Hi Temp Low Pressure transducer

Hey all, im looking for a low pressure (.043" wc Set Point) hi temp (400F) pressure transducer. Anyone ever use anything along these lines? The best ive found that is low presure (they make many hi temp hi pressure) only go to 250F operational temperature. Right now, the best ive come up with is to use a long sample tube to allow the use of the 250F sensor. Id like to mount right to the stack. Thanks

Matt

I think Foxboro (Invnesys) has something along these lines. To get the low pressure range, you may need to look at a differential pressure transmitter, and leave the Lo port open to atmosphere.

When you are talking to the suppliers, remember that there are two temp ratings to consider. One is the rating of the pressure diaphragm or other wetted parts. This can be quite a bit higher than the rating for the electronics.

Besides a long sample tube or pigtail, another option is to use a diaphragm or guage isolator, or a silicone fluid filled sensor tube. I don't see any disadvantage to using this to get the electronics away from the sensing tap. If this is a liquid application, just make sure to account for the fluid column height in your calibration.

Endress + Hauser used to have a 2-wire pressure transducer for around $175.00 that I used several times on a hi-temp vacuum application. My E + H rep had the factory install an oil-filled isolator that enabled me to connect to the hi-temp process. I don't remember all the details, but I do remember that the isolater cost around 2 1/2 times more than the transducer itself. But they worked great.

Matthias von Zorn said:

Hey all, im looking for a low pressure (.043" wc Set Point) hi temp (400F) pressure transducer. Anyone ever use anything along these lines? The best ive found that is low presure (they make many hi temp hi pressure) only go to 250F operational temperature. Right now, the best ive come up with is to use a long sample tube to allow the use of the 250F sensor. Id like to mount right to the stack. Thanks

Matt

Click to expand...

I think you will find that a capillary tube style will be the only type to handle the high temps. This is used to standoff the electronics from the heat.

Your best bet will be to use impulse tubing to remove a draft range DP transmitter body from the heat source.

Most industrial draft range transmitters have meter bodies rated in the 150°F - 170° range, depending on vendor. The 250° rating is usually for ranges higher than draft range, where the sensing diaphragm can be heavier and bulkier.

The rule of thumb is impulse tubing drops 100°F per foot of 1/2" tubing at 70°F ******t temperatures. There's no flow through the impulse tube, so it only has to lose conducted heat from the source to the atmosphere. If the surrounding ******t is higher than 70°F, the loss isn't as great, requiring longer impulse tubing. Do not skimp on the diameter of the impulse tube, because the pressure you're looking for is on the very low end. If there's radiant heat from the source, interpose a heat shield with an air gap between the shield and the transmitter to keep the body temperature of the transmitter down.

I doubt you'll find a diaphragm seal that can begin to give you any sensitivity at the very low 0.04" w.c. range you're looking at. Filled capillaries are only useful if there's a diaphragm seal large enough and flexible enough to provide the sensitivity needed to see 0.04"w.c., but capillaries introduce their own errors. The expansion/contraction of the fill fluid in the capillary over various ******t temperatures would probably produce errors greater than the value you're looking for. Most people are unaware of how much error can be attributed to temperature changes in the capillary fill fluid. The cost of seals and capillaries and installation is frequently more than the cost of the transmitter body. Capillaries have to be welded in place, and are always "specials". The rule is, don't use a capilllary and seal unless you have to. They are always high cost upfront, less sensitive, slower responding and have the additional error from the capillaries temp swings.

High temp vacuum can range as much as one bar, or one atmosphere, which is 10,000 times greater pressure than you're looking at (400"/0.04"), so it's likely not to be a candidate.

The pressure transmitter market is big & broad, but I don't think you're going to find a draft range transmitter in the $200 range that does 0-0.1" w.c.

Dan

Why does the word "a m b i e n t" get asterisked out?

danw said:

Why does the word "a m b i e n t" get asterisked out?

Click to expand...

Might be this

http://www.topix.net/content/cbs/1986917647102951890114639509220538089822?threadid=RV37NNBH35P66LNJ

Thanks DanW, I think im going to have to use a tube. Oh I dont plan on spending $200 for it, its closer to $1000 - $1200 for the sensor. I do have the budget for it as well, so its not a problem to spend. But, as you can imagine, I dont want to cook a 1200 dollar sensor.

Thanks again guys, everyone here is great. Not only for PLC question, but for general industry questions. There is such a wide variety of skills and experinces here!

matt