khan

Anybody able to advise me how to "adjust" a pump gauge. It's an old Specialzed pump (similar to the old Silca floor pumps). Anybody ever adjust these and can explain the process to me or point me in the right direction to resources? I appreciate the help!

Bianchigirll 

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LarDasse74

You can adjust it if you are sure the range is just shifted.. ie. a zero error when the real pressure is 0 psi the guage reads 20 psi and when the real presure is 100 psi the guage reads 120 psi. If the guage is 20 psi off at the start and some other amount off later on then I cannot help...

Disclaimer: I am not familiar with that pump so I can only speak in generalities.

There are two ways to adjust a pump like this - either move or bend he needle or possibly adjust the spring. To move or adjust the needle you need to disassemble the guage enough to get access to the needle then either pull the needle off and replace it at 0 psi, or just bend the neede so it pointe to 0 psi (but be careful the needle is not rubbing or can otherwise affect the function of the guage). As for adjusting the sring, I know my cheapo raleigh floor pump, when the guage is disassembled, has a plastic screw that preloads the spring. Turning this screw in or out can affect the way the needle points at ny given pressure... and since coil springs are generally linear, this adjustment should only result in an adjustment of the zero as described above.

If it not simply a zero error I am afraid I cannot help you.

khan

Thanks for the info so far. I'll see how far into the gauge I can get and what I can do when I get there.

merlin55

see the cut away view at https://www.freepatentsonline.com/6604428-0-large.jpg
the bourdon tube (item 28) can be carefully tweaked, or often the zero point is adjustable once you get behind the dial face. The air inside the bourdon tube makes the bent tube straighten out a bit which is how the pressure is measured...state of the art technology from the 1850's

Panthers007

I'd be quite concerned about the accuracy if the readings on the gauge had shifted - as it appears your pump's gauge has. It should be calibrated with a known-accurate gauge before any use. Or a new gauge installed in the first place.

I just read a thread here a few days ago. The person's pump-gauge was inaccurate - to put it mildly. He put in what read 100psi into his tires. And one of them blew apart when he hit a pothole. Turns out that 100psi was more like 200psi.

eddy m

Bourbon tubes get knocked off zero all the time. It changes the starting point of the scale but doesn't affect accuracy at all except to add a constant. I have a lot more confidence in my pump that shows 5 psi at rest then the one where the needle rests on a pin stop at zero. If your needle is resting at 20, just pump to 120 when you want 100. Trying to reset it will only risk damaging it, and won't improve accuracy.
If you want to be more accurate then that, you will need to find a lab somewhere to calibrate it for you.

em

eddy m

This thread got me thinking about the accuracy of my own pumps, so I went went knocked the zero stop off the gauge on my SKS. The needle now stops on about -15 psi. That's not much better than what the OP was so concerned about.

em

FBinNY 

You have a bourdon tube gauge, and as others have said it can't really be adjusted. You and use it as is and subtract the zero reading (20#s) from the reading to get a reasonably close reading.

If you have access to an accurate gauge, and can remove the crystal you can recalibrate the gauge.

Connect the gauge to a known pressure source at about the center of the scale (ie. 80psi for a 0-160psi gauge), remove the crystal and carefully pry the needle from the pin, lifting from both sides so as not to bend the pin. Now set the needle at the true pressure and press it back down on the tapered pin.

BTW- this is how gauges are set at the factory, the gauge is run through a few cycles to pre-stress the bourdon tube, then the pressuer is set at mid scale and the needle is pressed on.

neil0502

Verify what's right. Leave the pump alone. Do the math when airing your tires.

My $0.02.

DaveSSS 

Many years ago I made my own pump calibration device. It consists of a short length of pipe with an elbow and cap at one end and a high quality pressure gage at the other. I drilled the cap so a presta valve could be inserted like it is in a rim and tightened it in place with the standard nut. The rubber left at the base of the presta valve (cut from an old tube) seals the hole well. I can pump up this piece of pipe and get a reading to compare with my pump gage. There will always be some difference, but as long as it's not major, there is no problem. I know what gage reading I prefer based on experience.

Unless the face of the gage has twisted, you probably won't be able to fix the gage. Even if the needle comes to rest at zero does not mean it measures 100 psi correctly. It may register correctly now, at higher pressures.

eddy m

What did you use for a test gauge? It seems like all of them are bourbon tubes with the same limitations as the gauges on pumps. Is there a reasonably priced, accurate electronic gauge?
FWIW before electronic stuff was available, hospitals used mercury columns for blood pressure, not bourbon tubes. Race car teams use pyrometers to measure tread temperature to set tire pressure. It turns out it pretty hard to measure pressure accurately.

em

eddy m

It seems like if the gauge were off by 100% you should be able to figure that out just by how hard it is to bring the tire to full pressure. Probably no ordinary pump gauge is very accurate, and the accuracy changes when you bang the pump around. Do you check the accuracy of all your pumps? How would you even do that?

em

wrk101

+1 Pump gauges aren't accurate anyway. Get a separate gauge.

Panthers007

I've found this to be quite accurate - based on the results of several pump-gauges that were new and read 0 psi at rest:

https://www.rei.com/product/756362?cm...:referralID=NA

Mondoman

The "bourbon" tubes probably went the way of the medicinal whiskey...

tatfiend 

When I worked in manufacturing every measuring instrument was calibrated on a regular cycle. The calibration standards were also checked regularly by an outside calibration service using standards traceable to NBS/NIST. The only way to ensure accuracy over time and use.

My old Blackburn pump (RIP) did have an adjustable zero for the gauge. For home use a decent Presta or Schrader separate pressure gauge should ballpark you as far as accuracy is concerned.

Accu-gage claims their gauges are +- 2% in the center of the range, at least when new. Here is a link to their bicycle tire gauges.

https://www.getagauge.com/BicycleTireGauges.cfm