Pre-loading of pressure cells and its effects on calibration values

Figure 1. P6 pressure cell

1. Introduction
A Vibrating Wire Pressure Cell (see Figure 1) consists of a circular or rectangular flat jack, formed from two sheets of steel welded around their periphery. The narrow gap between plates is filled with hydraulic oil. The cell is connected to a Vibrating Wire (VW) transducer by a short length of stainless steel tubing, forming a closed hydraulic system. 
Due to the gap between the plates being filled with oil, a pre-loading offset is introduced to the measurements taken from the VW transducer compared to reading seen on the calibration certificate. The offset is introduced because the oil that is inserted into the closed hydraulic system slightly pressurises the transducer. The calibration certificate received with the unit is solely for the VW transducer before it has been inserted into the instrument and as a result does not include the pre-loading offset experienced with the final product.

2. Changes in calibration values that you would and would not expect
As a result of the pre-loading offset described above, the pressure cells will initially be under a slight pre-charged pressure. This can lead to what would appear to be large differences between the calibration certificate and the output received by the users site zero reading. The amount of difference will vary depending on each individual transducers calibration, the formula below gives a guideline of the parameters you would expect your pressure cell to fall into. 

    Standard Vibrating Wire P6 Pressure Cell - 50kPa divided by the Linear gauge factor (Located on the calibration certificate) gives the maximum difference value allowed in F²/1000.
     Push-in Vibrating Wire P10 & P9 Pressure Cells - 75kPa divided by the linear gauge factor (Located on the calibration certificate) gives the maximum difference value allowed in F²/1000.
If after performing the above calculations you believe that your pressure cell is giving you readings outside of what you are expecting please submit a ticket through the support site.

Warning: Every pressure cell should have a site zero reading recorded. To do so lay the cell flat on the ground and take zero reading, the result should be of a constant raw data value. Please be aware that due to the pre-loading offset described above the recorded value WILL differ to the calibration certificate zero reading.