d/p level transmitter

Guest, you haven't actually asked an answerable question, here. Please re-phrase it, making it clear what you want to know.

Hi,
The high side needs to be connected to the bottom of the tank and the low side to the top of the tank. Otherwise you will get a negative reading from the transmitter.

Guest,

Further to wrenchpuller's comment (assuming that this is what you're asking), if you don't connect as he suggests, you may also damage the transducer (depending on the type and the allowable negative differential).

Hi JohnDG and wrenchpuller,

Most modern differential pressure transmitters can read both negative as well as positive pressures. You just need to configure the transmitter to give the correct direction for the 4 to 20mA output.

For wet leg transmitters, I always connected the high side to the higher tap and the low side to the lower tap. My reason is to keep the reading positive. However, this results in a high pressure reading when the level is low and a low pressure reading when the level is high. This necessitates the reversing of the current output. This also confuses a lot of the new guys who attribute low pressure with low levels and high pressures with high levels.

One solution is to reverse the tapping. This results in a reading negative reading at low level and an almost zero reading at high level. This makes sense to them since negative numbers are "less" than zero, right? It also removes the necessity of reversing the output.

Dear Vulcan,

I've been realised many d/p tx. installations with steam flow applications but have not seen any permission by txmitter manufacturers that pressure inlets can be reversed.

Your comment is true for the electronical side of txmitter of course but the mechanical construction is not permit to apply the pressure inversely for the sake of proofing the instrument.

Inversed connection is possible only with transmitters that have equally spanned vacuum+pressure ranges (not bigger than -/+ 1 bar generally).

It is possible that i have not experienced with a tx. type that you mention of course.

If this is the case could you tell me please, the the manufacturer or model nr etc. of the transmitter?

Best regards.

I've used Foxboro's IDP series, as well as Rosemount's 3051, and 1151 series of differential pressure transmitters. I've never damaged them by reversing the process connections. We also have Honeywell but these are not reversed (I didn't install them).

I don't get what you mean that the mechanical construction won't allow reverse connections. As you can see below (Foxboro), the two ports are exactly the same so there should be no problem connecting them reversely.

Thank you for your reply.

I meant pressure inlets of transmitter by the word "mechanical construction".(I think i can not use english suitably).

I agree with you that txmitters will not damage if you apply pressure reversed but i want to tell that you can not obtain a proportional output at this situations.

I will examine the models what you installed.

Thanks again.

Hello Vulcan,

I have dig into the transmitters that you mentioned but i can not see they are working with reversed tapping (Damaging is a different matter, forget it) and you can obtain just a meaningless current value at the output, whichever you have configured it (4 to 20mA or 20 to 4mA).

I have decided that the your comment(#5) is your estimation (possibly you are relay on the technology excessively), because you say that " I always connected the high side to the higher tap and the low side to the lower tap. My reason is to keep the reading positive.". Means you actually have not tried revers tapping.

I'm sure you will see the point after a rechecking of the subject (or you done it already)...

Best Regards

The current outputs were far from meaningless.

I tried checking it once by using a transparent plastic tube that I marked with graduations at the 0, 25, 50, 75 and 100% levels. We carefully measured the graduations to obtain the pressure in mmH₂O. We checked the calibrations first with the high side on top and the low side at the bottom. Then we reversed the connections and tested again. There was no difference in linearity or accuracy (as far as we could tell by visual means).

We did this to settle the same argument that we're discussing right now. The result was, there wasn't any difference. Therefore, I decided that having a positive pressure indication was better simply because people tend to think there's something wrong if the reading is negative.

Dear friend,

Please do the same tests with a d/p tx connected an external orifice plate integral with a steam carrying pipe, or with a d/p tx which is connected to a pressurized tank with 2 tap. I want to emphasize that you can't obtain an output like '4, 8,16,20 mA' with normal tapping and '-20,-16, -8, -4 mA' (or 20, 16, 8, 4 mA) after you reversed the tapping, once you have configured the output.

Upwards going and downward going outputs together can be obtain only with the transmitters that have +/-1 bar span and those can not work under static pressure of say,10 bar.

I agree with you if they are the mention the transmitters with mmH₂O spanned because available ranges of them are much bigger than 100mmH₂O and can be used so as to zero -or center- the output anywhere in the available span. But you also can not get an output like i mentioned above, for two sides of the center. It is possible only a current centered with 12mA, going upward to 20mA or going downward to 4mA.

I wonder if i misinterpret the comment #5 ?? If so then .

Regards

We're talking about dp level here, not orifice dp measurement. If we're going to talk about orifice applications then, yes, you are correct but that's not the topic here.

hai vulcan

i had grasped u had gave a correct suggesstions regarding the dp transmitter can connect

At both side of the tapping point it will give negative reading we connected wrongly LP side to at the bottom of the tank

Dear friend

There is no problem.Before your reading is negetive value to zero value just like -1000 to -100mmH2O. YOU THINK LIKE THAT.In this time you change this taping this time this range become 100 to 1000mmH2o will come you are not changing span range. if you will change span range that time become problem

I agree with John and wrenchpuller, I wouldn't put the high leg on the low pressure end of the tank. It seems to me to be backwards. I don't argue that you can do it though and I would be surprised if it damaged the transmitter.

In closed tanks (vessels) where the pressure is other than atmospheric the low side of the DP cell is connected to the bottom tapping point and the high side is connected to top. This seems back to front and many people find it confusing. The only thing that has to be remembered is that the high side leg must be full to the upper limit of the vessel. This gives a reference leg for the transmitter of 100%. The low side of the transmitter varies with the level of the vessel. The zero % (4mA or 0mA) which ever your point uses will be at - (minus) the height of the tank (highest diferential) and the 100% (20mA) will be zero differential pressure. This call a depressed zero. the biggest problem is that as this seems back to front people keep blowing out the reference leg which then give an error as it is now trying to give a measurement with nothing to reference against. I hope this helps.

Here is the connections discussed here:

First sketch is the wrenchpuller's interpretation that gives rise a positive reading.

2nd is the confusing one that Vulcan and Doggoneit try to explain, with a reversed tapping and output.

3rd is more interesting by me, position of tappings seems true though. With this connection, you must configure the zero at the end of span to get a positive reading. In fact, the ΔP is lowering whilst the level is getting higher.

The most important point is where the dp tx will be placed against center line of the tank.

regards