Load Cell & Pressure | Bubble | Occlusion | Liquid Level | Flow Meters


· NIST – National Institute of Standard and Technology: A metrology laboratory and non-regulatory agency of the United States Department of Commerce. Sensors with “NIST Traceable” calibration have an unbroken chain of measurements leading back to standards maintained by NIST.

· CENELEC: the official electro-technical standardisation body of the EU, producing assessment and measurement standards

· ISO Certifications: a rigorous independently certified set of standards for quality systems, which is maintained by ISO  “International Organization for Standardization”.  The quality systems communicate strategic objectives and foster a culture committed to excellence in everything we do

· I2C (aka I2C): Short for Inter-Integrated Circuit, a communication protocol that is commonly used for communication between a sensor and a processor or microcontroller. Technical Note: Signal Conditioning I2C

· Noninvasive: Sensors that are noninvasive are able to take measurements of a fluid in a closed system (e.g. a disposable tubing set) without coming in contact with the fluid. SMD offers many types of noninvasive sensors, including pressure, bubble, flow, occlusion, and temperature.

Load Cells & Pressure Sensors

· Load Cell (aka Force Sensor): A transducer that converts force applied to it into an electrical signal. Typically made up of multiple Strain Gauges in a Wheatstone Bridge configuration.

Thin Film Strain Gauge· Strain Gauge: A device used to measure strain. Typically four of them are used in a Wheatstone Bridge configuration to make a Load Cell. 

· Sputtered Thin-Film Strain Gauge: sputtered thin film process to create the electronic strain gage circuit. This process involves the sputter deposition of thin films of dielectric and resistor alloy onto metal components. This process forms a molecular bond with the component with no adhesives involved thereby eliminating the adhesive-based errors associated with other bonded strain gauge load cells.

· Wheatstone Bridge: a network of restive “legs”, one of more of these legs can be an active sensing element. The Wheatstone Bridge is the electricalWheatstone Bridge equivalent of two parallel voltage divider circuits. A physical effect, such as a change in strain applied to a sensor, changes the resistance of the sensing elements.

· Dielectric: an electrical insulator that can be polarized by an applied electric field. When a dielectric is placed in an electric field, electric charges only slightly shift from an average equilibrium position causing dielectric polarization. Dielectric polarization creates positive charges, displaced in the direction of the field and negative charges shift in the opposite direction, this creates an internal electric field that reduces the overall field.

· Rated Capacity: Design value of the upper limit in the measuring range.

· Rated Load: Upper limit of load at which the transducer preforms to the specifications.

· Rated Output: Value obtained by deducting the output under no load from the output under the rated capacity.. Usually expressed in mV/V, mA, or equivalent strain.

· Overload: Load exceeding the rated capacity.

· Safe Overload Rating: Max overload which may not cause any permanent change to stated specifications, expressed in percentage of the rated capacity.

· Ultimate Overload Rating: Max overload which can be applied without causing any structural damage, expressed in precentage of the rated capacity.

· Overload Protection: A system designed to prevent a sensor from being damaged by an applied force that is higher than the maximum rated load.

· Rated Output Voltage: Rated output expressed with the output voltage at the open end. It is expressed together with the excitation voltage.

· Rated Output Current: Rated output expressed with the current flowing when connected to a prescribed load impedance. It is expressed together with the excitation volatge and load impedance.

· Rated Output Equivalent Strain: Rated output expressed with a vaule converted into strain, with the gauge factor 2.0.

· Input/Output Resistance: Resistance of input or output terminal. Use of the thermis limited to the indication of the nominal resistance of an input or output terminal.

· Stress: Force applied per unit area of a material, typically expressed in N/m² (Pa) or lb/in² (psi)

· Strain: The response of a material to an applied stress, typically expressed in terms of change in length over the material’s original length. 

· Gauge Factor: The ratio of the change in electrical resistance to the mechanical strain in a strain gauge. 

· Maximum Capacity (aka “Range” or “Capacity”): The maximum recommended applied force for a given load cell as specified by the sensor’s datasheet.

