Testing; Calibration; Check The Offset - Delta-T Devices SWT5 User Manual

4.2

Testing

4.2.1 Calibration

When delivered Tensiometers are calibrated with an offset of 0 kPa
(when lying horizontally) and a linear response. The offset of the
pressure transducer has a minimal drift over the years. Therefore, we
recommend you check sensors once a year and re-calibrate them
every two years.
Return the Tensiometers to us for recalibration, or contact us about
available calibration accessories.

4.2.2 Check the Offset

If there is no pressure difference between the cup interior and the
surrounding the signal should be 0 kPa.
There are two ways to check the offset.
1. Connect the Tensiometer to a readout device. Place the SWT5 in a
beaker and fill the beaker with de-ionized water up to the centre of the
sensor body (see 3.4.) Wait until the reading is stable. If there are
bubbles inside the cup this might take a while.
Now the reading is the approximate offset. The value should be
between +0.3 and -0.3 kPa.
2. To check the zero-point more precisely shaft and sensor body need
to be disassembled.
The pressure sensor diaphragm is inside the small hole on the
pressure sensor body. It is very sensitive and must never be
touched! It can be destroyed even by slightest contact! No
contamination should get on the sealing and gasket.
Before reassembling cup and sensor body carry out the degassing
procedure (see chapter "Refilling").
After taking off the shaft shake the pressure sensor to remove water
from the pressure transducer hole. The offset is acceptable when the
reading is between -0.3 and +0.3 kPa.
34  Service and maintenance
SWT5 User Manual v2