5 Common Pressure Measurement Fails


failed pressure measurementWhen you’re dealing with pressure measurement, you need to get it right the first time. There are a host of dangers waiting in the shadows of incorrect pressure sensor ordering, installation, and application. To help you stay well clear of said dangers, here are 5 common pressure measurement fails:

Chemical Incompatibility

Pressure transmitters are, by necessity, contact sensors. In other words, they have to be in contact with the target media in order to function. Therefore, chemical compatibility is a constant concern.

Carefully consider the sensor’s wetted material. 316L SS is a very common offering (for good reason), but it’s not compatible with everything. Lean on your MSDS and compatibility charts to make sure you’re not setting yourself up for massive failure.

Incorrect Pressure Range

A standard piezoresistive pressure sensor’s accuracy is based on the pressure range. The error band is a percentage of the full scale range (our standard is ±0.25% of full scale). That means you need to match your sensor’s pressure range as closely as possible to what you expect to measure in the real world.

In other words, if you’re measuring up to 100 psi, don’t buy a 10,000 psi sensor.

There are times when ordering a higher pressure range makes sense. Calibrating a high range sensor to a lower measurement range is called down ranging. It’s useful when you’re worried about occasional spikes in pressure above the normal range, or when you want to boost longevity with a thicker transducer.

However, there are reasonable guidelines for down ranging pressure sensors. Remember, the accuracy of the sensor is still dependent on the original full scale range, not the calibrated range. So going with a sensor that’s more than 150% of your expected range starts to really diminish accuracy for most of our customers.

Conversely, some have justified that a 2X overpressure rating means, for example, that a 100 psi transducer can measure a 200 psi range. This is not the case. While it won’t blow the sensor out of its fitting, it will overstress the diaphragm and destroy the calibration. Overpressure and burst specs are safety features meant for abnormal operating conditions.

Too Much Torque

A pressure transducer’s sensing element is a very thin diaphragm with electronics mounted on the back. The diaphragm is susceptible to careless handling and compression during installation. Compression is a result of applying too much torque when installing the sensor.

The electronics on the back of the diaphragm react in a consistent and known way to its minute movements as pressure rises and falls. However, when the diaphragm is compressed from over-tightening the sensor, its shape is changed and it no longer reacts to pressure as expected.

You can imagine how this can drastically alter your pressure measurements. If you’ve tightened your sensor too much – to the point that it affects the diaphragm – it’s time to order a new sensor.

Incorrect Application of Sealed Sensors

A sealed sensor uses a closed chamber as its reference (zero) point, while a vented sensor uses atmospheric pressure. That means that as the pressure in the atmosphere changes with the weather, the closed chamber will expand and contract – much like your eardrums when you’re congested.

This has the effect of moving the zero point around, and messing with your pressure measurement. In high pressure ranges, this all falls within stated error band on the sensor and is therefore unnoticeable.

However, in low pressure applications – especially anything sub 100 psi – a sealed pressure sensor becomes problematic. The changing atmospheric pressure will act on the sealed chamber within the sensor causing unreliable measurements.

Sealed sensors are advantageous for keeping fine particles, like dust, moisture, or chemical sprays (paints, adhesives, etc) out of the electronics of the sensor. If you need this kind of protection in a low pressure application, you’ll need to take steps to normalize for atmospheric pressure.

Too Much Input Voltage

On occasion, we come across someone who has attempted to power the sensor with too high a voltage. This is likely due to what the installation tech/electrician is familiar with at the time. For example:

  • We’ve seen 110V wired to a sensor, which is designed for 24V
  • We’ve also seen AC current applied to a sensor that needs DC
  • Finally, some people have even wired the input voltage to the output wire

All of these scenarios result in a fried sensor and voided warranties. Moral of the story: know the voltage and current limitations of the sensor, and get familiar with the sensor’s wires. Take time to understand what you’re working with and you’ll likely install the sensor successfully without incident.

Contact us to make sure you avoid these common pitfalls. We'll help you navigate the steps to successful pressure measurement.

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top photo credit: darkbuffet via flickr cc

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