The economics of volume meter testing systems - Static test benches versus trolley and tunnel types
Two basic physical configurations of automatic and semi-automatic test systems are routinely used for volume meter testing. They are static test benches and those with moveable meter trolleys, which are sequentially placed into test tunnels (gantries) for processing. Both types are capable of operating fully to the requirements of IEC 736, but there are significant differences and trade-offs to be considered in performance, throughput and manning requirements.
Historically, automatic test systems were originally developed with moveable trolleys, as the capital cost of the electronics and computing power necessary to run such systems was high compared with the cost of producing the mechanical system to support the meters on test. Some manufacturers thought it was necessary, therefore, to make the fullest possible use of these high cost parts of the system, even at the expense of additional mechanical and electrical complexity.
In recent years the situation has changed markedly. The cost of electronics, and particularly computing power, has fallen significantly relative to the cost of the mechanical support systems. This has led to the need to re-evaluate the relative costs of the two types of system, taking into account the advantages and disadvantages of each.
BASIS OF COMPARISON
A fixed test bench able to test 40 single phase KWH meters simultaneously is compared here with a similar trolley system consisting of two 20-position trolleys, a test gantry and a separate warm-up station. These two systems are similar in capital cost, as the hardware requirement is roughly equivalent and the software and computing costs are identical.
They are compared principally for potential throughput and manning requirement, but other secondary features are also considered.
Figure 1 shows the flow of meters through a single-gantry trolley type bench using two 20 meter trolleys and a separate warm-up station, and compares this with a single 40 position fixed test bench. It is assumed that the test requirements are identical for each system, and as a basis for comparison the UK regulatory requirements for repaired single rate, accuracy class 2, Ferraris disk meters are used to determine the types of test required. Other regulatory environments may well give different results.
The functions required are
- Pre-heating of meters
- Adjustment of meters at 5% I basic cos = 1
- Adjustment of meters at I max, cos = 1
- Adjustment of meters at I max, cos = 0.5
- Starting watts test at 0.5% I basic
- No load running test at 110% of V Nominal
- Accuracy test of meters at 5% I basic cos = 1
- Accuracy test of meters at I max, cos = 1
- Accuracy test of meters at I max, cos = 0.5
- Register (dial) test.
Items 1-4 are normally carried out with meter covers removed, to allow adjustments to be made. Items 5-10 are final tests, normally carried out with covers on. No adjustment to the meters can be made during these tests.
With a trolley type system, the pre-heating, starting watts and no load running tests are normally carried out with the trolley attached to a small warm-up power unit, with no provision for automatic monitoring of meters or measurement of their accuracy. The meter trolley is then moved into the main test gantry for adjustment and accuracy tests.
This method of working gives rise to one particular procedural problem. It is a condition of the test regulations that final tests are carried out sequentially, with no adjustment of the meter being made between the elements of the test. The reason for this is that the adjustments made to an induction meter during calibration are somewhat interactive, and an adjustment made to correct low load inaccuracy may affect other parameters such as starting performance. It is therefore a violation of the test regime to adjust the meter after checking starting watts. This problem obviously does not arise with a fixed bench, as tests can be carried out in any convenient order.
As shown in Figure 1, the throughput with the trolley system over a single 8-hour shift, using the above procedure, is 100 finished meters (5 runs in total). The throughput with the 40 position fixed system is 120 meters (3 runs in total) over the same period.
Figure 1 also shows the manning requirement for the two systems. It can be seen that the requirement for the two-trolley system is more or less one person continuously, with occasional assistance required at peak times.
The 40-meter fixed system has a significantly lower manning requirement – slightly over 55%. This is borne out by experience in actual test environments, where it has been found that two skilled operators can easily cover three fixed test benches.
INITIAL CAPITAL COST
There is little difference in initial capital cost between a 40 position fixed bench compared with a trolley system using a single gantry and two 20 position trolleys. Expansion costs are also similar, in that the cost of expanding a single gantry to two gantries on the same power unit, and doubling the number of trolleys, is only slightly less than adding a second 40 position rack to a fixed bench (as long as the original power units are specified with such expansion in mind).
MAINTENANCE AND RELIABILITY
There are few basic differences between the two systems, except that trolley type systems must have a reliable means of connection between the trolleys and the test gantry. This normally consists of a low insertion force, high current connector which is either automatically or manually inserted each time the trolley is connected to either the test or warm-up power unit. This connection system must be ultra reliable and robust or it will cause unreliability of results. Even the best type of connector available is less reliable than a fixed connection.
Single 20 position gantry +
Two 20 position trolleys +
|Throughput (meters/8-hour shift)||100||120|
|Manning requirements||110% man - day||55% man - day|
|Adaptability to alternative meter base arrangements when quick connectors are used||Alternative trolleys may be purchased||Not possible without considerable difficulty|
|Maintenance requirements||Trolley plug-in connectors are liable to damage if misused. Otherwise limited to routine maintenance and calibration||Limited to routine maintenance and calibration|
|Working environment||Space is required to wheel trolleys between loading, test and warm up stations||Limited to bench footprint plus access only|
|Functionality||Potential problems arise with creep and starting watts tests being carried out during warm-up and before calibration. Also with some electronic meters it may still be necessary to carry out these tests in the main gantry||All functions, including warm-up, creep and starting watts may be carried out in whatever order is the most convenient, and on both electronic and Ferraris meters|