48 hr. The final choice of manufacturer and model should be based on

1. The ability of the equipment to perform the required task and any reasonable variations of the task.

2 The stability of the manufacturer and the proven reliability of his products

3 The compatibility of the equipment with existing test equipment (in terms of interfacing with other equip­ment and maintenance and calibration).

4 Price and delivery Price should be considered after the first three considerations since problems with any of the first three usually result in losses far exceeding any price differential in competitive products.

(b) Special Test Equipment. Justification The deci­sion to design and build special test equipment is usually made when it has been determined that there is no commercial test equipment to perform the particular measurement. Less frequently, the decision is made when commercial equipment is so general purpose and the particular measurement is so specialized that it is cheaper to design and build a special instrument than to purchase the general-purpose equipment The most common pitfall in this latter situation is finding that, after the special test equipment has been built, a design change in the equipment being tested results m total and irreversible obsolescence of the special test equipment Even where there is no choice but to design specialized test equipment, obsolescence through design change of the assembly under test presents a real hazard. The solution is to design all special test equipment to be as flexible as practical

Design and Building Special test equipment is pro­duced in small quantities, typically only one of a kind being built The labor costs for design and construction are the major items of expense, and material costs are not significant This must be kept in mind. For instance, other considerations being equal, it is not profitable to devote 2 hr of design effort to avoid the use of an SCR that is $10 more expensive than another SCR Similarly, incorporating available designs as elements in the special-equipment design can save time and money Duplication of circuits in the unit under test is often necessary to ensure compati­bility between the test equipment and the tested unit Although the designer has greater freedom in some respects, he has some constraints that are more stringent than those imposed on the product design engineer. Accuracy of the equipment being used to measure or to generate quantities normally must be 10 times greater than the tolerance of the device under test, i. e., the test equipment itself can contrib­ute no more than 10% to the error that is allowed for the unit under test. There are occasions when even 10% test-equipment error cannot be tolerated. There are other occasions when state-of-the-art measurements are made and a 10 1 accuracy ratio cannot be achieved Although

special test equipment involves construction of a single item or, at most, a limited quantity, workmanship cannot be compromised, and rugged construction is generally a must Engineering breadboards and prototypes cannot be simply stuffed into a box and shipped out for use. Inadequate mechanical ruggedness as well as poor solder connections in breadboards and prototypes are certain to create problems in normal use One technique, normally associated with production in quantity, can be applied to advantage in the production of special test equipment, namely, the use of printed circuit boards. These provide ruggedness and also have the short direct paths between discrete components needed in high-frequency and pulse circuits. Pnnted — circuit-board construction even on a single-unit basis can be as economical as a terminal-board layout. Definite cost savings can be realized if two or more units are built Moreover, the printed circuit board ensures similar perfor­mance of the units.

The approach in designing special test equipment is similar to that in designing a commercial product Initially the design goals are established in close cooperation with the quality-control engineer. Design alternatives are studied, and the best is selected for the particular measurement problem. In this choice a number of factors must be considered the complexity or difficulty of the measure­ment, working environment, skill level of the operator, expected useful life of the test equipment, cost per measurement over the useful life, operator convenience or human engineering, ease of maintenance and calibration, cost of maintenance and calibration, accuracy, safety, and initial cost. Once the approach has been established, the design engineer develops the detailed design, including breadboards of critical circuits if the problem is electrical Construction is best accomplished under the direction of the design engineer by people familiar with the peculiarities of test-equipment construction The completed equipment or first item is evaluated by the design engineer and, with more-complex design problems, further evaluated by the quality control engineer After his acceptance the equip­ment is then passed through the Calibration Laboratory for formal entry into the calibration cycle. Finally, the equipment is released to the process-control engineer for application

