Related Programs and Projects
To satisfy requirement of users, three sets of research apparatus will be developed, whose key part is a kind of distributed sampling shunt with ultra-low load effect. The apparatus includes: a wide range ultra-high precision current meter, a calibration device for load coefficient of sampling shunts, and a highly stable constant current source. The project will carry out research in material characteristic of new resistance wire and self calibration method for load factor determination, preproduction of a new kind of resistance element with high precision and establishment of database of typical temperature coefficient of resistance alloy material. It will built an accurate and reliable foundation of national electrical measurement system and improve the national traceability system of the accurate high current measurement.
The current unit Ampere is one of basic units in international unit system (SI). It plays a core role in electric measurement. However, there is not any effective method to regularly maintain an accurate constant current as a standard up to now. In practice, the current is measured through the voltage drop on a resistor using Ohm's law. The main problem of this method is the measurement error caused by resistance change due to current heating. Generally, the relative resistance change caused by current heating is called resistance load factor. Therefore, the key point in precision current measurement is to reduce load factor of resistors.
In this project, it is expected to get the following three achievements:
1. Home-made and imported current shuts are of discrete forms. They have a considerable thermal resistance to environmental temperature. A new design of a shunt is of network type, which includes a series of element resistors whose contact area to the environment is increased and correspondingly whose thermal resistance is reduced effectively. At the same time, the load of the whole shut is shared by multiple discrete resistors; the load of a single resistor becomes very light. Owing to the above two aspects, the load factor of the whole shunt is greatly reduced.
2. A long connecting wire should be used when multiple small discrete components are coupling into the network. The resistance of the wire affects the accuracy and load factor of the whole resistance seriously. It has been proved in theory by the project team that there exists a special node in the network. If the potential terminal of the shunt is drawn out from this node, the influence of connecting wire resistance can be eliminated. This conclusion has been evaluated and proved by experiments.
3. Because the distributed structure of the shunt is flexible, a method of self calibration for load factor is developmentThe calibrate accuracy of the load factor is at an order of about E-8. This method can be applied to calibrate load factor of sampling resistors and similar products.