what could cause a pressure gauge to read low?
what could cause a pressure gauge to read low?
(OP)
Hello friends!
I have a deaerator. It is fitted with a 0-1 barg analog dial gauge (wika, not sure of model right now). It is reading low. Digital sensor (4-20mA) reads ~0.22barg, consistent with thermometer reading 105-106 C. But the analog is reading ~0.12barg. What could be the cause? Analog gauge is brand new.
I have a deaerator. It is fitted with a 0-1 barg analog dial gauge (wika, not sure of model right now). It is reading low. Digital sensor (4-20mA) reads ~0.22barg, consistent with thermometer reading 105-106 C. But the analog is reading ~0.12barg. What could be the cause? Analog gauge is brand new.





RE: what could cause a pressure gauge to read low?
What is the height difference between transmitter and gauge, do they share the same connection point?
RE: what could cause a pressure gauge to read low?
Many people do not check the calibration (especially the zero) out of the box. I often find that practice to be a mistake. Instruments get pretty beaten up in shipping and in warehouses. When I'm dealing with a 0-10,000 psi gauge I typically don't care about a few psi intrinsic error. If I have to differentiate 0.1 bar from 0.22 bar then I always verify the calibration on a new analog instrument prior to use.
David Simpson, PE
MuleShoe Engineering
In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. Galileo Galilei, Italian Physicist
RE: what could cause a pressure gauge to read low?
Wika can have an open/close vent valve on the top of the housing on its higher end gauges.
Others have a rubber plug that keeps the fill fluid inside the case during transport and one is supposed to poke a hole through the rubber plug when the gauge is put into operation.
RE: what could cause a pressure gauge to read low?
RE: what could cause a pressure gauge to read low?
Here is a pic of both the digital transmitter and the analog gauge. Similar installation height. I did not see a hole for liquid filling, also no obvious way to adjust calibration.
Is it possible that the analog gauge is defective out of the box? Or could it be lying at the low end of the range?
RE: what could cause a pressure gauge to read low?
The gauge is connected below the floor, what makes you think the two points are at the same pressure?
Sorry it's a long time since I worked on a de-aerator .there are a bunch of trays inside the column so I assume some pressure difference.
If you suspect the gauge is out test it against a column of water 88.44" high (hose full of water) (0.22 bar) and reposition the pointer.
Note - the gauge has a syphon which may give a slight negative effect.
Wika are a reputable company, they won't sell you a defective gauge but it's possible it got bumped or stressed during installation and has a zero shift, check it against a known pressure e.g. water or Hg column, re zero.
Do not turn the gauge by the case, use a wrench on the square stem.
RE: what could cause a pressure gauge to read low?
Under normal operating conditions neither is submerged in water. What they should be sensing is the pressure of the steam in the headspace above the water. I always assumed that gas pressure would read about equal in such an arrangement. But wait, there's more. The top of that dome is vented, so if anything, it should be the digital sensor that reads low due to proximity to the vent...
I guess you're right, the only way to really tell is to test it against a standard. I will think how to rig up a water column for this.
How do you go about re-positioning the pointer? Take off the case and look for adjustment screw? Nothing is showing on the outside and I am hesitant to start disassembling it.
RE: what could cause a pressure gauge to read low?
RE: what could cause a pressure gauge to read low?
The tool looks like a miniature version of a tie rod end remover.[img https://www.amazon.ca/OTC-7315A-Universal-Tie-Remo...]
The end of the screw is turned down to a diameter a little less than the diameter of a meter shaft.
Link
You are using a 1 barg meter to measure .22 barg. Some meters are very accurate over the entire span but I never expect good accuracy in the lower quarter of the span.
If the meter is accurate at the low end, great, but I am never surprised if it is not. Your error is 10% of full scale. Not great but....check the calibration.
The pigtail may be adding a few percent error also but not much.
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: what could cause a pressure gauge to read low?
RE: what could cause a pressure gauge to read low?
They might charge quite a nominal fee to check out gauge and correct it if its a slipped pointer.
You might want to find out who your instrument vendors are and ask.
RE: what could cause a pressure gauge to read low?
Any bourdon tube gauge has adjustments for zero, span and linearity, I'm not sure if they still teach the technicians how to calibrate those.
RE: what could cause a pressure gauge to read low?
Pointer puller... Ok. Is there any way to make one myself if I fail to locate one?
If you had any info on this, I would be very grateful. My searches so far have failed to turn up any instructions on how to do this.
I did find one video where the poster bends slightly the omega shaped link inside the gauge to reset a bad zero.
RE: what could cause a pressure gauge to read low?
RE: what could cause a pressure gauge to read low?
But you asked, so:
1. Homemade pointer puller. This site touts rolling your own, but requires downloading a freeware (read that Malware) app to get the pdf. Caveat emptor.
[img http://www.homemadetools.net/homemade-needle-pulle...]
2. Mechanical pressure gauge calibration procedure:
If Wika has a calibration procedure, they do not publish it in their gauge manual.
Attached is a section from the 1976 Ametek US Gauge manual with illustration and an explanation.
Ashcroft publishes a method for adjusting linkage:
[img http://www.ashcroft.com/installationandmaintenance...]
In essence there is a gain/range/fast/slow pointer adjustment and a zero adjustment (pointer).
RE: what could cause a pressure gauge to read low?
danw2, thank you so much for the documents!
RE: what could cause a pressure gauge to read low?
Apply a test pressure of 0.22 barg.
Remove the needle and install it indicating 0.22 barg.
If you want better than that, buy a good quality indicating, electronic DP instrument.
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: what could cause a pressure gauge to read low?
RE: what could cause a pressure gauge to read low?
Mike Halloran
Pembroke Pines, FL, USA