Level measurement using Hydrostatic type.

Hi,

There is no restriction on the mounting of the Electronic devices. I means to say you can mount anywhere at suitable location.

Thanks Pappu.

Is it because that the vent line, which ends inside the transmitter box is meant for opening to atmospheric pressure? Is this the reason why it would not make a difference?

Thank u

Whoa, there. Some of the words are getting tangled up with others.

Is this a differential pressure transmitter, being used for liquid level measurement?

Yes!

If your sensor is using a diaphragm-type technology it will be affected by atmospheric pressure. The venting allows the sensor to adjust to different locations.

To better explain, understand that diaphragm-type sensors are actually capacitive sensors, where the capacitance changes as the diaphragm flexes. If the sensor you are using is a diaphragm-type, there will actually be 2 sensors;

1) the reference sensor that maintains a constant dielectric value and spacing between plates.

2) the working sensor that will vary as the diaphragm changes.

The output is given based on the difference between the two.

Now consider, the sensor was manufactured at an altitude of 100ft above sea level, but you're use for it is at 3500ft above sea level. The barometric pressure will have changed from around 14.5 to about 12.7. Without providing a vent in the reference sensor, there would be a change in pressure between the plates of the capacitor causing an offset error in your output. How great depends on the range of the sensor. Generally above 500psi, it is not noticable but below 500psi it will begin to affect the accuracy of the sensor.

Hi friend I want to know the difference between pl3701p and pl3701c they are using as level sensors and I want to know the theory of diaphram sensors type

vented cable

This is the first time I've ever heard of such a thing. If I were to guess, I think you're referring to the impulse piping or capillary that connects the diaphragm to the transmitter.

If that is, indeed, the case, then good instrumentation practice says that the transmitter needs to be at the same level as the diaphragm. That means that, when the tank is empty, the transmitter will read the pressure as zero.

If the transmitter is above the diaphragm, you'll get a negative pressure reading when the tank is empty. If the transmitter is below the diaphragm, you'll get a non-zero positive reading.

If, however, you're referring to an electrical cable, then the transmitter can be at any height.

Not to dispute anything you said because by the poster's comment regarding the amplifying circuit being outside you're probably right, but there reasons why an electronic cable is used as the vent.

Many older gauge-type diaphragm sensors used capillary tubes vented out through a pigtail connector (connector and cable molded as one) to prevent intrusion of the media.

Even ones without capillary tubes and pigtails have to be vented through the cable. When the mating connector is coupled, they are generally environmentally sealed so the venting is channeled out through the cable.

It makes sense.

The arrangement maintains the Ingress Protection rating of the transmitter housing, thereby protecting the electronics, while providing the reference atmosphere needed for a differential pressure transmitter used in a level-measurement situation by using the interior of the display unit as the reference pressure.

OK. That's good.

Here's another vote for placing the display anywhere.

Well, as I said, this is the first time I've heard of vented cables and from the website our OP provided, it seems to be used mainly in marine applications (which I have no experience in).

I can see two reasons for doing this. One is that the vent needs to be free in case the compartment gets partially flooded. Since the diaphragm needs to be near the bottom of the tank, putting the vent at the diaphragm location can cause it to read erroneously if the compartment floods.

The other reason is that the tank and the diaphragm can be in two different compartments. If the vent is in sealed or partially-sealled compartment, the vent can't do its job of compensating for ambient pressure compensation.

My take, therefore, is that you can located the electronics anywhere as long as it under atmospheric pressure.

If you are measuring a liguid much heavier than air then you can have the vented cable / pressure sensor mounted where you like, within reason, and not incurr a significant error.

A good reason for mounting the sensor above the liquid level is to be able to easily change or calibrate the sensor.

In your case if the sensor is mounted below the liquid level it is good practice to mount the amplifier with vent cable above the liquid level - so as if the sensor fails the liquid does not contaminate the amplifier electronics or vice versa.

John.

Thank u guys for all the replies.

vented cable - This is a special cable which has a vent pipe running inside along the cable upto the transducer. Pls refer enraf PL3700 series pressure transmitter.

http://www.enrafmarine.fr/pdf%20file/FC/PL3701P%20screwed.pdf

I am planning to use this for tank level measurements in a marine vessel. Some of these are used as top mounted and some as side mounted in the tanks.

But as these are special cables, I have to order them from vendor for the length from transducer to the transmitter box. I am aware that I should install the transmitter at an area away from a flood-prone area. So I am taking this transmitter into another compartment where it is protected. But my question was whether I need to still maintain the installation at a higher level than that of the highest level of the tank.

