sealing in calibrated CALIBRATION equipment

A

Thread Starter

Anand Iyer

Hi all,
I have done calibration of instruments using calibration equipment like dead weight testers and wallace and tiernan pneumatic calibrators and other tools over time since 1990 and still calibrate but use modern (!!!) digital calibration tools(!!!).

These calibration tools were calibrated by standard laboratories and certified as happens today too.

Now in the older equipment the zero and span adjustment screws were sealed with a sticker marked with something like, 'calibration void if sticker removed or damaged'. The same happens today except that in many cases there is a prominent "ZERO" switch which when pressed, the zero adjustment is done.

Does anyone have any idea as to

1. Does the zero adjustment affect the calibration, thereby making the old certification null and void!
2. If the calibration is not affected by the zero button, that is the calibration certificate is not void then why is this so?
3. If the calibration is null and void and yet you want to use this calibration tool then ,Should the zero button be sealed/made dysfunctional?

Anand
 
T
This depends on the true nature of the zero button. On some instruments the zero button performs a tare function, ie, it does not set the zero reference calibration point, it just zeroes out the display for relational measurements. If it does change the zero reference point then it would of course invalidate the calibration.
 
M

Marcos Thoni Bergamo

Anand

It seems that the zero adjustment is a standard procedure to use your instrument. It is quite common among electronic pressure gauges, refractometers, etc..

The calibration of your instrument was probably carried after that zero adjustment. The certificate is valid only if you repeat the adjustment before use.

This way you can reduce the overall uncertainty minimizing offset error, using a simple and reliable standard: pressure = 0.

If you would like to discuss it deeply, please contact me directly: [email protected]@@santistatextil.com.br (just remove the duplicates @@ - spam protection)


Regards


Marcos Thoni Bergamo
Santista Textil S.A.
Brazil
 
J
Simple question, lengthy reply:

Are we talking about instruments like HART pressure transmitters?

Transmitter calibration can mean three things:
- Sensor trim (i.e. correcting how many inH2O the digital display and communication shows e.g. against a deadweight tester)
- Range change (i.e. setting the input values corresponding to 4 mA and 20 mA)
- Output current trim (ensuring that current output matches the multimeter reading)

For a smart instrument zeroing means changing the range such that the output becomes 4 mA at the presently applied input. E.g. if the original range set in the transmitter was 0-100 inH2O and with 25 in H2O applied to the transmitter you press the zero button the range will be changed to 25-125 inH2O. I.e. by zeroing you have only changed the range, you have not made a sensor (calibration trim). You have not changed the value you read on the digital display or over the communication, only the 4-20 mA.

If the calibration certificate was issued showing applied pressure vs. displayed pressure than the calibration certificate is still true. If the certificate showed applied pressure vs. output current it no longer matches.

Your transmitter shall have either a hardware jumper or software lock that disables writes to the device such as zeroing.

Take a look at chapter 9 of the book "Fieldbuses for Process Control: Engineering, Operation, and Maintenance" (buy online in hardcopy or download immediately in softcopy). If your email does not support this hyperlink feature correctly, please copy the URL and paste it into your Internet browser. Mind the line wrap, make sure to get the complete path all the way to the 6: http://www.isa.org/Template.cfm?Section=Shop_ISA&Template=/Ecommerce/Product Display.cfm&ProductID=3036

You can even download chapter 1 for FREE in softcopy form NOW. Mind the line wrap, make sure to get the complete path all the way to the 5: http://www.isa.org/Template.cfm?Section=Shop_ISA&template=/Ecommerce/Product Display.cfm&ProductID=4585

Jonas Berge
==================
[email protected]
www.smar.com
 
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