# HELP: install Level transmitter on an AIR tank

#### Aatefsari

Good day gentlemen,

we have a closed Tank used for AIR compressor. I already installed Level transmitter on the middle of the TANK.

My question is: Can I calibrate the LT If I have the length of the tank?

question 2: the low impulse line should be empty or filled with liquid?

Last question is: I have the formula to calculate the ranges, but I want to confirm that, can you please help me if you have the formula?

The tank is used as AIR receiver from our AIR compressor, and we are using the LT to measure the water if there will be. S please help me to get this.

Thank you a lot in advance.

#### David_2

You're trying to measure the level of condensed water in an air receiver tank?

What technology is the level transmitter?

What exactly have attached to the middle of the tank?

Please provide a sketch of these physical connections.

#### Aatefsari

Yes I am measuring the condesned water inside the air receiver tank.

I am using differential pressure transmitter to measure the level of the water. High side connected to the bottom of the tank and the low side connected to the top of the tank by using instrument tubes.

what I ment when I said “ the transmitter installed middle of the tank “ is:
my transmitter mounted above the minimum level and below the maximum level. Same as this picture

The above picture same as what I have exactly. I hope it is clear without a sketch. If not clear, please tell me.

Thank you a lot Mr.David.

#### David_2

Just to confirm, connections are tubes or pipes. The DP transmitter did not come with remote seals connected with capillary tubing, correct?

#### Aatefsari

Just to confirm, connections are tubes or pipes. The DP transmitter did not come with remote seals connected with capillary tubing, correct?
The connections are instrument tubes only ( one to HI-side, the other tube to LO-side )

Correct. It is connected through manifold same as this picture:

#### David_2

1. Locating the elevation of the pressure transmitter

The transmitter can not 'see' or sense any liquid beneath the elevation of its ports.

Any liquid below the elevation of the transmitters' ports, labeled X in your diagram, cannot be detected, seen or sensed by the pressure transmitter.

The implication of that physics fact is that you need to mount the transmitter at whatever elevation you consider 'zero'. In the diagram above, zero level would be at the top elevation of X.

If you expect to read, sense or detect level H, then the transmitter has to be mounted much lower than shown in your diagram. As shown, the transmitter cannot read or sense the full level H because the lower elevation of the H is far below the elevation of the pressure transmitter's ports.

2. dry leg or wet leg?

If water condenses out and gradually fills the receiver tank, then water is likely to condense and gradually fill the low side "dry leg" pipe/tube. I've seen lots of low side connections that use a pipe tee at the bottom of the low side pipe, with a valve below an extension nipple at the very bottom. Condensate can accumulate in the extension nipple before it rises to the elevation of the Tee which is typically the elevation of the low side pressure port.

If the condensate is drained from the extension nipple often enough, then the transmitter never reports a false low level due the condensate level in the extension nipple rising high enough to touch the low side pressure diaphragm,but it's a maintenance headache.

Opening the drain valve will lower the pressure sensed by the transmitter on its low side; the dropping low side pressure which will increase the transmitter's reported pressure (reported pressure is high side minus low side) so be aware of that. Draining a dry leg should be done when any control using the reported level signal is held in manual mode in order to to prevent a level spike during draining from initiating unwanted action.

wet leg
If you fill the low side pipe with water and make a "wet leg", then there's no maintenance needed like there is in periodically draining a dry leg extension nipple. I suspect that the humidity in the receiver is high enough that the water in the wet leg will not evaporate, but I have no personal experience. Should the water level drops in the low leg, the transmitter would report a false high level value. The re-filling procedure, using a pipe Tee at the top of the low leg, requires zero air pressure in the receiver which is probably not a common situation, a special maintenance situation. But if there's so much moisture in the receiver that condensed water fills a substantial percentage of the receiver, why would water in the low leg tend to evaporate? Anyone else out there have direct experience?

3. Configuration

Air pressure cancels out because the high side sees the same air pressure as the low side.
high side liquid hydrostatic pressure + air pressure minus low side hydrostatic pressure (hopefully zero) + air pressure

Dry leg configuration:
LRV = 4.0mA = 0.0
URV = 20.0mA = Max level (transmitter elevation up to max level) or something slightly above the max level.

Wet Leg configuration
Same deal, air pressure cancels out.

Wet leg LRV = 4.0mA = the negative value of the height h, which will be a negative value because when
the high side = 0 and
the low side is a filled wet leg with height h, then,
high side minus low side = 0 minus h = negative h = 4.0mA

Wet Leg URV = 20.0 mA = max level minus h, where max level is height from the transmitter port to max level elevation.

If max level = h, then
4.0mA = LRV = negative h
20.0mA = URV = 0.0

If Max level < h, then
4.0mA = LRV = negative h
20.0mA = URV = max level minus h

Example:
h = 8m
max level = 6m

LRV = -h = -8m
URV = max level minus h = 6m minus 8m = -2m

When level = 100%
high side = 6m
low side = 8m
6m - 8m = -2m = URV = 100% = 20.0mA

When level = 50%
high side = 3m
low side = 8m
3m - 8m = -5m = 50% = 12.0mA

When level = 0%
high side = 0m
low side = 8m
0m - 8m = -8m = 0% LRV = 4.0mA