level transmitter indication

[Iradah]

Hello all,

I have a question about the level transmitter indication.
I have a level transmitter with the type rosemount 1151 GP that is 1 ft above the bottom of the tank.
there is 1.5 ft dead level above the zero of the transmitter. the transmitter gives a level % out put. the range of the transmitter is 0-345'' H2O for a range of 1-41 ft.
The problem is that the level transmitter is giving negative level % output. I have replaced the tramsmitter with new one and insured that it is calibrated, but still getting the negative indication. since the tank outlet is 1.5 ft above the zero of the transmitter, the minimum output should be ~ 3.75 % and not negative. Please advise what to do to get right readings.

 

[danw2]

Being a gauge pressure transmitter, the low side port can not be connected at the bottom of the tank. If it were a DP model with the low side connected to the bottom, the result would be negative readings.

I'm not sure what happens when the LRV is higher value than the URV, whether the indicated percentage is positive or negative. Have you checked the LRV and URV?

 

[Iradah]

Can you expaline what you mean by LRV and URV? how can this affect the readings and give negative values? thank you.

 

[danw2]

LRV = lower range value in inches of water = 4.0mA
URV = upper range value in inches of water = 20.0mA

If this is a pressure vessel and there's vacuum over the liquid, you could have negative values with respect to the atmosphere.

 

[danw2]

Where are you reading the negative values?

On the transmitter's display?

On a loop indicator?

On a DCS?

 

[Iradah]

LRV = 0" H2O = 4.00 mA
URV = 345" H2O = 20 mA
The negative readings are on the DCS.
The tank PSV is set at 3" H2O. can this be the reason behind the negative readings?

Thank you.

 

[danw2]

The DCS is configured incorrectly.

 

[Iradah]

Can you explain more about that, they put a signal of 0 " H2O and check on DCS, they found that it is 0% level. then they put 345"H2O and checked with DCS, it was 100% level. they alos checked for the middle point 172.5"H2O and found it 50% level at DCS? how can the DCS be wrong?

 

[danw2]

If
LRV = 0" H2O = 4.00 mA
URV = 345" H2O = 20 mA

AND
"They put a signal of 0 " H2O and check on DCS, they found that it is 0% level. then they put 345"H2O and checked with DCS, it was 100% level"

Then it has to work properly when put back together unless

a) The physical substitution test (put a signal of 0 " H2O and check on DCS) isn't what eveyone wants to think it is, used the wrong terminals or looked at the input tag as a raw value, not the final value tag used where the indication is bad.

b) Something isn't correct in the DCS configuration.

There must be a math statement because 41 feet elevation is a head pressure of 492 inches water column. Your URV of 345 is indicative of a fluid with specific gravity of 0.701. A column of 492" of fluid with an S.G. of 0.701 will produce a head pressure of 344.9" w.c.

The input point gets a math treatment (input*1.426 and scaled for percent) somewhere, in order to make the measurement equal to level elevation. What else is in the math ?

The DCS probably isn't 'wrong', it's doing what it's been configured to do and you get negative values instead of positive values.

 

[Iradah]

DCS converts the 4-20 mA to level % out put from 0-100%. the relation is linear. so I think that the DCS is correct.
I tried to understand more about how this transmitter works. it measures the pressure inside the tank with reference to atmospheric pressure. I was told that the transmitter was calibrated to zero while disconnected from the tank. it looks like the transmitter gets affected by the pressure inside the tank, but tank pressure is set at 3 "H20. getting negative readings means that we have vacuum inside the tank? please advise how you think the pressure of the tank is affecting and what I can do to correct the negative readings?
Thank you.

 

[danw2]

The vent/relief valve will produce a level level error of 0 to 3", but in the positive direction.

Please provide examples of how "the problem is that the level transmitter is giving negative level % output" is manifested.

For instance, what are the values for this 3 column tank level table?

stick 4-20mA DCS
25%
50%

"Stick" means whatever manual method you use to measure tank level when you don't believe the instrument.
4-20mA value means the transmitter's output at that stick level
DCS means the value shown on the DCs at that stick level.

Use your own % levels, but at least two are needed to figure out what's going on.

 

[danw2]

The "test" terminals provide a 4-20mA output signal without breaking the output circuit, when a milliammeter is placed across the terminals.

 

[Iradah]

Having 3"H2O above the vent will add 0.14 ft to the actual level in the tank.
the negative reading appears whent the tank is epmtied. however, when we empty the tank we pump out the condensate from an outlet that is 3 ft above the bottom of the tank and 2 ft above the zero of the transmitter. this means that when we empty the tank we should have around 3 ft remaining which should give 5% level in DCS.
When the transmitters were replaced and calibrated, We took several manual dip readings and compared them to DCS. Please see below the table of the readings:
Dip reading, ft Level % Expected level, ft Dip - Expected Error %
2.625 3.526 2.410 0.215 0.082
2.333 3.085 2.234 0.099 0.043
7.083 12.000 5.800 1.283 0.181
11.250 22.520 10.008 1.242 0.110
15.375 32.060 13.824 1.551 0.101
21.427 46.700 19.680 1.747 0.082
26.125 58.130 24.252 1.873 0.072
30.896 69.550 28.820 2.076 0.067
33.406 75.850 31.340 2.066 0.062

Dip reading is the manual dip level % is the DCS reading, expected level is the level I expect to have at the level % reading. this is level%/100 * 0.4 + 1
Dip - Expected showed that the level % indication is less than the acutal level %.
After we emptied the tank, the level % became -0.62%, we filled the tank again and it gave positive readings, again emptied and gave -0.62%. is it possible to pump out the condensate to less than 1 ft level although the outlet is at 3 ft?!
I appreciate your advice.