I installed the 121GW iOS. I think that's the only application available for this meter, am I right?
Actually, I just published a third-party app (
"Meteor for 121GW") for the 121GW on the App Store. I think there also used to be another third-party (manufacturer?) app called "EEVBlog Smart Data Logger", but I don't see that one on the Store anymore.
You're welcome to try my app (it's free), but honestly if you can't connect with the 121GW app then you're unlikely to succeed with mine. All BLE iOS apps use the same system library to detect nearby devices and connect to them when requested, so there's really nothing any third-party app can do to find a meter that doesn't show up in the main app.
Well I tried both ways. I looked for the meter on the iPhone in Settings, Bluetooth. The iphone does not find the meter. Then I tried just the application, it never finds the iPhone as well. I delete the app, and reinstalled it, no help.
A BLE device like the 121GW meter doesn't require (or support) Bluetooth pairing, so there's no need to go to the iOS Settings page to connect to it. Instead, you need to go through the (BLE) scan-list page that comes up in the 121GW app when you first start it up. (This page has the title "Select the 121GW to connect to:"). This page will list any nearby meter that's currently "advertising" its presence over BLE.
If your meter is turned on and you don't see any meters listed on that page in the app, then make sure that you've pressed the "1ms PEAK" button for long enough for the "BT" symbol to appear on the meter's LCD. Once you see the BT symbol on the meter LCD, try the Refresh button in the app to see if anything shows up in the list. If that doesn't work, try turning the meter off and back on, and then repeat the 1ms-long-press / BT-symbol / app-refresh cycle once again to see if that helps.
If none of that works, then you should verify that your phone's Bluetooth is actually turned on, and not disabled by something like Airplane Mode. If Bluetooth is on, then you should try turning it off for a few seconds and then back on again, and then repeat the process above. (Turning Bluetooth off and back on fixes a _lot_ of problems on Android, but I've also seen it fix temporary problems on iPhones.)
If the app still can't see your meter, then the remaining explanations are pretty rare. One is that another device (iPhone, iPad, Android phone) nearby is connecting to your 121GW before your iPhone gets a chance to see it. (Any connection will turn off the meter's advertising, making it invisible to the 121GW app). Or, you could be operating in a _really_ noisy radio environment, and your phone is having trouble getting a clean signal. Or, finally, the Bluetooth radio in your meter could be broken - to check this, there are several apps available on the App Store that will let you detect all nearby devices and will at least tell you if your meter is actually advertising.