If you don't vent a canner, then it doesn't read accurate when processing.... this is what the USDA says :
Air trapped in a canner lowers the temperature obtained at 5, 10, or 15 psi and results in underprocessing. To be safe, vent all pressure canners 10 minutes before they are pressurized.
I don't see how it would be accurate to not vent the air out first. I would use 2 quarts of water, then vent it for 10 min. and see what happens when the gauge reaches 15 lb. See if that is when the counterweight rocks.

We use a test unit from Presto. We get it calibrated every year. It is like a bicycle pump, and you put on the gauge to be tested on a stem. Then you test to see if the pressure on the person's gauge matches the test unit gauge. We test it at both 11 and 15 lb. pressure.

I know you can ship the gauges, but I am not sure they are accurate when you get them back, just due to the jarring of the shipping. That is why we tell people to get a brand new gauge tested. We see new ones right out of the box be off as much as 3-4 lb. So, that new gauge would have to be thrown away.
If you do ship one, be sure to pack really well in foam rubber. Put it between thick foam rubber, then put a rubber band around it, or tape it together. That is how Presto sends them to the extension office.

I personally wouldn't be afraid to can with a new canner without testing the gauge. I usually can at 12 pounds rather than 10, just to be sure.
Tomatoes can be water bath canned instead of pressure canned.
Stanley

Melissa - Does your canner use a weighted regulator? If so, then there is really no need to test the gauge. The weighted regulator controls the pressure and the gauge is just for show. I would have no problem using a brand new unit. If your canner doesn't use a weighted gauge, I would be on the lookout for any pressure cooker/canner at a garage sale or thrift shop with one. The vent pipe is a standard thread and can easily be swapped out with another manufacturers so that you can use the weight. I've retrofitted my All-American with a Presto vent pipe and weight.

Linda Lou - A gauge will read accurate regardless of venting. Your copy and paste of USDA instruction concern temperature, not pressure. Do you "vent" the canners before hooking them up to your test gauges?

Maybe you would take a risk, but honestly, we see a lot of new gauges that are bad.
If it was 4 lb. off, then canning at 12 lb. would not be safe.
With a dial gauge canner, you process at 11 lb. not 10 lb.

Melissa,
If you have acess to calibrated gauges all you need to do is compare your gauge to the calibrated gauge.(that is what calibration is) If your gauge reads the same as the calibrated gauge you are good to go. If not, cycle the gauges through the complete range of your gauge several times THEN do it again and this time make a chart showing what your gauge reads when the calibrated gauge reads 11, 12, 13, 14 and 15 pounds. Use this chart as the canning pressure you need to use for your canner. Gauge testing is getting harder to do every year due to the liabilities involved. I do my own because I have a dead weight tester left over at time of retirement. The gauge on my canner is near 10 years old and still holding accurately. For a cheap gauge that's outstanding.
You are definitely on the right track and should have no problem!
Jim

Thank you everyone for the advice. I have a field trip for work tomorrow so it will be convenient for me to stop by my university lab to see whether I can put my gauge in the same line as a calibrated one. If not, I'll call industrial places.

Here is what confuses me. Presto said water boils at 15 psi (pounds per square inch), which makes sense to me, since 14.7 psi is atmospheric pressure at sea level. At work we call this psia, or absolute pressure, as opposed to psig, or gauge pressure, which is measured with reference to atmospheric pressure, and is the only way, I think, to measure pressure (we add 14.7 to get the reading in psia). So if the gauge reads 14.7 at boiling, and I can at 11, why doesn't water boil at lower temperature? I assume that it's 14.7 psia and 11 psig aka 25 psia, but I don't see how the gauge works. Sorry to confuse, I confess that I haven't actually put the canner on the stove so maybe I will understand when I look at it more.

Melissa

Melissa,
Water DOES NOT BOIL AT 15 PSI but at 14.7 psiA or 0 psig.
Don't let the terms PSIA and PSIG confuse you. It's like the terms Celcius and Farenheit. They are merely terms used to say the same thing using different SCALES or numbers. At sea level water boils at 212 degrees F at 0psig/14.7psia. The gauge on your canner reads in psig therefore water starts to boil at 212 deg./0psig, when your canner reaches 11psig the water temp is >240 deg. When you first start your canner heating you will hear a lot of activity inside, but when you reach canning pressure and the pressure is stable you will hear very little. You will also see that to keep the pressure at 11 will take very little gas flame or heat.
If you take the gauge off your canner and erase the numbers, which are 0-15psig, and replaced them with the numbers 14.7-29.7psia you would have the same thing.
The scale PSIA is not used very often anymore except in the steam generating field or boiler industry. Almost ALL gauges you see elswhere will be scaled in PSIG unless labeled differently. (and they MUST be)
Jim

Jim, I agree with & understand everything you say here. My question was, WHY did Presto tell me that I could check my gauge by seeing whether water boils at 15 psi?! That's the part that didn't make sense. Good to know that gauges must be labeled if psia. Too bad power plant engineers don't have to indicate which type their sensors are recording in our computer system, because I work in the performance department at the power company, where we have to work with all different gauges and no standard.

