Dials will never be accurate and I suggest that you don't waste any money on getting it checked or replaced. You can search the internet for the gasket if its still available. If its very old, it may lack a essential safety valve that protects against excessive pressure buildup. Most canners today use a weight set to get a more accurate pressure from 5 to 15 pounds, depending on the amounts of weights used. The weights are what regulates the actual pressure and is far more accurate than any dial gauge. Suggest that you also do a SEARCH in this forum to find any references to your brand of pressure canner. You will find any hundreds of posts about pressure canners.

Here is a link that might be useful: Search results for National 7 canner

A SIMPLE web search brought up this site for parts...

Here is a link that might be useful: National # 7

Yes, you can get the dial tested and buy a new one and have it tested, as well. The canner can be used still. There is no way to convert them over to a weighted gauge canner that I know of and do it safely. I tried to retrofit one. You can use it as long as you get the gauge tested and then must have it tested each year before use. That may affect whether or not you decide to use it, depending upon whether or not your county extension offers the service. You can get a new gasket and over pressure plug, too.
Good luck to you.

The site I linked to states that the Presto weights can also be used on the National 7, as shown on the web link. They also sell the gasket as indicated. I still don't think is worth the effort to replace a dial gauge as its totally useless and not worth the $20 to replace it. County extensions are few and far between and every year its even more difficult to find one that can 'check' the dial. They don't CALIBRRATE these as there is no means to do such a thing on such a poorly constructed dial gauge. None of these dials are adjustable in any way, and so they only give you a conversion chart to see what the pressure MAY be by looking at the dial and referring to the conversion table to see a possible pressure. Even for that, the dials can go out of whack in one single process, so why bother. Use the right thing for the right job, the weights!!

That is interesting because last year I emailed that same company and was told the new vent pipe would not fit the old National 7. I ordered a new vent pipe anyway to see, and was sent the old, more fat one. They said that was the one that would fit. Well, it did, but it sure did not accept the weights. So I gave up on trying to fit the weights on it. The vent pipe threads on the old canner was totally different and larger in diameter. It was also shorter. I sold the parts I ordered since they were no different than the old ones that came on the canner.
A dial can be accurate. If they could not be accurate, we would not be able to test them and tell people to use them. We cannot take that kind of risk for liability.
I am not saying that I feel they are more reliable than weights, but the dials can still be used if tested and found to be accurate. We tell people they can be used as long as they are not more than 2 lb. off in either direction, either high or low. No, we cannot adjust or calibrate a bad dial gauge.
I would contact the company on the link and ask them about the vent pipe and weights.

Thanks everyone for the replies, I'm going to order a dial for it and stuff. I'll see if my extension office tests them, I have a few areas around I can check if ours doesn't. I just don't have the 60+ dollars to shell out for a new one right now. There is another one in my mom's attic, it might not be as old, I'm gonna check it out too.

LindaLou, I have the dial on my canner and thankfully, my extension service still tests it for me. It pays to be nice to the person that runs my extension office!

I agree, the dials are accurate as long as they are regularly tested. My old canner is 40 years old, I've replaced the gasket, but that dial gauge is tested every year and it's still fine, within 1/2 pound of being accurate.

If a gauge is bad, they can be replaced and the new one can be tested even before putting it on the canner. If it's off, it can be returned. It's much easier, simpler and more cost effective than buying a different canner or trying to retro-fit an old one.

Annie

Does anybody have a copy of the owners manual for a National Pressure Canner No. 5? Would it basically be the same as using a No. 7? Can anyone give me a few pointers?

My mother has passed on to me the utility of pressure cookers and canners, they have become a hobby. She lets me use her Presto No. 7, that she has owned since 1947. Recently purchased a No. 5 at an estate sale for $12.00 in like new condition with all accessories.
Never knew to check the dial gauge until this year, 2013 from info on the internet, checked it for the first time, it is fine. You could buy your own gauge to check against or just convert to weight, its so easy! No. 5 and No. 7 are very similar. Also, so as not to baby sit it anymore while canning, I switched out the vent tube for one from a Mirror Matic tube and weight, with no problems or complications what so ever, works great and the dial gauge reading checks with the weight. Parts cost me about 5.00 on Ebay. Seals are available anywhere from .99 to 10.00 or so. Yes, you can and can and can!

I know this is an old post, but wanted to share for posterity. Some good info here, some not so much.
I like these old canners and have my grandmother's and a couple more.
Do yourselves a favor, don't waste your money on new gauges, or even having them checked at the extension office. Buy a new vent tube/stem, Presto #1058, and install it in place of the pressure regulator knob stem or the gauge. Then you can install the new 3 piece Presto weight set #50332. Then ignore the gauge, or even plug it off. You won't be sorry, no more baby sitting the doggone gauge! And the weight is more accurate too. I am so glad I did.