Documentation The documentation problem on spe­cial equipment is as complex as that for a new product to be marketed commercially The first documentation takes place m the engineering notebook or similar document, where various design approaches are explored and details supporting the design logic are recorded. When problems arise as the result of measurements made by test equip­ment, whether special or commercial, the first area ques­tioned is the adequacy of the test equipment This is rightfully so, and it is only sound engineering practice to have the validity of the design well documented In many cases the normal documentation associated with construc­tion and calibration of test equipment is not adequate for this purpose. Documentation of the design details, with one exception, must be complete and under formal control Because of the unique nature of test equipment and the importance of decisions that are based on its performance, the designer must clearly express all the details of his design to the group constructing the equipment and the group calibrating and maintaining the equipment. The exception to this would be the assembly details that are normally associated with the new product. Since special test — equipment production usually consists of only a few items and since they are constructed under the direct supervision of the test-equipment engineer, those assembly details not essential to the accuracy of the equipment need not be documented. Calibration instructions and specifications must be provided for the Calibration Laboratory. The equipment must not only perform correctly when initially released but also must continue to perform within specifica­tion, be periodically recalibrated, and be successfully repaired when necessary, all without the direct and continu­ing direction of the test-equipment engineer. When more complex test-equipment designs are involved, the quality — control engineer’s test or inspection instructions may not be adequate. Then a detailed instruction manual, similar to the manual for any commercial piece of test equipment, must also be provided by the test-equipment design engineer. This manual would provide the same type information customarily put into instruction manuals for commercial equipment, such as oscilloscopes, frequency counters, and spectrum analyzers.

Follow up and Feedback. Performance and continuing evaluation of special equipment is monitored in two ways. The maintenance record maintained by the Calibration Laboratory provides an accurate record of long-term performance. The process-control engineer provides rapid feedback of any critical problems. During the first months of use, he must be particularly alert to spot problems not readily apparent in an engineering evaluation. One of the common weaknesses of test equipment that is not always found in an engineering evaluation is its response to certain modes of failure in the unit under test. Under some conditions destruction of the test equipment is possible, and protection must be designed into it. The long-term performance information from the Calibration Laboratory maintenance record can be used to revise calibration recall intervals, i. e., to base them on actual performance In addition, this record may also indicate more subtle reliabil­ity problems

(c) The Calibration Laboratory. Both commercial and special test and inspection equipment must perform as intended by the original design engineer so that the test technician will feel confident of the results he obtains The Calibration Laboratory ensures this performance through its various activities and provides the basis for his confi­dence

The fundamental responsibility of the Calibration Lab­oratory is to ensure that the units of measure at a particular facility have the same dimensions as those defined and maintained at the National Bureau of Standards There are four basic areas of responsibility

1 Traceability of all units of measure to the National Bureau of Standards (NBS). Traceability implies both derivation of the local unit from the national unit and a known accuracy relation between the local and national units.

2. Maintenance, both preventive and corrective, must be performed to ensure continued performance of equipment within its specifications.

3. Documentation must be initially validated and there­after maintained to ensure that the equipment can be calibrated and restored, if necessary, to the required level of performance.

4. Recall control must be established to ensure that the equipment can be relied on throughout its life to be within its specifications with a reasonable degree of confidence

Organization and Staffing. The Calibration Laboratory can take many forms from a single person who performs all the basic functions to individual departments for each of the various tasks. In a medium — or large-scale operation, the first logical division of effort should be between the maintenance function and the standards function Meaning­ful results in standards work can only be obtained when meticulous care is taken in making measurements by someone who is not only familiar with the techniques involved but also takes great pride in his work. The maintenance function also requires special talents, but not to the same degree as standards work. In either case the individual must live by the rule that the quality of work, not quantity, is of prime importance. Where there are a number of people on the Calibration Laboratory staff, there must be technically competent leadership. In small organizations this is provided by the supervisor or manager. In large organizations engineers who are specialists m the areas of electrical and mechanical measurements provide the leadership.

