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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.
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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!
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Pascal Bouquet