Fault Finding Pneumatic Instruments

1. Introduction
This article explores various avenues to take if you do not believe your pneumatic instruments are functioning correctly. Instruments that use pneumatic technology are those such as pneumatic Piezometers and pneumatic settlement cells with a Soil Instruments pneumatic logger and the various issues that can occur. As with any instrument, if they are not well maintained the readings you are receiving can be greatly affected.

2. Problems that may be experienced
Issues for pneumatic Piezometers can be varied and are usually one of the following:

    Severed tubing either on the input or output tube leading to dirt or moisture entering the tubing or diaphragm.
    Receiving varied readings from the instrument, such as a Piezometer giving a reading of 6.15 meters of water column and when repeated it gave 6.5 meters.

    Receive no readings from your pneumatic instruments, by this we mean you attempt to pump gas through the system at the same pressure or higher than previously used and no reading is returned to the logger.
There are two key reasons that you would either receive varied readings or no return reading when pumping gas through the system. These are commonly results of dirt or liquid infiltrating the system. If large amounts of liquid or dirt do become lodged either in the tubing or at the diaphragm this can greatly affect the accuracy of an instrument and will need to be cleared prior to quality readings being achieved.

3. Fault finding and resolving your issue

Severed tubing can cause many issues with readings from instruments however if you are receiving unexpected or no readings first we recommend that you check the joins that were made where the tubing was cut. Please ensure that nitrogen can get through where the connections have been made. If it is clear that nitrogen can pass through the joins then there may be an issue with dirt or liquid within the tubing, if not we recommend that you re-make the joins in a fashion that you can guarantee that the nitrogen can pass through.
If you are receiving variable readings or no readings this could be resultant of dirt or fluid infiltrating the tubing and diaphragm. To attempt to remove water from the tubing we recommend a nitrogen gas bottle is connected to the Piezometers and settlement cells and pumping nitrogen through the tubing at a slightly higher pressure through the Piezometers and settlement cells than normal. The nitrogen should be pumped through the faulty instruments for two to three hours in an attempt to remove any residual water in the system.

TIP: The diaphragms within the Piezometers are made to withstand pressures of up to 100 psi and nitrogen gas that is pumped through the system attempting to clear any water or dirt should not exceed this.

If, having pumped nitrogen gas through a system to remove any water, you are then able to get steady readings however upon when you next return you receive no readings or variable readings there is a possibility that either there is a further hole in the tubing of that the membrane of the Piezometer is broken. The best way to check this would be to check any connections where the tubes have been cut to ensure they are not leaking. We would then recommend pumping the system with nitrogen until there is no water left in the tubes. Once this has been completed we would recommend coming back to these cells and pumping these cells again to see if they have refilled with water. If they have it could be assumed that there is a leak within the system.