New Equipment Control All new equipment, com­mercial or special, should pass through the Calibration Laboratory before it is released to the end user An initial calibration is performed to ensure that the equipment is indeed within its specification. At the same time the documentation is being checked out to make certain that it is adequate for future calibration and maintenance. The performance and repair record, discussed later in this section, is initiated at this time, and the equipment is placed on a recall schedule. These steps are taken to establish a firm base for all future control of the equip­ment.

Calibration Standards The traceability of any unit of measure, including a known accuracy relation to the unit as defined by the National Bureau of Standards, is the fundamental responsibility of the Calibration Laboratory The number of intermediate steps between the end user and the NBS depends on the accuracy requirements. Traceabil­ity does not imply direct comparison of the local standards against the NBS standard A valid traceable standard can exist even though the measurement has been passed from NBS through many intermediate laboratories provided the accuracy degeneration is correctly defined.

In each calibration laboratory some instrument or piece of equipment represents the most accurate repository at that local level for a particular unit of measure This then becomes the primary standard for that unit in that laboratory. Several echelons of measure may still exist between this local primary standard and the end user of test equipment. In some cases the local primary standard must be used directly to calibrate test equipment, more often it is used to calibrate other standards called “secondary standards” or “working standards.” Depending on the accuracy required of the local primary standard, it may be calibrated either by an independent laboratory or directly by the NBS Every measurement made degrades the transferred value of the standard involved in that measure­ment to some extent. This then becomes the disadvantage of using intermediate laboratories in establishing calibration standards. In very accurate measurements, this degeneration often cannot be tolerated. Despite this basic disadvantage, all local primary standards should not, and often cannot, be calibrated directly with the NBS. Calibration at NBS is usually much more expensive than traceable calibration through an independent laboratory, and it usually involves delays that may not be tolerable where alternate equipment is not available. In addition, NBS will not work with the less-accurate standards that are often all that are required for a particular local primary standard.

One word of caution on standards of any type no device, either mechanical or electrical, remains indefinitely stable, regardless of the ultimate source of the instability. For this reason all test equipment is placed on a calibration recall cycle. Standards, too, are subject to instability. The shiny new standard is not nearly as valuable as the old standard that has a proven record of stability. The standards of any calibration laboratory increase m value with time as their history of stability is documented

Use of Outside Laboratories. The integrity of traceable calibration is not jeopardized in the least by using outside laboratories for calibration of either the local primary standards or the user’s test equipment In the case of standards calibration, the first consideration needs to be that the required level of accuracy for that standard can be achieved by a laboratory other than the NBS. Each intermediate step between the user and the ultimate reference at NBS degrades the transferred value of the unit. The particular outside laboratory chosen to perform the calibration must be selected with care, especially if a local primary standard is to be calibrated. There are a number of obvious things to look for good equipment, certificates establishing traceability, neat and well-organized laboratory areas, and available and well-used reference material. These are all items that may easily be checked. More difficult to determine is the level of competence of the personnel Regardless of the quality of the physical facilities, reliable results m precision measurements cannot be obtained without highly qualified personnel

The choice of an outside laboratory for routine calibration of standard test equipment is not as critical, although the selection should still be made with care The reason for using an outside laboratory instead of NBS for calibration of local primary standards is that the turn­around time is shorter than at NBS On the other hand, the decision to use an outside laboratory for calibrating test equipment is based on economics. If the number of calibrations per year of a particular type is limited, use of an outside laboratory that already has appropriate stan­dards and trained personnel is more practical. When the cost of performing your own calibration is being compared with the cost of having an outside laboratory perform the work, the cost of calibrating local primary standards must also be included. It is not at all uncommon for the price of a single NBS calibration to exceed the cost of the equipment being calibrated.

