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www.experts-insitu.com In Situ experts in hydraulics – Newsletter N°27, may 2012 All rights reserved IN SITU ‘ Pressure Measurement Why measure pressure? The pressure in a hydraulic system is a very important parameter. From it we deduce the forces applied to the cylinders and the torque on the pumps and motors. Make sure to select the material carefully and place it correctly in the circuit...! Here are some elements to guide you in your selection of pressure measuring devices... 1 > The working environment > All the external and internal constraints that can disrupt the operation of the device : type of fluid, seal compatibility, fluid temperature, type of connection on the fluid side, outside temperature, protection against water, shocks, risk of explosion (device equipped with electronics), type of power supply, connector type on the electrical side, vibration resistance, storage conditions, obstruction .... [...] Be aware that the manometer is a pressure device outlined in the directives 97/23/CE; it should be respected and used accordingly. 2 > The type of reading you desire > In Absolute pressure : measurement starting from empty (0bar). > In Relative Pressure : measurement starting from atmospheric pressure (1.013 bar from empty). You also need to choose the measurement unit. Bar, psi (equivalent to 0.07bar), Pascal often read in MPa, which itself is equivalent to 10 bar. In general the device must be located as closely as possible to the place where you wish to take the measurement, because if we insert a nozzle or a capillary tube the responsiveness of the measurement will be altered. However, this is an advantage for the life expectancy of the device! Different methods can be used, but the measurement precision, the reading method of this precision as well as the costs are very variable. 3 > The < Cl > classification This concerns de class of accuracy in % of the measuring range. In the case where the manometer has a stop, this will only be valid from the 10% measuring range. For example : you have a 0-400 bar manometer classed Cl1.6 with a needle stop, then The accuracy classification from 40 bar to 400 bar will be: +/-6.4 bar. www.experts-insitu.com www.experts-insitu.com 4 > The classic mechanical needle manometer This is the one which springs to mind because of the low cost and easy installation. We find models with diaphragm, Bourdon tube, C tube or helix..... The larger the dial, the more accurate the reading will be. For a proper reading of the values, stand directly in front of the dial to avoid parallax errors. This type of device must be kept away from vibrations to ensure a long life. Be aware that the presence of liquid (glycerine, silicon oil...) in the device allows to reduce vibrations. We can also divert with a capillary tube; after it has been put into place, the vent opens to bring the device to atmospheric pressure. The digital gauge: in fact this is more a pressure sensor with an integrated display, see point 5 below! For manometers equipped with insulators, make sure to open them to make the measurement. And remember to lower the pressure before closing ! 5 > The pressure sensor : This consists of a membrane, a ceramic cell or other device in contact with the pressurized fluid and an electronic part. It requires an additional module to interpret the reading; this offers the possibility to record values or min/max continuously and therefore gives an impressive frequency of readings (usually a value in less than 10ms!). On this unit you should check the measuring range, the maximum pressure to be measured, the burst pressure, the tolerance on the voltage supply, the linearity, the type of measurement signal (voltage, current, bus CAN), approval type, torque, authorized mounting position.... Today systems which are equipped with certain dialogue protocols even allow a calibration and data feed through the wiring of the instrument! Through electronics certain adjustments are possible to compensate for drifting due to temperature change or other phenomena. Note that some devices can incorporate other information than fluid temperature, which can allow further measurement compensation. > Regular calibration is the only way to ensure accurate measurements for all these devices. To this we must add the monitoring of mechanical efficiencies which will influence the accuracy of the result ! Find the unit conversions and the calculations using pressure in the iPhone Application. In French/English/Italian and soon in Chinese! www.experts-insitu.com www.experts-insitu.com Pascal Bouquet