Last updated on April 22nd, 2025 at 02:36 pm
A ratemeter is a measuring instrument that accepts input pulses and displays a value that is proportional to the rate at which the pulses occur, averaged over a time selected by the user. Another way to express this is:
Rate = ( input frequency/scale factor ) x time factor
During the setup of a ratemeter, the user selects the desired display parameter (e.g., GPM) and enters the number of pulses the sensor generates per unit. The meter applies the appropriate time factor to the equation and displays the reading. For example, the flow application shown below has a turbine flow sensor with pulse output connected to a pulse input ratemeter. The ratemeter displays GPM:
Gallons/minute = (pulses per second/pulses per gallon) x 60 sec/minute.
Another common application for a ratemeter is measuring motor speed. The second example shows a ratemeter connected to a magnetic pickup that detects shaft rotation. The scale factor here is the number of pulses per rotation. For a readout in revolutions per second, the time factor is 1.
In the third example, a photoelectric position sensor on a conveyor generates one pulse for every object that passes it. To display a production rate in units per hour, the meter applies a time factor of 3600.
Ratemeters are available in a variety of case styles. Some are designed for panel mounting. DIN rail mounting and enclosed field mounting are also popular. The KEP BATM series has six case styles, including panel mount, NEMA cased, and explosion proof.
Panel-mounted meters are usually powered by AC mains or a low-voltage DC supply. Field units located close to the sensor often include a battery or are loop-powered. Many digital ratemeters offer additional functionality such as analog output, serial communication and alarm output.
An analog output (dc V or mA) is used to transmit a linear rate signal to a remote indicator, data recorder or other peripheral. This output can also feed a rate signal to a process controller or PLC.
Advanced ratemeters often include a digital communication port for connection to a SCADA system or corporate network. This is useful for production control, quality monitoring, and other support functions. The meter’s low data rate makes RS-485 an effective and easily implemented interface with Modbus RTU protocol. USB and Ethernet are communication options on some newer instruments. The Trumeter Vista Touch is a flow rate indicator with a graphical touch display that includes all three digital interfaces.
An alarm output allows the ratemeter to activate an external alarm or two-state control device when either a high or low limit is reached. Normally, this port is a transistor output for low voltage switching (<30Vdc) or a relay that can switch AC mains voltages.
Another useful feature in a ratemeter is multi-point linearization. Many flow sensors are non-linear at the low end of their range. Linearization improves sensing accuracy and can extend the usable measuring range.
Examples of ratemeters available from Weschler include (clockwise from left): the Simpson S661, a basic 1/8 DIN panel mount meter with 2 relay outputs; the Trumeter APM-Rate, a panel mount meter with digital and bar display, analog output and 2 alarm outputs; the KEP BATRTM, in an explosion-proof housing, can be loop powered, with optional analog output and optional RS-485 communications; the KEP BATRATM, in a NEMA 4X enclosure, with a Hi/Lo alarm output (battery power, analog output and RS-485 optional).
Totalizer
Some ratemeters include a totalizer function that measures the cumulative volume, mass or count. On meters with a single display, the total may be viewed by toggling the readout. Both parameters are simultaneously visible on meters with a dual display, such as the panel-mount version of the KEP BATM (left). The totalizer function can also be purchased as a separate instrument in this product family. Similarly, Laurel offers a ratemeter, a totalizer and a combined ratemeter/totalizer in their versatile Laureate Counter line (right).
A ratemeter with an analog input will measure the output of a smart sensor or signal conditioner. The KEP INT69 is an integrating ratemeter that accepts 4-20mA, 0-20mA, 0-5V, 0-10V or 1-5V signals. A square law function is available for inputs that require square root extraction. Like the Laureate, it can be ordered as a ratemeter, totalizer or combined ratemeter/totalizer.
Batch Control
A ratemeter’s analog output may be used to drive a batch controller. A batch controller is a device that regulates the dispensing of materials to produce a desired mixture or product. Some batch controllers include ratemeter and totalizer functions. The KEP BATCHtrol2 (upper left) accepts both analog and pulse inputs. The MB2 Mini-Batcher (lower left), obtains its input directly from a pulse output sensor. The batch control function is also available in other types of instrumentation. Some advanced digital controllers, such as the the Yokogawa UT55A (right), can be programmed to operate as a batch controller. The Laureate Counter mentioned above can also be configured to do batch control.
In summary, rate measurement is a basic function essential to many processes and production systems. A variety of ratemeter products are available to measure flow rate, speed, and other rate variables. This capability is also included in other types of process instrumentation. Modern digital instruments make accurate rate measurements and offer extended capabilities to simplify system integration, increase thruput, and maintain quality while improving yield.