Frequency of Calibration. A frequency of calibration is established to give the equipment user reasonable assurance that the equipment will remain within its specifications between calibrations. The frequency depends on the type of test equipment and its application. Because the severity of operating conditions can be so widely variable, calibration intervals for a class of instruments are usually based on their history of performance. This ensures an optimum recall interval for the particular conditions of environment, use, and abuse. Initially, intervals are estab­lished by reference to handbooks and manuals that describe normal recall intervals for typical applications. Too long a calibration interval is undesirable because it diminishes the possibility of the instrument’s remaining within its specifi­cations throughout the interval. The cost of repairs per call-in usually increases in this circumstance. On the other hand, too frequent call-in has disadvantages. Calibration costs per year on the instrument are higher than they should be. In addition, the instrument is out of service more frequently than is necessary, resulting in inconve­nience to the user and the need for backup equipment that would not otherwise be required.

The significance of calibration stickers should be considered. Too frequently a current calibration sticker is considered proof that the instrument is functioning cor­rectly, and, conversely, an expired sticker is proof that the instrument is no longer within specifications. Barring errors, the sticker is only proof that the instrument was within specifications at the time of calibration. It also signifies that, if the instrument is within the calibration interval, there is a reasonable probability that it is still within specifications. If the calibration interval has expired, there is a diminished probability that the instrument remains within its specifications. With both electrical and mechan­ical test equipment, there is always the chance of a subtle failure that is not readily detectable There is no substitute for intelligent use of test equipment by a user who is alert to subtle irregularities.

Call-in Techniques. It is often more difficult to break loose a piece of equipment from the user for a trip to the Calibration Laboratory than it is to repair and calibrate the instrument. Two direct approaches are generally used. When the instrument is calibrated, a sticker is put on it which gives, among other information, the date when the instrument is due for recalibration. The user himself can thus see when the equipment must be returned for recalibration, and he can schedule his use of the equipment to allow turn-in before the calibration has expired. The Calibration Laboratory keeps a record of the date the instrument is due for recalibration. Each month lists of equipment due for recalibration are circulated to alert the users so they can plan around the temporary loss of the equipment. Delinquent lists are also published by the Calibration Laboratory when equipment is not turned in as required. The attitude of the supervisor or foreman in the area using the equipment can be helpful in ensuring the timely turn-in of the equipment. Another call-in technique that can be used in some special situations is the running­time meter Where equipment degeneration is based on running time rather than elapsed time since previous calibration, the running-time meter can be used.

Performance and Repair Records The Calibration Laboratory must generate at least two records the calibra­tion sticker and the equipment-history card. The calibration sticker is placed on the instrument after calibration and contains, as a minimum, the date calibrated, the date due for recalibration, and the identification of the person who performed the calibration. Information such as equipment — use limitations and equipment accuracy may also be desirable. The equipment-history card contains all the basic data and history of the instrument. In small — to medium — size operations, this record is normally a maintenance or history record that is maintained on a one-card-per — mstrument basis In large organizations the information may be entered into a computer. The basic information that must be maintained in this record includes the description of the equipment, the identifying serial number, the name of the person who performed the last calibration, the date of the last calibration, the date recalibration is due, and the condition the equipment was in when received for repair or calibration Besides providing the basic informa­tion for calibration recall, the history information is useful in troubleshooting, and the information on equipment condition is used to establish realistic calibration intervals.

Obsolescence Determination Most equipment even­tually reaches the point where it becomes more economical to replace it than to continue it m service There is no problem in determining that new measurement require­ments have exceeded the capabilities of existing instrumen­tation More difficult to determine is when equipment should be removed from service owing to lack of use or the high cost of repair. The history card is the basic tool for making this determination Data on the equipment condi­tion when received and the extent of repairs necessary at each calibration can be used to decide when it is cheaper to invest in a new instrument with low maintenance cost When this judgment is being made, maintenance costs associated with equipment abuse should be excluded since, in most cases, these costs would continue even with new equipment. Disposal of equipment for lack of use is highly dependent on an organization’s specific situation. The advantages of removing the equipment from an organiza­tion’s capital assets should be considered, as well as the cost of storage, equipment deterioration, and the risk of equipment obsolescence with prolonged storage.