Demonstration model of a moving iron ammeter. As the current through the coil increases, the plunger is drawn further into the coil how to use clamp meter pdf the pointer deflects to the right. Wire carrying current to be measured.
Furthermore, the poles of the permanent magnet are arcs of a circle. Early ammeters were laboratory instruments which relied on the Earth’s magnetic field for operation. It is generally represented by letter ‘A’ in a circle. North when a current flowed in an adjacent wire.
This made these instruments usable only when aligned with the Earth’s field. The uniform air gap between the iron core and the permanent magnet poles make the deflection of the meter linearly proportional to current. These meters have linear scales. Because the magnetic field is polarised, the meter needle acts in opposite directions for each direction of current.
For this reason moving-coil meters are only usable directly for DC, not AC. Moving magnet ammeters operate on essentially the same principle as moving coil, except that the coil is mounted in the meter case, and a permanent magnet moves the needle. Moving magnet Ammeters are able to carry larger currents than moving coil instruments, often several tens of Amperes, because the coil can be made of thicker wire and the current does not have to be carried by the hairsprings. Indeed, some Ammeters of this type do not have hairsprings at all, instead using a fixed permanent magnet to provide the restoring force.
An electrodynamic movement uses an electromagnet instead of the permanent magnet of the d’Arsonval movement. The iron element consists of a moving vane attached to a pointer, and a fixed vane, surrounded by a coil. As alternating or direct current flows through the coil and induces a magnetic field in both vanes, the vanes repel each other and the moving vane deflects against the restoring force provided by fine helical springs. The deflection of a moving iron meter is proportional to the square of the current. Consequently, such meters would normally have a non linear scale, but the iron parts are usually modified in shape to make the scale fairly linear over most of its range. Moving iron ammeters are commonly used to measure current in industrial frequency AC circuits. In a hot-wire ammeter, a current passes through a wire which expands as it heats.
Although these instruments have slow response time and low accuracy, they were sometimes used in measuring radio-frequency current. These also measure true RMS for an applied AC current. In much the same way as the analogue ammeter formed the basis for a wide variety of derived meters, including voltmeters, the basic mechanism for a digital meter is a digital voltmeter mechanism, and other types of meter are built around this. There is also a range of devices referred to as integrating ammeters.
A picoammeter, or pico ammeter, measures very low electric current, usually from the picoampere range at the lower end to the milliampere range at the upper end. Ammeters must not be connected directly across a voltage source since their internal resistance is very low and excess current would flow. Ordinary Weston-type meter movements can measure only milliamperes at most, because the springs and practical coils can carry only limited currents. The resistances of shunts is in the integer to fractional milliohm range. Nearly all of the current flows through the shunt, and only a small fraction flows through the meter. This allows the meter to measure large currents.
To make a multi-range ammeter, a selector switch can be used to connect one of a number of shunts across the meter. It must be a make-before-break switch to avoid damaging current surges through the meter movement when switching ranges. It also avoids any inaccuracy because of contact resistance. And if the movement resistance is 1000 ohms, for example, R1 must be adjusted to 4. Switched shunts are rarely used for currents above 10 amperes.
Zero-center ammeters are used for applications requiring current to be measured with both polarities, common in scientific and industrial equipment. A special type of zero-center ammeter for testing high currents in cars and trucks has a pivoted bar magnet that moves the pointer, and a fixed bar magnet to keep the pointer centered with no current. The magnetic field around the wire carrying current to be measured deflects the moving magnet. Some recent types have a parallel pair of magnetically soft probes that are placed on either side of the conductor. The needle’s resting position is in the centre of the scale and the restoring spring can act equally well in either direction.
This page was last edited on 13 November 2017, at 16:08. Flow can be measured in a variety of ways. Positive-displacement flow meters accumulate a fixed volume of fluid and then count the number of times the volume is filled to measure flow. Other flow measurement methods rely on forces produced by the flowing stream as it overcomes a known constriction, to indirectly calculate flow. Flow may be measured by measuring the velocity of fluid over a known area. The density of a liquid is almost independent of conditions.
This is not the case for gases, the densities of which depend greatly upon pressure, temperature and to a lesser extent, composition. The energy flow rate is the volumetric flow rate multiplied by the energy content per unit volume or mass flow rate multiplied by the energy content per unit mass. Gases are compressible and change volume when placed under pressure, are heated or are cooled. A volume of gas under one set of pressure and temperature conditions is not equivalent to the same gas under different conditions.