OK, here is where my gauge stands. I went to school and compared it to two different gauges used in experiments, date of last calibration unknown. When the calibrated gauge read 11, my gauge read 10. The third gauge read 11 like the calibrated gauge. So I conclude that my gauge reads one pound light, though it's possible both calibration gauges were wrong. I know calibration by the extension or Presto is the best idea, but I would like a few opinions on whether I am most likely safe if I can at 11 psi on my gauge (which would be 12 on the calibration gauge). Thanks to everyone for the advice so far.

Melissa

Melissa,

After re-reading your post, I think I can see where you got confused in their instructions. They aren't worrying about when the water boils but when the regulator begins to rock. It is preset to rock at 15 pounds and if your gauge reads 15 while it is rocking, then it is accurate at 15 lbs. If that is the case, I suspect it would also be accurate at 11 lbs. but Linda Lou says that they test them at 15 and 11. Can a gauge be correct at 0 and 15 but incorrect at 11?

Linda Lou - Have you ever encountered a gauge that gave one accurate and one inaccurate reading? Do you adjust them or advise the owner to get a new one?

Melissa
Does the power company you work for have an Instrumentation Dept.? Somewhere in the company there has to be someone who calibrates and certifies the gauges and intruments that control and keep track of boiler pressures etc. A power plant cannot run properly or safely (legaly) without this kind of service. (Many gauges etc are used for "indication only" and dont need calibration) That is who you should look for. The gauge that you compare your gauge to must be calibrated. You have compared your gauge to two others and found that both of the other gauges said the same thing (11 pounds) while looking at your gauge (10 pounds). Interesting don't you think?
With the on going gauge problem in this country I don't understand why canner manufacturers don't make a weighted gauge with 11,12,13,14, pound settings or weights. For me the only thing I want to work with is a dial gauge but I am lucky in being able to cal. my own. I have a problem with weights that "jiggle".
Malon
It is possible to have a gauge accurate at 0 and 15 pounds and be off the mark at 11 pounds (a little), this is called "hysteresis" and it can be off either + or - the 11 pound mark. Some gauges can be adjusted but for the most part those on canners cannot be. It is possible to remove the hand on a gauge and replace it at an exact mark but then the gauge can ONLY be used for that mark (i.e. 11 pounds). Not a good thing to do in case the canner is sold or someone else, unaware, uses it. It is best to make a calibration (deviation)chart to go with THAT gauge.
Melissa - in your case I know what I would do but won't say that here.
Good luck.......Jim

'I have a problem with weights that "jiggle".'
Jim,
Since it sounds like you have scientific knowledge, I'm just curious what you mean by that. I'm not able to get my gauge calibrated. I have the set of 5/10/15 weights and used the 10 pound weight the one and only time I have used the pressure canner (to make turkey stock). When the gauge said 10.5 pounds, the weight was dancing rather than jiggling. Do you suppose that means the gauge runs high?
Melly

Jim - thanks for the information on gauges. I don't work with them much other than adding R134 to my vehicles when needed. My old All-American had only a gauge to regulate pressure and I disliked that system. My extension office tested it when I purchased it used but I had to leave it with them for 2 weeks. I retrofitted the lid by removing the folding vent pipe and switching it out with a Mirro or Presto vent pipe that I picked up at Goodwill. Now I can use the standard 5/10/15 pound weight and not worry about staring at the gauge for long periods of time.

Melly - My gauge, after being checked, reads around 10.5 pounds when the weight is jiggling.

Jim, that's a great idea & I want to make it happen, but all the maintenance & instrumentation is at the plants, so I don't have a contact who could arrange it. Funny, I went to engineering school because I loved hanging around with & learning from mechanics, but now I don't get to deal with mechanics at all.

What if I can find someone to sell my a new, calibrated gauge and a fitting to put it into the canner? Any reason that wouldn't work? Maybe Presto could even mail me a new gauge, but if I can't trust the new gauge it came with, why would I trust a new gauge purchased separately? Grr.

Melissa

I think my situation is resolved. Monday I have an event for work that will end at 2, so instead of going back to the office I will just take the rest of the day off & drive out to the one extension office in VA that tests gauges. I will make it worth the trip by stopping off at a berry farm on the way back. I haven't done any blackberries yet this summer. This weekend I can still take advantage of tomato season by BWBing a double batch of Annie's salsa, and since my Clear Jel finally came in I can defrost my prepared sour cherries & make pie filling. Next year I will send my gauge to Presto in March.

