Pressure Measurement Life Lessons

Feb. 26, 2013

Just as a large ship is steered by a rather small rudder, pressure sensors play a much larger role in process monitoring and control than their size indicates.

While pressure sensors are generally small in stature, premature failure can cause big headaches in process monitoring and control applications.

Just as a large ship is steered by a rather small rudder, pressure sensors play a much larger role in process monitoring and control than their size indicates. Likewise, they may not be the most expensive or complex cog in the machine, but their failure can have big consequences.

Pressure transducer failure can be problematic for a range of reasons, but two primary concerns are:
1. Damage to the system can be substantial when a pressure sensor fails and pressure falls or soars out of spec.

2. Pressure sensors are often installed in out-of-the-way spots that make them difficult to replace, adding to downtime and repair costs.

RELATED: Checklist for Pressure Sensor Selection

So, in the spirit of avoiding pressure sensor replacement as long as possible, here are five tips and tricks that will increase the longevity of pressure sensors in most application environments.

1. Select an effective pressure range

A pressure-sensing element is a moving part, which means it can be over-worked, strained, and broken if it is continuously used at or beyond the maximum capacity of the sensor. This may seem counter-intuitive—the inclination may be to think that if the application pressure is a steady 100 PSI, then a 100 PSI sensor would be the best choice. However, this is not ideal.

Instead, think of a pressure-sensing element as an elastic band. If it is stretched and held at the maximum length, it would soon degrade and eventually break. Pressure sensors work on the same principle, but instead of elastic rubber, there is a thin diaphragm made usually of metal. The diaphragm is meant to flex with the rise and fall of pressure, which affects the output of the sensor and produces a measurement.

A better approach is to select a higher pressure range, and adjust it to meet your specification. This process is called down-ranging—it will increase the longevity of the sensor.

2. Protect the sensor diaphragm before, during, and after installation
The pressure sensors diaphragm is designed to be sensitive to pressure forces; this does not mean that it can withstand other types of force applied to it. In fact, it is just the opposite. The diaphragm is the most fragile part of the pressure sensor and requires special consideration when it is handled.

Anything that happens to the sensor diaphragm that affects how it moves will change how the sensor performs. For example, sharp impacts can cause a dent in the diaphragm, which will damage the accuracy of the sensor. If the diaphragm is not physically damaged, it can still cause the sensor output to begin drifting.
Drift will happen if the bonding layer between the sensing elements and the sensor diaphragm is disturbed. The output will drift until it settles again.

During the manufacture of pressure sensors, complicated and elaborate methods are used to cure and age the bonding layer so that it is inert. A sharp impact can cause stress fractures in the bonding layer that tend to negate the curing and aging process that the sensor has undergone. This causes the sensor output to drift until it becomes inert once again.

Sharp impacts can cause a dent in the diaphragm, which will damage the accuracy of the sensor.

Thus, when handling a pressure transducer, take care not to drop it or let anything hit it on the diaphragm. Flush-mount style sensors are particularly at risk because the diaphragm has no protection, which is why submersible hydrostatic pressure sensors use either a nose cone or a heavy-duty cage. Any testing needed prior to installation should be performed with this caution in mind.

During installation, watch out for burrs in the fittings that could scratch the sensor diaphragm. A small scratch in the wrong place can cause performance issues over a period of months. In addition, over-tightening the sensor can cause output shift, but this is typically seen in the form of a constant output shift—which can be handled by an offset on the controller. However, this shift will not be the same for every sensor and will be different every time the same sensor is removed and re-installed.
If and when the need arises to clean the pressure sensor, you must take extreme caution if anything is put inside a pressure port where it could come into contact with the diaphragm. Some diaphragms are only 0.002” thick. Even a slight touch can cause physical damage to the diaphragm and ruin the sensor.

Whenever possible, flush and clean pressure sensors with solvents that are appropriate for the material of the sensor and the application media.

3. Manage the application environment
Temperature: Most data sheets offer compensated, operational, and storage thermal ranges. While they are all important to the life and performance of your pressure transducer, the one to be most concerned with is the operational temperature range. The best approach is to ensure a sensor meets all the expected thermal requirements of the application. This avoids a potentially troublesome work-around.

