Instrumentation question

My company is wanting to purchase a tool that will calibrate pressure transmitters. We have a lot of Foxboro and some Seimens pressure transmitters. We also have a few DP pressure transmitters. My supervisor is looking at a "wally box" made by Wika. But I was thinking of a Hart communicator, since all of the transmitters are hart devices. Does anyone have any opinions as to which is the best way to go here,and why? Or does anyone have any suggestions.

The answer to your question depends on what the requirement for 'calibration' means.

25 years ago, before 'smart' transmitters, it was routine practice to calibrate a pressure transmitter as often as quarterly (every 3 months) because the transmitters drifted, sometimes as much as 3-4%. Even though temperature compensation was attempted, but the difference between winter and summer readings or even day and night readings could be attributed to temperature drift as much as anything else.

Calibration involved
- shutting off (or disconnecting) the process from the transmitter,
- applying a an actual wet 'zero' pressure with air or hydraulic fluid (zero is sometimes zero, other times it is an offset value)
- making an adjustment so the 4mA read 4.000 mA and not 3.933ma.
- applying a span pressure (air, gas or hydraulic fluid)
- adjusting the electrical span (20mA) so it reads 20.00mA
- going back and forth between zero and span and iteratively adjusting the zero and span until both hold their settings.

And it still didn't take out the drift due to temperature change.

Sometimes the cal was done in the field, with 5 valve manifolds installed (on DPs) for applying the cal pressures, other times on the bench.

The Wally box was one of several tools that most large instrumentation shops had for calibrating their transmitters, for applying air or gas pressure to the transmitter.

Smart transmitters have been around since their introduction in 1983, and in wide use since the early 1990's. Smart transmitters typically use HART for configuring the transmitter.
The tag name, engineering units (PSI or KPa), and zero and span values can be setup using a HART communicator.

Modern smart transmitters from the major reputable industrial manufacturerers have negligible drift, that is, close to zero drift. There is no detectable difference between a pressure reading on the coldest winter day or hottest summer day when reading the output of a primary source calibrator like a dead weight tester.

Technically speaking, setting the zero and span with a HART communicator is not "calibration", it is "configuration". Most HART transmitters have non-standard commands (the DD for that device must be in the HART communicator) that allow one to 'force an output', whereby the millianp current output can be checked and adjusted. Force zero output, check for 4.000 mA, adjust if necessary. That does qualify as "calibration".

I am not aware of a major manufacturer who has calibration beyond the tweaking of the 4-20mA output. In other words, the transmitter 'reads' the applied pressure, converts it to a value and that's the value it is. There are methods of 'live zeroing' where whatever pressure is applied is considered zero, but that means the transmitter still sees that applied pressure but considers the applied pressure as 'zero' and that becomes 4mA.

If your transmitters have HART, it pays to have a HART configuation tool, either a handheld or a PC with HART app, like Siemens's PDM. Since smart transmitters can be field configured for different ranges, it pays to be able to range the transmitter for each situation. No pressure need be applied, no adjustment need be made, just keyboard entries on the HART communicator.

Smart transmitters can't be internally calibrated on the input side, and can only be calibrated on the output sid (4-20mA). The realization of this comes when reading the manufacturer's documentation for the pressure transmitter. There are procedures for adjusting (tweaking) the 4.000 and 20.000mA outputs, but no means of altering what value the transmitter converts the applied pressure to.

The elimination of drift has significantly cut down on the necessity for calibration.

That being said, there are regulatory requirements (whether company instituted or government instituted) where a 'calibration' is necessary.

What constitutes "calibration" in your case?

Configuring zero and span ranges (probably not, unless the purpose of the Wally box isn't clear)?

Would forcing an output through HART and checking the 4-20mA output suffice?

Do you have to apply a zero and span pressure, or apply additional pressure points and confirm the transmitter's response?

If you can live with forcing zero/span and checking/adjusting the output, then a HART communicator with a good digital meter (milliamp) scale will do the job.

If you need to apply pressure then, then you probably need the HART communicator to make the adjustments (if transmitters are HART enabled) and a pressure source, which could be a Wally box or a number of any other pressure source calibrators.

Dan

Go hart..

I can't tell you how much trouble this will save you, just trust me.