Malon, you're right that they told me the regulator should be rocking when the gauge reads 15 psi. I thought they meant when the water first boiled. But of course, in case of a linearity error I would rather not trust that for canning at 11. Hysteresis is a problem that I don't think calibration would necessarily solve. If the gauge is reading different on the way up vs. on the way down, there's not much you can do. Seems like the canning procedure is designed to eliminate as much of this as possible, so if I can keep the gauge steady I think I'll be ok. I'm having hysteresis problems at work, it makes my data plots appear as very wide bands and my target curves can't id whether the current state is headed up or down. Maybe my next job will be in control theory where things can be more realistic than having only one input to a target output. And here I thought the only way canning would define my career path would be my need for a job with time off to can during the summer!

Thanks everyone for the advice, I will report on this new exciting testing device being hidden away at the edge of the state.

Melissa

Melly,
I have never found a good definition of a "jiggle" or a "dance" with regards to a canner weight and that is my problem
How many "jiggles" equals a "dance"? I do know that a certain number of jiggles/time equals a certain pressure and that if the number of them increases or decreases then the pressure is doing the same thing BUT how much? Only a gauge can tell me that. That is why I won't can without a gauge plus the fact that I have to can at increased pressure due to elevation.
Malon,
You will find that a "10"pound weight gauge will not be exactly 10 pounds. Reason: to can safely the temp in a pressure canner must be AT LEAST 240 degrees F and 10 pounds won't do that. It is close but not close enough! Ever notice that a canning recipe says "can with 10 lb weight....
for gauge canners can at 11 pounds" for.....? That's why your dial gauge reads more than 10 psi with the "10" lb weight.
Melissa,
I know, all too well, your problem with hysteresis! It can drive you nuts.
Don't be surprised if that elusive, exciting testing device consists of nothing more tha a small black hand pump with a certified gauge attached to it. It would fit neatly in your purse.
Find a friend of a friend who knows someone in the instrumentation dept. at your plant.
Good canning..........Jim

The jiggle is a wiggle and isn't like a dance. A dance is a bit more pressure raising the weight to a point where it emits more steam and will not jiggle, but will spin and 'float' more.

Ken,
Well, I have watched a pressure gauge vary from set point so I am sure that a weighted gauge will do the same but no way to KNOW how much. I think the next time I have some spare time I will borrow a weight and vent assembly and watch just what happens to the pressure as the jiggle/wiggle changes. I have a certified gauge with a range of 0-20lbs in 0.1lb increments and would be interesting to watch. I guess I prefer something to watch, i.e. a gauge. Many years as a metrologist has taught me to trust a gauge more, nothing is left to interpretation.
Jim

I agree. The weights are nice, but if they get clogged, they can become inaccurate. I used to also be in calibration at a company I worked for, and was responsible for all electronic measuring instruments. I have this pressure fryer for making chicken (like KFC used) and if it jiggles too much, its to hot, so I have to turn down the heat to a very slight 'wiggle', as a jiggle is just a bit too much. Very hard to distinguish when there is no dial gauge attached. The end result for me if its too hot in the pressure process is burned chicken. The physics involved with the weighted gauge is such that if its a specific weight, it should be able to hold back the steam pressure until it build to a point where the pressure releases through the weighted seal. Yes, this can vary and can also be misinterpreted. An accurate dial gauge and weight are probably the best of both worlds.

My gauge was correct! I've only found one certified testing device in VA, and it's over an hour from me. The Family & Consumer Services lady had to travel that day, so she was kind enough to bring it to a nearby county & teach the 4H lady how to use it to test my gauge. And my gauge turned out to be fine. I highly recommend the King & Queen county and King William county extension offices in VA.

One other question for you MFPs, how do you become certified as a Master Preserver? The extensions around here have so little information on preserving, it's more agricultural & 4H.

Melissa

Melissa, it is getting very hard to find offices that provide training any longer. I was fortunate enough to be able to do it here. Acutally, I have done it twice, once with the extension agent that retired and then with the new one, just a few years ago.
It is getting hard to even find an office that will provide information, or test gauges.

That's too bad, Linda Lou. I'm glad we have you & others on here to help--imagine trying to learn to can on your own without the internet or an extension agent. Next summer I will probably drive out again to get my canner tested and pick blackberries, so maybe I can talk to the extension agent there about getting some training and maybe volunteering at my own county office. I could borrow her testing device to test gauges for people around here. Or maybe the state university that runs the extension could help. Thanks for the advice on getting it tested, I'm excited to can my first batch of water and then start tomatoes.

Melissa