Being too picky???

Gents,

While I know this is a technical forum... can you tell me if I'm being too picky?

I have inheritied the maint. responsibilities of several water storage tanks whose level is monitored via pressure transmitters located at the bottom of the tanks. The tanks range in height from 23 to 45 feet. ALL that is needed in terms of data is the level of the water inside as measured from the bottom.

Rather than scale each xmitter and the HMI graphics to the max height of each tank, they are all scaled to 50'.

This does work (with the loss of resolution on the smaller tanks). Can anyone else give any reasons why this should NOT be done. Or... am I TOO PICKY?

Thanks... Von

Well,

From an engineering perspective, the greatest resolution achievable is the desired result, prompting the change. However, from a congruency standpoint, I can see why they are the same. It all depends on the customer, which may be you if you are now responsible for the maintenance of these units. I'd say, do what you like.

IMHO, you are not being too picky.

The purpose of an HMI (other than as a control interface) should be to display information in as intuitive a fashion as possible based on the skill level of the intended audience. So, if your operator is looking to identify gallons remaining in the tank, that is what it should show. If it should show feet of water from the bottom, show that.

My guess is that the HMI designer, at the time of design, did not have all the information you currently have.

The most important question I believe you have now is what is the cost/payback for changing this.

Steve

When dealing with multiple tanks of differing sizes (and because of the HMI issue) we usually scale tank pressure transmitters to read 0-100% level. This way on the HMI side it is easy to set up the pretty graphics for the tank levels. To me this is the larger issue the resolution is secondary, especially since your input is most likely 12-15 bit resolution (maybe a big deal if these are very large diameter & you are calculating volumetric data). Of course if you are trying to be that precise something other than a pressure transmitter may be needed depending on what the liquid is, how the tank vents are setup and what the ******t [why did this comment out a m b i e n t] temperature swings are.

Darren

dash said:

what the ******t [why did this comment out a m b i e n t] temperature swings are.

Darren

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edit: sorry 'bout that I was just curious...

******ce

Paul

If pinpoint accuracy isn't crucial then you could have fewer spares on the shelf already calibrated for the application by having them calibrated the same. It makes sense to me for it to be done that way in order to make life easier for the maintnenance team. If the water is being metered out for some process and exact measurments are needed then I would calibrate each on to it's tank.

The people changing these out on weekends and at night may be general maintanance and not trained in instrument calibration.

Two separate issues:

As JTN points out, a common range for the transmitters means one spare fits all, no configuration/calibration needed.

I've done exactly that (identically ranged transmitters on different level tanks) on several tank farms, at the owner's request, to minimize replacement maintenance issues.

The 2nd point, does not particularly make sense: having all tanks represented on the HMI as 50 footers, when they aren't.

It would bother me to see a 24' full tank shown as a half full 50' graphic.

But sometimes, whoever uses it grows so accustomed to whatever it is, they don't want to change.

I'd leave the transmitters, and change the graphics if changing the graphics is OK with whomever it is that has to use the data.

Nice test, OkiePC, for the inanity of bleeping ******t (a m b i e n t), and not heroin.


Dan

Von,

I have seen this problem before. Most likely, the pressure transmitters are factory calibrated to produce 20 miliamperes at 50 feet head pressure. The PLC programmer probably had this info and not any on the actual height of the tanks. The real tank height is not usually known until the tanks are designed, ordered, and manufacturer's drawings finished, long after the PLC programmer has finished his work.

The easy fix is to go into the HMI program and rescale your level indicators to show 100% full at some lower number than 20 miliamps (whatever the full value is for each tank).

The hard fix is to recalibrate the pressure transmitters, usually possible, but not easy and requires the proper equipment.

If you are using SMART transmitters then calibration should not be a problem. (Just plug in the communicator and dial in the required range).

You should be able to use one transmitter to cover all required calibrations.
How does the operator know when a tank may be overflowing when all are calibrated to 50 feet?

I would say calibrate for the correct range for each tank and so set the system up for the operator so that he/she gets the correct information which will improve his workload.

Rather put the responsibility on the maintenance personnel after all thats what they are there for isn't it?

If you are not using SMART transmitters then ensure that your maintenance personnel are trained to be able to calibrate a transmitter which is really a very simple thing to do.

Thanks Guys,


Your responses mirror my thoughts. I will likely reprogram each tank's equipment over time as I perform P.M.

Thanks again to ALL!

Calistodwt said:

If you are using SMART transmitters then calibration should not be a problem. (Just plug in the communicator and dial in the required range).

You should be able to use one transmitter to cover all required calibrations.
How does the operator know when a tank may be overflowing when all are calibrated to 50 feet?

I would say calibrate for the correct range for each tank and so set the system up for the operator so that he/she gets the correct information which will improve his workload.

Rather put the responsibility on the maintenance personnel after all thats what they are there for isn't it?

If you are not using SMART transmitters then ensure that your maintenance personnel are trained to be able to calibrate a transmitter which is really a very simple thing to do.

Click to expand...

I would say the graphics should be changed to show information correctly. Is this occurring at a company you work at or are you doing work as an integrator at another company. If you are doing this at your work and you feel there will be enough gain by recalibrating the transmitters to change the way things are then do it. If it is for someone else I would find out if they have a good reason for having it the way it is now - if accuracy is not critical there are good reasons for having it this way.

A lot of places hire general maintenance types who fix anything that breaks, not instrument specialists. They may be comfortable changing out a transmitter but not changing parameters or turning pots on one. I have seen places where the people are capable of learning how to do this and you can train them and they can understand and learn it, but when one of these transmitters only breaks once every 3 years, they aren't going to remember the details on how it is done. When this thing breaks and they have other work going on they aren't going to have time to sit down and read the notes or manual to figure it out again. Sure it's not hard to do to us, but we do it regularly.

A lot of integrators tend to only look at their little portion of the picture. When you see something that looks like it should be done differently start asking people why it is done that way, and try to figure out the big picture. Maybe it will make a more accurate control system, but will it cost more down time or add to their training budget that is already too slim, and will the control system being more accurate add anything to their bottom line? When it comes down to it only two things really determine whether something should be done - is safety involved, and will this add anything the big pictures bottom line? (And sometimes I throw something pretty on a graphic just for good will - that goes a long ways for getting future work!).

We do as Danw for the LT's , set all the same scale (same reasons), and as Lancie1 for the HMI graphics.


And disagree with Calistodwt (& agree with jtn) as in many cases, the calibration/maint. people are following a set of written instructions that have to be very simple and NO chance of errror. And yet ...

Personally, I always scale a level transmitter 4-20mA to be 0%-100% physical level, with the full 100% being at the bottom of the overflow or top edge of a straight sided vessel, or, I guess anywhere you want to be the maximum level reached. To me, it's easier when looking in logic for knowing exactly what the level is and not having to know the tank height etc... It's also easy on graphics to use a multiplier to turn the 0-100% level into gallons or pounds or whatever.

I guess I could see the point of scaling all spares the same if you didn't have a HART or other type of transmitter where the range could more easily be set.

Also, on more critical control loops, I would prefer not to be trying to control in a 4-11mA range or whatever the smallest 'range' may be.

Just my preference.

Not on topic - drug testing

NOT ON TOPIC

OkiePC said:

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edit: sorry 'bout that I was just curious...

******ce

Paul

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