Hi all,
Just wondering can anyone tell me how to calibrate a magnahelic.
emma h,
to calibrate the Magnehelic gauge first make sure the instrument is perfectly leveled in both dimensions. Next disconnect both high and low pressure ports and adjust to zero with the screw on the face of the instrument. Usually that is all to get it calibrated. If the instrument needs its range to be adjusted, unscrew the bezel with the clear plastic lens, take away the screws that hold the scale, and very carefully take the scale out forcing the pointer sideways to avoid damage. Behind the scale you will find the spring that moves the magnet which actuates on the helical pointer axis, attached to the holder with a clamp on the upper side and sealed with lacquer. Connect the gauge to a known source of a pressure as near as possible to the upper extreme of the instrument range, loosen the clamp and move the spring in order to lenghten or shorten the spring cantilever, according to the adjustment you need, letting the instrument stabilise between adjustments. After adjusting and fixing the clamp, vent the pressure, reinstall the scale and readjust the zero. Iterate both zero and range adjustments until the instrument is in spec. The whole procedure is extremely lenghty and boresome, and you really need to evaluate if a new instrument is not the better option. On top, usually if one of these gauges gets its range out of spec, it is due to a pressure surge and normally the diaphragm is damaged and the gauge cannot be calibrated again.
Regards
Alfred
Hi Alfred,
Thanks a million for this I will give it a try.
Hi Emma
I am afraid you are being led astray here. Over the years I have been involved with calibration of hundreds of magnehelic gauges.
It is what the gauge is used for that dictates whether it must be calibrated. If it is used (most of them are) to measure room differential pressures in a regulated process, then the magnehelic is a critical instrument and must be treated as such.
No matter whether it is brand new, it must be check calibrated against a traceable standard (UKAS) before use, and once in use, recalibrated on a calendar bases at intervals that you must develop a written justification for.
For example, a new installation with no historic data, the regulators would expect to see them recalibrated at 6 month intervals. Once there is data to show they were stable over 6 months you would extend to 12 months, and so on. You can extend to what ever your data justifies, but it must be documented.
Your are required by cGMP’s to have a written justification for the classification of all instrumentation as critical or not critical.
Regards
Alex Kennedy
I dont think that you are being ‘led astray’ here emma h. Both are correct. Alfred just gave a lovely explanation on the method one would use to re-calibrate a Mag. gauge. alexkennedy has given some usefull information noted from experience.
Thanks to the both of you for the help.
[quote=Alfred]emma h,
to calibrate the Magnehelic gauge first make sure the instrument is perfectly leveled in both dimensions. Next disconnect both high and low pressure ports and adjust to zero with the screw on the face of the instrument. Usually that is all to get it calibrated. If the instrument needs its range to be adjusted, unscrew the bezel with the clear plastic lens, take away the screws that hold the scale, and very carefully take the scale out forcing the pointer sideways to avoid damage. Behind the scale you will find the spring that moves the magnet which actuates on the helical pointer axis, attached to the holder with a clamp on the upper side and sealed with lacquer. Connect the gauge to a known source of a pressure as near as possible to the upper extreme of the instrument range, loosen the clamp and move the spring in order to lenghten or shorten the spring cantilever, according to the adjustment you need, letting the instrument stabilise between adjustments. After adjusting and fixing the clamp, vent the pressure, reinstall the scale and readjust the zero. Iterate both zero and range adjustments until the instrument is in spec. The whole procedure is extremely lenghty and boresome, and you really need to evaluate if a new instrument is not the better option. On top, usually if one of these gauges gets its range out of spec, it is due to a pressure surge and normally the diaphragm is damaged and the gauge cannot be calibrated again.
Regards
Alfred[/quote]
Hi,
Are theire guideliens on the positioning of Magnahelic Gauges throughout cleanrooms or do the need to be adjacent to the room?
Hi Can Anyone please suggest best calibration instruments available to verify and calibrate the magnehelic gauges.
Thank you
Roger