pressure transducer

[lewlec]

Hi all

Any one recommend a good pressure transducer? Need to measure the water level in a tank, height is about 2 metres so it needs to be very accurate and a 4-20ma or 0-10v signal is needed. Also would like it to be mounted externally rather than submerged. Any suggestions would be appreciated.

Thanks
Gary

 

[Richard Burns]

Use an ultrasound sensor in the top of the tank instead!

Sorry I do not know about suppliers of pressure transducers but it seems that you would need to mount the tank on these transducers which seems complicated and would require a system to add the measured weights, however I think tankers have this sort of arrangement, just not sure of the accuracy probably only to a tonne or so!

 

[Marvo]

If you need it accurate and you want to go with pressure rather than level sensing then you should find a transducer with barometric compensation.

Is the water clean (is it potable or waste)? How accurate do you need the level measurement? Do you need constant measurement or are you using it as a switch when a certain fluid height is achieved?

 

[lewlec]

Hi

Thanks for the reply, the transducer will be connected to a plc and a hmi in the programme. It will be used to show the level of water in the tank, dry run protection for the pumps and activate the pumps to fill the tank. Also to activate a solenoid for mains water as back up if the tank gets too low. Does a ultrasonic give a analogue output? Or relay? It is for potable water

 

[Richard Burns]

I am pretty sure that the level sensors used in the plant I used to run gave a 4-20mA output to a PLC.
Though I seem to remember that they had to be locally programmed for the shape of the bottom of the tank, flat, curved, parabolic, cone,etc.

 

[lewlec]

Ok, it's something to look at.i was hoping to fit a tank connector to the tank and mount the transducer externally so it's easy to maintain or for any troubleshooting. I have seen a couple to suit this app but I was just wondering if any one had any experience with these and give any advice.

Gary

 

[Richard Burns]

About the only recommendations I can give for pressure transducers is to ensure the area is kept clean because they do get messed up in dirty environments, ensure that the tank is level and filled evenly to avoid spikes, have a method for lifting the tank so that the transducers can be removed for maintenacen or replacement.
However this is based on a weighing conveyor that may be in a different league of operation.

 

[tomfj]

to measure level in the tank there are no of options: differential pressure,ultrasonic,radar,capacitance,conductivity,sightglass,bubbler,floats,switches etc

for a pressure transducer the theory is that the pressure measured is equal to the liquid height above the transmitter multiplied by its specific gravity. There are 2 options differential pressure transmitter with a connection at the top and bottom of the tank if the tank is closed at the top,one side of the transmitter(high) is flanged/pipped on to the bottom of the tank while the other(low) is just pipped to the top.
gauge pressure where the tank is open to atmosphere and you dont have to worry about vessel pressure and you only have a connection at the bottom of the tank

Most instruments give back a 4 to 20ma signal. if not, its easy to fit a converter, pepperl & fuchs are a fairly common panel mounted type. issues for pressure tx include dirt in the tubing which can cause errors (its the small bore impulse tubing). Also you only measure from where the tx is located if the tx is mounted above or below the bottom of the tank you will have to adjust your zero pt .

The ultrasonic is non invasive reliable and accurate but needs a clear path to the surface also if there is a lot of vibration it can give false readings,also you have to allow a certain distance from the meter as it needs space to allow the signal to transmit, bounce, and then return before the next is sent

i would check out the rosemount site if i were you asit has a couple of good docs to show options and to explain them. (Rosemount are very good, reliable,accurate, easy to use instruments but they are expensive enough)

 

[lewlec]

Many thanks for the info, will have a look later.

Gary

 

[timbobelfast]

Hi all

Any one recommend a good pressure transducer? Need to measure the water level in a tank, height is about 2 metres so it needs to be very accurate and a 4-20ma or 0-10v signal is needed. Also would like it to be mounted externally rather than submerged. Any suggestions would be appreciated.

Thanks
Gary

Have a look at Vega gear here Measurement technology from VEGA: Sensors for measuring level, point level, pressure, density and flow

 

[Robotstar5]

Used a lot of the Vega sensors in the paint shop for level monitoring of paint and water, good kit, another good make to look at is IFM they seem to make a sensor for everything and their sales team will advise you on a site visit.

 

[Steve1412]

We use Hycontrol for ultrasonic depth measurement. They are very reliable and do give a 4-20mA output. We also have a few pressure sensors. These are Druck PTX 1830. They are a little expensive and have given us a few headaches especially if they come out of the liquid they can sometimes give spurious readings!!

 

[Sparkattack]

We use alot of sandhurst instruments, mainly for clean water tank level control. The type that sits in the bottom of the tank. Also Druck Tx's and milltonics ultrasonic units which have relays to start/stop pump.

 

[wozz]

as this thread seems to be at a end i have a quick question, in this situation everyone went for something other than a vertical float switch, was there a particular reason for this?

 

[Marvo]

He wanted a sensor to perform multiple functions including giving a visual height indication on an HMI as well as dry run protection and pump switching control. A standard vertical float switch wouldn't have this capability.

 

[wozz]

ah i srr so a vertical float would have only been good for dry run protection?

 

[tomfj]

In my opinion if he wanted to use float switches he would need multiple ones to indicate different levels. but these would have an accompanying lack of resolution/accuracy and more complicated cabling. whereas with an analog device like the Vega the signal can be passed through a barrier to provide switching (Such as a P&F KFD2-CRG2-1.D) while also giving an accurate level indication all done in a panel with less wiring.
A big issue with multiple level switches is error signals. In general one faulty switch can freeze the entire system as they generally work in series. so a jammed float switch or one that has gone past its limit and is stuck in the wrong position will introduce a system fault.
if a float type is required i have seen a couple of types one with an arm like a ball cock valve but with a sensor and the other with a tube that moves vertically on a central core. In each case it was only once and i couldn't say how effective they were

 

[Bobster]

In my opinion if he wanted to use float switches he would need multiple ones to indicate different levels. but these would have an accompanying lack of resolution/accuracy and more complicated cabling. whereas with an analog device like the Vega the signal can be passed through a barrier to provide switching (Such as a P&F KFD2-CRG2-1.D) while also giving an accurate level indication all done in a panel with less wiring.
A big issue with multiple level switches is error signals. In general one faulty switch can freeze the entire system as they generally work in series. so a jammed float switch or one that has gone past its limit and is stuck in the wrong position will introduce a system fault.
if a float type is required i have seen a couple of types one with an arm like a ball cock valve but with a sensor and the other with a tube that moves vertically on a central core. In each case it was only once and i couldn't say how effective they were

I don't think this is an Ex install, it's not going to need a barrier if it isn't

 

[SheffSimon]

I don't think this is an Ex install, it's not going to need a barrier if it isn't

It's not that kind of barrier, it's a kind of signal conditioner which will power up the instrument, not ATEX kit.

 

[lewlec]

Hi

Thanks for everyone's replies, spoke to a company called ifm and they do a sensor that is accurate to 0.01 % so I may go for that although a bit pricey at £450. Danfoss make a cheaper version but up to 6 weeks waiting.

Thanks
Gary