So appreciate your comments.

Your link didn't say anything about vented cables but I visited the website and found it there.

This is a new one for me but it should operate under the same principles as other pressure-type level transmitters.

It's okay to have the amplifier above the transducer since there's nothing in the vented cable to influence the reading. I couldn't find a technical manual but I think the vent is connected to the low side of the transducer. If you're going to put the amplifier in another compartment, make sure that compartment is open to the atmosphere. If it's sealed or partially sealed, the level reading may be incorrect. By how much depends on the pressure in the compartment.

Referring to the enrafmarine specs you posted it looks to me that this installation is for a atmospheric vented vessel ( see elbow vent on top ) in this case the transmitter is no more than a pressure transmitter who responds to the hydrostatic laws i.e

pressure = specific gravity x height ( don'forget to match the units) so assuming that the sensor is in the center of the flange you must take the distance between this center and the real bottom into acount to calibrate your Zero or leave it as a suction cavitation prevention when pumping out.

I think You can use any pressure transmitter whitin the range providing they have the neccerary documents and certifications for Marine and Ex installation

".....assuming that the sensor is in the center of the flange you must take the distance between this center and the real bottom into acount to calibrate your Zero or leave it as a suction cavitation prevention when pumping out....."

Appreciate if you can explain "..leave it as a suction cavitation prevention when pumping out". i didnt understand.

Ahah! Another snippet of information. This thread is beginning to rock-and-roll now....

Electricity doesn't respond to gravity, and the diaphragm attached to the flange is the place where hydrostatic ends and electric begins. The transmitter is mounted within the housing that is attached to the flange, so there should be no issue with mounting the display anywhere in particular.

Hi all,

Some more info on the application.

The transmitter used is a hydrostatic pressure type to measure the level of the tank. To address the question from Slack, I think(!!!) it is a DP type with the liquid pressure acting on one side and the vent or capillary pipe (which is kind of sealed with the transducer and is forming part of the cable) going from the transducer to the transmitter box.

Unfortunately the details about the "special cable" is not mentioned anywhere in the website. It is only mentioned in their technical manual – it says

__________________________________________

ELECTRICAL CONNECTING CABLE (except atmospheric pressure model): [???? – WHAT IS THAT REFERING TO???]

o Transducer is delivered with minimum of 1 meter to max of 50 meters of cable.

o Correct length to be determined by customer [ ??? THIS IS TO BE INFORMED DURING ORDERING STAGE TO VENDOR TO AID THEIR CALIBRATION]

o Cable is composed of 3 wires (0V, supply, signal), 1 braid ground, one vent pipe (gauge application) and one aluminium shield.

o Transducer can only be connected to the amplifier.

So hope you guys can advise me with this info whether the mounting height of the transmitter at a higher level than the transducer or tank top level is a must.

Thank you all in advance.

Hi Engineerin....etc... I've used these vented cable transducers many times purely because you can have a sealed transducer even though its reading gauge pressure.

The vent 'wire' in the cable must not be blocked, but having the backside of the diapragm vented in this way means that you can mount the electronics is a safe position and that if the diapragm is damaged and leaks, the electronics as well as the tank contents will not be spilled into the environment, essential with some tank contents.

So to re-iterate, mount the electronics with the vent tube above the maximum level of the tank contents so that if the transducer fails there is no spillage of contents.

Your question about "...what is atmospheric pressure one..." is purely because the manufacturer makes the same style of transducer in differential pressure (gauge pressure with the vented pipe) as well as absolute pressure (measuring atmospheric pressure only).

Don't worry about the statement that the correct length must be stated when ordering as this is for the very accurate calibration and also the response time of the transducer.

Just mount the end of the cable with the electronics above the maximum fill level of the tank.

John.

Hi friend please I want information about pl3700 series

Even the transducer may be anywhere. If you use a tube with air blown in and an opening on the bottom of the tank then the transducer, e.g. on the top, will show the bottom pressure. This application is used for water level measurement in impounding reservoirs or dams (dont know the right expression ...) where a simple tube over a big distance is much cheaper/easier and more maintenance friendly than a special vented cable.
Regards Uwe

Another cost advantage is that it has not to be a "wet" transducer.

Hi guys,

Thank you all for your comments. I have learnt more than I set out to begin with.

Appreciate your guys taking your time out to share your knowledge.

Will come up with more questions (!!) soon.

Thanks.