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Behind the Scenes: Grace Under Pressure

Nov. 17, 2015
Omega Pressure's engineering manager explains just what intrinsically safe means and how it designed its pressure transmitters to stay safe and cost less.
Omega Pressure's engineering manager explains just what intrinsically safe means and how it designed its pressure transmitters to stay safe and cost less.

To meet the safety needs of hazardous environments, Omega Pressure has released two intrinsically safe pressure transmitters: PX51 and PX509, which are extensions of previously existing models PX41 and PX409. Engineering manager Cyrus Grimes explains how the company made their product safer while still keeping costs down.

Why would someone need intrinsically safe pressure transmitters?

When you have hazardous vapors, like in chemical processing, oil and gas, and mining, you need the right equipment to make that environment safe. One of the most robust methods is using intrinsic safety, which is what these are rated. Intrinsically rated means the device can suffer two faults and still be safe. So it can be shorted and suffer a catastrophic failure and still not release enough energy to generate a spark or ignite any of the explosive or combustive elements in your environment.

What’s the difference between the two models?

PX51 is a bonded foil sensor, with pressure ranges from 15 to 30,000 psi and an accuracy of .25% full-scale accuracy. It’s available in gauge, sealed gauge, and absolute pressure.

The 509 family, also available in barometric and compound gauge, are oil filled and have a silicon sensor inside the unit, isolated by the diaphragm. It has a much higher accuracy of .08%, but is limited in range from 10 in. of water, which not many companies offer, up to 3,500 psi.

If you need higher pressure you want to go with the PX51, but if you want higher accuracy and can accept lower pressure, you go with the PX509.

What did you have to change to increase the safety rating?

We designed the amplifier, the critical part of the sensor, to be 2/3 of the power rating under any condition, which extends the life and makes them low power. We also put them into environmental chambers and measured any hot zones and what the maximum temperature occurred on the devices to meet the intrinsic safety requirements, and didn’t spark or get hot enough to ignite any materials. In terms of environmental protection, the transducers are fully welded, 316 or 17-4 stainless steel, and are corrosion resistant.

How did that go?

Some issues of heat generation occurred on the amplifiers when inducing faults. These parts get all the electricity and in some cases they did overheat. To reduce the max temperature these parts went through, we went to a larger part. The larger footprint allows the heat to be distributed more evenly.

Keeping costs down when dealing with a product that needs to work in hazardous areas can be quite a challenge.  What did Omega do to balance cost and performance?

You want to start out as lean as possible. You can build something that will succeed on the first try very easily, but you don’t want to over-engineer something, because the costs are higher and footprint is larger.

We also employed a modular way of thinking so there’s a commonality of parts. We took proven performers with good track records, in this case PX41 and PX409. The only difference is the electronics are rated differently. The amplifier is designed to work with both families, and it’s used in two of our load cells. This required some extra engineering, but with four product families supported by one amplifier, it helps with inventory, assembly and when you design it right up front, then you can apply it to other tech in future very easily. They all went through FM approval at the same time, which took a little longer but cost a lot less.