Transmitter calibration

Although I am a little confused at what you're stating and asking, I thought I'd give a try...!

Here goes! This type of data that you're giving, seems to me you are measuring a sort of interface in a sealed vessel. First, the leg is a giveaway that you're creating some type of inert gas or vapor cushion or barrier or Isolation or purge,whatever it's called at your end and that it's a sealed vessel (pressuized of course). Second, the negative numbers imply that the measured medium lays below the second layer which is a lower density fluid...? Please give more detailed info; I ca't even believe what I'm saying relative to what you have stated. Believe me; I am more confused than you are!

I'm a little confused as ronald but I'll give it a try too.

In a pressure type level transmitter with two legs, a negative value means that the high (H) side is connected to the lower tap and the low (L) side is connected to the upper tap.

With a wet leg in this configuration at low level (just above the lower tap), the low side has a much higher pressure than the high side. Since a differential pressure transmitter calculates pressure as H - L (High minus Low), your measured pressure is negative.

Now, if the level rises to just below the upper tap, the low side still has a higher pressure than the high side though it's now much smaller. Your measured pressure is still negative but less (in magnitude) than when the level was low.

If the level goes higher than your upper tap (not good practice), your pressure could possibly become zero.

'Hope that makes sense to you .

Thanks to all,

I could understand what did you say and I will confirm from filed how is connected.

mean while if there is change in density of media what is the formula to be used for calculation?

I have this written specifically for such an application. Unfortunately, it's at the office (50 miles away). Will try to remember to get it to you on Monday... unless someone posts it up before then. I may be able to remote-into my DocLib to snap a screenshot of an e-copy I had uploaded.

Vulcan explained the function (for lack of better terms) as I would have.

if there is change in density of media what is the formula to be used for calculation?

If there is a change in density then your level indication will not be accurate. To maintain accuracy, you'll need another instrument to measure the density and use this to adjust the level indication (compensating for the density change).

If your medium doesn't change density (or the change is very small as to be negligible), you can calculate the pressure at the transmitter with this formula:

The pressure figure here (which I forgot to put before finalizing the drawing) is in inches (or centimeters, millimeters, whichever you're comfortable with) of water. This is for the leg that is at the lower tap point. For the higher tap, you only need the height of the fill fluid and its specific gravity.

If I may suggest, you can simplify your calibration if you just drain the tank to the low level height and zero the transmitter. Then you fill the tank to the high level height and span the transmitter. You'll have no need for the actual pressure reading and just rely on percent (%) level.

What I did for my factory was to fabricate a level calibrating jig using a 2 inch pipe that was the same height as the tank. I'd connect the transmitter to the pipe, fill it to the brim and calibrate the transmitter. I used much less water for calibration and work time was faster.

In a general sense, that's it. Found my old notes.

Hi dear frind

I have a big problem on one transmiter ,it has 2 capillary installed on 2250 mm from them and inside the tank we have 60 bar pressure of gas .I calibrated with formula and atm. but not hapen any thing . so now I put zero on dynamic status ,but it not answered me wll . please gide me how I can calibrate the transmiter ,and what's youre sugest .my email is mehdifiroozi2008@yahoo.com

thanks and regard

mehdi

Hello mehdi,

Sorry but I don't reply directly via email. In any case, I can't understand your definition of the problem. You may be doing something wrong but I don't know what it is.

I suggest you register with CR4 so that we can discuss the problem and others can join in as well.

regards,

Vulcan

sir,

kabayan pasensya na eh maitanong ko lng.hirap kasi mag ingles.buti andyan ka.

paano mag compute ng maximum deviation %fs pressure transmitter 4-20Ma output 0-101.5 psig input.

salamat kabayan,

speed

Hello dear,

Your DP cal range of -3.62 to -1.2kpa in a pressuried closed vessel is possible and it means you have an operating process condition of vaccum (suction) inside the vessel.-3.62 depicts the negative pressure at vessel empty and -1.2kpa at 100% level.Accordingly the transmitter will give 4 -20 mA signal for 0 to 100% of level inside.Of course the density must have been considered while calculating the said cal range.

M S Vilkhoo

AGM(Inst)

Siel Chemcial Complex rajpura(Punjab) India

Pressurizing the tank has nothing to do with the calibration (all tanks on earth are pressurized).
ALL differential level transmitters negate dual equal pressures.

A span of -3.62kpa to -1.2kpa = A differential span of 2.42kpa (all spans are displayed as positive)

All differential transmitters with a WET low side leg will "FEEL" a higher pressure on the low leg at all times except when the tank is full (OFTEN EXPRESSED AS A NEGATIVE).

Pressure on the LOW side MIMICS a vacuum (negative pressure) on the HIGH side (and vise versa).

Therefore to calibrate the instrument to read correctly this higher low side pressure must be considered (in this case as a set of negative numbers).

To easily calibrate this particular instrument on the bench :

Method (A):

1) Put 3.62 kpa on the LOW side and then ELEVATE the zero to 0.00% output.

2) Put 1.2kpa on the LOW side and span the instrument to 100.00% output.

Differential of 2.42kpa.

....OR.....

Method (B):

1) Put 0.00kpa on the HIGH side and then ZERO to 0.00% output.

2) Put 2.42kpa on the HIGH side and SPAN (RANGE) the instrument to 100.00% output

also a differential of 2.42kpa.

3) Put 3.62kpa on the LOW side and ZERO to 0.00% output.

....OR.....

To easily calibrate the instrument on the tank in the field:

1) Block the process to the HIGH side ONLY and vent to atmosphere.

2) Zero the instrument to 0.00% output

3) Pressurize the High side to 2.42kpa and SPAN (range) the instrument to 100.00% output.

4) Block in all calibration or vent ports and then open the instrument to the process.

OUTPUT should now display tank level.

CHANGE INSTRUMENT TAGS TO REFLECT WHICH METHOD YOU USED TO PREVENT THIS PROBLEM AGAIN.