We use the HART and a pressure generator and a certified test instrument. When we calibrate we are really just comparing the field instrument to the certified instrument. If the readings are within tolerance the instrument is returned to the field, if it fails, we send it out for repair.

Since we went to smart instruments we just verify the readings are correct. We do this on an annual basis for quality and safety critical instruments, even thought the instruments are supposed to have be drift proof for several years.
On non-critical instruments, we check them when we suspect a problem.

I've used the Fluke 744 for HART calibrating. You can purchase a certified pressure sensor that connects to it. It can do RTD's as well. Using Emerson's AMS or other calibration documentation software, you can upload and download to it as well, giving you 'as found' and 'as left' results tracking. It can do most HART functions but not everything a Rosemount 275 can do. I also have a HART 'modem' for connectivity directly to a laptop for direct calibration and documentation.


http://www.fluke.com/products/view/home.asp?SID=0&AGID=0&PID=19351

cntrlfrk said:

I've used the Fluke 744 for HART calibrating. It can do most HART functions but not everything a Rosemount 275 can do.

Click to expand...

Cntrlfrk: Have you ever successfully added a DD (device descriptor) to the Fluke?

At a local ISA meeting last month the topic came up, with a couple people upset that they have been unable to successfully update their Flukes with a DD.

Have you tried to add a DD?

Dan

A Hart communicator will be useful for entering scaling values, but what about the actual calibration? Hart communications devices have nothing to do with the calibration of the actual sensor.

We use a Deadweight Tester for all of our pressure calibrations here.

danw said:

Have you tried to add a DD?

Dan

Click to expand...

No, I haven't. I was frustrated when it couldn't calibrate a Fisher DVC though. That may have been corrected since. I haven't used the Fluke 744 for a couple years since I've changed employers, but it was a good device.

A few of us dropped into a food processing plant and calibrated several hundred devices in less than a weeks time.

A good unit if you can afford it!

rdrastHart said:

communications devices have nothing to do with the calibration of the actual sensor.

Click to expand...

We used certified pressure modules (and DP modules), with the HART capability you could view the module pressure reading and the transmitter reading side by side on the screen, store it as 'as found' data, then perform a sensor trim with the same unit, and store the new 'corrected' values as 'as left' data. With the RTD's, we simply sourced the 3 wire transmitter from the 744 and verified the data at the PLC/ control level matched the sourced value. Good for ducumenting for meeting regulation standards.

http://us.fluke.com/usen/products/700P00.htm?catalog_name=FlukeUnitedStates

rdrast said:

A Hart communicator will be useful for entering scaling values, but what about the actual calibration? Hart communications devices have nothing to do with the calibration of the actual sensor.

We use a Deadweight Tester for all of our pressure calibrations here.

Click to expand...

The HART communicator can get you into the cal modes. The pots of yesteryear are gone. The Siemens has pushbuttons and a display, but many others don't. Access is through the HART communiccator.

As Ken Moore mentioned, a large part of calibrating is applying a reference pressure (which can come from a deadweight tester, a hand pump, a Wally box, whatever is deemed suitable), and comparing the transmitter reading/output to the reference. If it's within spec, do nothing, except note/log/document it.

One cannot mess with the input side on any smart transmitter. The most any modern smart transmitter allows you to do is to declare "this applied pressure equals my zero point (4mA)" and "this applied pressure equals my span point (20mA). The transmitter takes its internal reading, whatever that is, and considers it zero or span, respectively, and then linearly interpolates reading in between.

The HART comnmunicator easily gets you into those modes, if you don't have something like a Siemens DSIII which has a couple push buttons that get you into mode 2 (zero) or mode 3 (span) that does that.

On the output side, the HART communicator can be handy to force the output to zero or full span 20mA, so that it can be tweaked while reading the current output. On some models, it's more than handy, it's required.

Dan

Accuracy!!

What are your accuracy requirements for the sensors you are calibrating?


Your calibration tool should be more accurate than your sensors, otherwise your sensors will be calibrating your calibration tool. This is one of the main considerations of buying a calibration tool.

Also, consider calibrating all the way through to your operator's display if possible. There are additional inaccuracies added by an analog input. This should also be considered or handled separately. It depends on your situation.

I have used the Druck units with mixed success. One has worked well the DPI610, one was a complete lemon the DPI605. I don't know why the 605 sucked and the 610 was fine, it had great functionality when it was working it just needed to be fixed every year.