#### Technical FAQ

 Sensitivity of strain gauge sensor and load cell is measured in $mV/V$ (millivolt per volt) and it is equal to output voltage of sensor with $1V$ power supply and nominal load. Standard sensors are made with sensitivity from $1mV/V$ to $3 mV/V$, the normal/common value is $2 mV/V$ and $1.5 mV/V$. The datasheets include the nominal values. The exact value differs can differ slightly from nominal value. For example, the sensor with nominal sensitivity of $2mV/V$ can have exact value of $1.9836 mV/V$. The exact value is stated in the protocol supplied together with the sensor. Sensor output voltage depends on three factors: sensor sensitivity $C (mV/V)$ excitation of sensor $U_C (V)$ load force $F_x$ Output voltage of sensor with nominal range $F_n$ can be calculated with this formula: $$U_S=\frac{C\cdot U_C\cdot F_x}{F_n}$$ Example. Sensor sensitivity is $C = 1.9836 mV/V$, the range $F_n = 5kN$. Sensor is supplied by $10 V$ DC voltage and loaded with force $F_x = 3.5kN$. Output voltage will be: $$U_S=\frac{1.9836\cdot10\cdot3.5}{5}=13.89 mV$$ If $F_n = 0kN$, output voltage is $U_s = 0mV$. If nominal range $F_x = F_n = 5kN$, then $U_s = 19.836mV$. That means the output voltage depends on load and it is changed linearily in range $0…19.836 mV$. Note: Sensor zero balance is omitted from calculation. Signal conditioner has two objectives/purposes: Excitation of sensor Amplification of sensor output voltage Sensor excitation has to be accurate and stable because of output voltage depends on it. If supply voltage fluctuates e.g. with temperature changes, output voltage fluctuates also. Sensor would be considered as temperature unstable. Amplification of output voltage is necessary because its value is small, from $10$ to $30 mV$ even in case of full loading. Input range of computers, PLCs and other electronic units is usually $100 mV$ and more. It is useful to amplify the output voltage in order to use all possible range of input of the device and to increase accuracy. It is possible to buy special cards as part of systems (PLC) where force sensors and load cells can be connected directly. The card excites the sensor and has analog input with range of few mV. However, the cards are quite expensive, so usage of load cells signal conditioner can be considered to be the cheaper alternative. Yes it is. Difference between force sensor and load cell is only in their calibration. Note: Force sensor is calibrated in $N$ (Newton) or $kN$ (kilo Newton), load cell is calibrated in grams, kilograms or tons. Commonly known formula $F = m \cdot g$ describes relationship between force and weight. Gravity g is acceleration with value of $10$ ($9.81$ is more accurate). Force sensor with range of $1 kN$ can be used for weighting of $100 kg$ because $100kg \cdot 10 = 1000 N = 1kN$. Formula works the both ways, so load cell intended to weight is possible to use for measurement of force. Requirement to connect more than one sensor to electronic unit can occur e.g. in case of construction of weight scale. The sensors, placed under each corner of the scale plate, have to be connected to electronic unit. In that case the sensors are always connected paralelly. It is important to be aware that parallel connection increases excitation power supply demand. If 4 sensors are connected parallel, the signal conditioner has to provide electric power 4 times more. Maximum number of the sensors connected, or minimal load resistance of the power supply are stated in the sensor parameter description. For every sensor we produce, a calibration certificate is issued from our in-house test facility. Upon agreement with the customer, the sensor can be sent to an external independent testing institute, which issues its own calibration certificate. All our force sensors and load cells meet the requirements and technical standards valid for Slovakia and EU. Check Documents section for Declaration of Conformity (CE).
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