There are two aspects of the operational temperature that are important—the media temperature and the external temperature. In some cases, they are in fact the same. Such is the case with submersible sensors. Be careful to use the more extreme of the two when selecting a sensor with matching thermal specifications. Generally speaking, it is cheaper to get the right sensor than to find a way to cool or heat it.

Chemical Compatibility: The media and external environment must also be compatible with your sensor, just as temperature must. The surface that the media touches is called the wetted surface, and is more important because an incompatibility here will cause sensor failure much faster. However, for the sensor to last, it must be constructed of material that is unaffected by any chemicals and corrosive gasses in or around the application area.

For submersible sensors, the entire housing and the cable constitute the wetted material. One of our customers recently experienced a worst-case scenario of sensor corrosion. A ground loop was sending current directly through the liquid media. This caused accelerated electrolysis that ate through the entire sensor housing and circuitry—even though the sensor was chemically compatible with the liquid.

The diaphragm is the most fragile part of the pressure sensor and requires special consideration when it is handled.

Moisture: If the sensor is in an outdoor, humid, or otherwise wet environment, the sensor case should have an IP or NEMA rating appropriate for the environment. If it does not, then install it in an enclosure that does have such a rating.

Be careful of airtight enclosures that are exposed to changing temperature extremes. Condensation can occur inside the enclosure and create the very situation you are trying to avoid. In such cases, consider using desiccants to absorb the moisture or install a vent hole that has a breathable path. Make sure the vent is impervious to moisture by using a material such as Gore-Tex® or something similar.

4. Wiring
There are a few forms of electromagnetic interference (EMI) that can affect the life of the sensor. EMI will do more than reduce the performance of sensors in the short term; it can degrade or destroy them permanently. When wiring the sensor, make sure it is wired and grounded correctly per the manufacturer’s instructions.
Power surges and electrostatic discharge (ESD) are common examples of EMI that can damage sensors. Effective grounding of the sensor will protect it from these and other forms of EMI. Many manufacturers offer internal protections for surges and other types of noise, but almost all of these are dependent of proper sensor grounding.

5. Recalibrations
The output of all sensors will drift over time—some more than others. Ideally, the magnitude of the drift is so low that it does not affect the practical performance of the sensor in its given application. The magnitude of the drift will be greatest when the sensor is newly manufactured. Over time, the rate of the drift continues to decline. This effect is known as aging.

Manufacturers, as previously mentioned, go to great lengths to neutralize the drift by artificial or accelerated aging to a level of drifting that will be acceptable to the user. These effects in the field will only be seen over time, if ever.

When wiring a sensor, it is vital to make certain it is wired and grounded per the manufacturer’s instructions.

After an extended period, it is possible for the sensor to drift outside the acceptable performance range. The user can typically fix this with a zero balance, or an offset adjustment made to the sensor. Therefore, a sensor with an adjustment feature can extend longevity. Take advantage of these adjustments before disposing of the pressure transducer, or send it back for recalibration.

Another method to handle drift and other performance-diminishing effects is to have regularly scheduled recalibrations done as part of a preventative maintenance plan. These can be done in-house, if the right equipment and expertise is available, or they can be sent to the manufacturer or calibration house approved by the manufacturer.

This approach is not always feasible, depending on the application and recalibration costs, but it is an effective option if reasonable. The manufacturer, in particular, will know the signs of other issues with the sensor that could cause a failure in the near future. This is proactive solution for failure prevention.
These tips are fairly simple and easy to implement and require marginal effort and cost. If followed, they will result in fewer failures and longer lasting pressure sensors. Moreover, they will limit expenses and reduce stress as a result of dealing with sensor problems.

Elden Tolman is a product development engineer at Automation Products Group, Inc. (APG). After earning a bachelor’s degree in Engineering from Utah State University, Mr. Tolman has worked at APG for nine years. Currently he manages nine product lines, with hundreds of options ranging from outputs to process connections. He actively provides technical support to a range of large end-user companies in various industries. Mr. Tolman can be reached at 435 753-7300 or [email protected].

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