Omega is a reliable source for load cells and pressure transducers that provide superior quality data in an increasing number of processes. It is essential for the pressure or force of that process to reach a sensing element in order for load cells and pressure sensors to provide the information required by customers.
The sensing element reacts to the pressure or force of the process, producing an output signal that can be interpreted by a data-collection device or a readout device. Therefore, the sensing element is considered to be the heart of the transducer or load cell.
The Pressure Measurement System Theory
The pressure measurement system consists of a sensing element with four strain gauges applied to it. The strain gauges are configured in a Wheatstone bridge, where all 4 resistors (labeled R1 thru R4 in Figure 2) are equal, and change by equal magnitude proportionally upon the application of strain.
Greater output is achieved with greater force or strain (input). A Wheatstone bridge device needs four wires for its connection, positive and negative sensor output, and positive and negative excitation.
A typical pressure sensor works producing a strain gauge output as the deflection of a diaphragm is caused. The output can vary from 1 to 3 millivolts per volt (mV/V) to as much as 10 to 30 mV/V, depending on the strain gauge technology. The output of the sensor must be multiplied by the voltage used to power the device in order to calculate full scale output.
For example, for a 3 mV/V sensor, if 10 volts DC is used as the excitation voltage, users can expect to get 3 mV/V x 10V=30 mV at full scale.
Figure 3. Typical Reaction of Diaphragm When Pressure is Applied.
The PX4600 pressure transducer is considered to be one good example of how a pressure sensor works. The process pressure that the customer is trying to measure will be brought to the diaphragm element via an access port.
The diaphragm is deflected due to the pressure, stressing the Wheatstone bridge arrangement on the other side of the diaphragm, and producing a mV/V output. This millivolt signal is then read by a device that can accept a millivolt signal or to a signal conditioner or an amplifier for further signal processing.
A USB connector is fixed at the end of the cable of the PX409-USBH for direct input into a laptop computer. The onboard electronics process the signal into a communication protocol, which can be used in an easy and convenient manner. The free software available on Omega’s website can be used for a plug and play experience. It is possible to connect a unit to a laptop which will display and gather data providing power to the sensor itself.
Figure 4. Example of Wheatstone Bridge Sensing Element on a Broad Mountable Transducer.
Figure 6. DPGM409
A digital output is used by the DPGM409 digital pressure gauge in its wireless transmitter versions. This enables acquisition of the readings from a remote line of sight location without the need to run signal wire. This signal will be accepted by a wireless receiver and then the data will be displayed or logged.
An unamplified output is present in most lead cells. Unamplified outputs are common where the environment is too extreme for electronics to survive, or with devices that are too small to be equipped with signal conditioning electronics.
This is the case with the PX1009, PX1005, and PX1004 products, which are unamplified due to the extremely high and extremely low operational temperatures at which they are designed to function.
Unamplified sensors comprise of a rather short transmission distance capability, usually no longer than 6.1 to 9.1 m (20 to 30'), due to small signal strength. This also makes them susceptible to electromagnetic noise from the surrounding environment.
Internal signal conditioning electronics are used by amplified sensors to create a stronger signal. This allows them to be less susceptible to environmental noise and capable of going longer distances to their receiving units. A smaller operational temperature range is present in sensors with internal amplifiers due to temperature restrictions of the signal conditioning electronics inside the sensor.
Current output sensors can provide high accuracy despite sending their amplified signal as much as 304.8 m (1000'). In general, accuracy under 30.5 m (100') can be maintained by voltage output sensors.
The third type of sensor is a digital output sensor, categorized by output. This type of output is capable of providing the longest transmission distances and the lowest noise available. A wide range of communication styles are available, such as the DPGM409 and the PX409-USBH or RS485 devices.
Figure 9. Typical 5-point calibration
Total Error Band
This is the band maximum deviation for any output when considering all defined sources of error, such as humidity or temperature or vibration. It is expressed as a percentage of the rated output.
Static Accuracy, with the combined effects of linearity, repeatability, and hysteresis, is expressed as ±% of span, and is in reference to the BSL. The static error band is considered to be a good measure of the accuracy that can be expected from a load cell or a pressure sensor at a constant temperature.
BSL (Best Straight Line)
BSL is the maximum error deviation from a terminal-base line, divided in half. The outputs from zero and full-scale are used to create a line in order to determine this line. Distance from this line is used to measure the other data points.
The line that has the same slope as the terminal-base line is the Best Straight Line, but this line is offset so that the errors are equally split on either side of the BSL. Performance for linearity is described by the Best Straight Line.
This is the maximum deviation of the calibration curve from a straight line drawn between the rated and no-load outputs. Non-linearity is expressed as a percentage of the rated output and measured only on increasing pressure load.
The maximum difference between output readings for the same applied pressure, approached from opposite directions, is known as hysteresis. Hysteresis is determined by comparing outputs for a pressure value, first attained by approaching from lower pressure and then by approaching from a higher pressure. Lower hysteresis is obtained when the two readings are closer. This error is difficult to correct.
Repeatability is the maximum difference between output readings for repeated pressure loads, under identical environmental and load conditions. Better repeatability is obtained when the readings are closer. It is not possible to correct this error.
This information has been sourced, reviewed and adapted from materials provided by OMEGA Engineering Ltd.
For more information on this source, please visit OMEGA Engineering Ltd.