· mV/V: The output voltage of a load cell divided by the input voltage applied to the sensor. Usually expressed at the force sensor’s maximum load capacity. For example, a 2kg load cell with a rated output of 2.0mV/V and a supply voltage of 10V would have a 20mV output signal with 2kg force applied.

· Accuracy: The degree to which a sensor measurement corresponds to the correct value. Incorporates, temperature effects, non-linearity, hysteresis, and non-repeatability.

· Non-Linearity: The difference between the ideal and actual mid-scale sensor reading divided by the full range of the sensor, expressed as a percent. Typically referred to as “Terminal Point Non-Linearity”.

· Hysteresis: The occurrence in which the value of a physical property lags behind changes in the effect causing it. In the case of a load cell or pressure transducer, hysteresis is defined as the upscale reading (usually at mid-scale) minus the downscale reading, divided by the full scale reading expressed as a percent of the full scale output.

· Excitation Voltage: Voltage applied to the input terminal of transducer.

· Recommended Excitation Voltage: Maximum excitation voltage with which the transducer can preform to the specifications.

· Safe Excitation Voltage: Maximum excitation voltage with which the transducer may not preform to the specificationd but when the commended excitation voltage is recovered, the transducer preforms to the specifications.

· Non-Repeatability: The maximum difference in output when the same load is consecutively applied to a load cell.

· Repeatability: Maximum difference between output varieables inititated by repeatly applying the same load under the same conditions. Usually, it is measured using the rated load and expressed in percentage of the average rated output.

· Zero Balance: Output under no load with the tranducer placed in the prescribed posture. Usually expressed in mV/V, µm/m or percentage of the rated output.

· Temperature Effect on Zero Balance: Change of zero due to change of ambient temperature, expressed as change of zero per 1° C in percentage of the rated output.

· Temperature Effect on Output: Change of gain due to chnag eof ambient temperature, expressed as change of zero per ° C in percentage.

· Resolution: The lowest detectable change in output from a sensor, usually dependent on the equipment used to read a sensor. See our Technical Note on Resolution & Accuracy

· Sensitivity: Ratio of changing output to changing load. Usually expressed in mV/V or m/m per 1-V excitation voltage

· Calibration constant: Ratio of the rated load to the rated output.

· Naural Frequency: Frequency due to free vibration of the transducer under no load.

Bubble Sensors

· Bubble Sensor (aka Bubble Detector): A device that is typically used to detect the presence of air in a liquid filled tube or pipe. Most commonly used in medical devices but has many industrial applications as well.

· Response Time: The amount of time it takes for a bubble sensor to take a reading. See our Bubble Flow Rate Calculator for more information.

· Durometer: A measurement of the hardness of a material, typically used for plastics, rubbers, and elastomers. There are multiple durometer scales, including Shore A and Shore D.

Occlusion Sensors

· Occlusion: A blockage in a tube that causes a change in pressure or flow. 

· Occlusion Sensor: A sensor that is used to detect occlusion, commonly used in medical devices.

· Creep: The tendency of a solid material to slowly deform under mechanical stress. Observed in all solid materials, creep is orders of magnitude more significant in plastics and viscoelastic materials, e.g. silicone tubing.

· Stress Relaxation: The tendency of a viscoelastic material to decrease in stress response when a constant strain applied to the material. One of the major difficulties when attempting to measure pressure non-invasively.


Liquid Level Sensors

· Float Switch: A type of liquid level switch with a component that floats on top of a liquid. 

· Normally Open (NO): A switch, relay, or reed switch, is normally open when it is open circuit in its normal (unactuated) state.

· Normally Closed (NC): A switch, relay, or reed switch, is normally closed when it is closed circuit (i.e. short circuit) in its normal (unactuated) state.

· Specific Gravity: The ratio of the density to a substance to the density of a reference fluid, typically water.


Flow Meters 

· Turndown Ratio: For a flowmeter, the ratio of the maximum flow rate that can be accurately measured divided by the minimum flow rate that can be accurately measured. Our Ultrasonic Flowmeters have a typical turndown ratio of greater than 100:1. 



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