I have 3 of these now and they work great. I mount two of them inside the body of my guitars to track the humidity. One of them keeps going through batteries though. I think that's the fault of the cheap Amazon batteries I've been using in it.

The Short: I have one of these data loggers to monitor the humidity in each of my guitar cases and they work well. It is easy to read the humidity in all cases at once with app, they have long battery life, they are small and can be tossed into the storage compartment, and seem to be accurate when compared with my reference hygrometer. That is all I really need to say.

The Long:
I use the BlueMaestro Tempo Discâ„¢ Bluetooth Temperature, Humidity and Dew Point Sensor Beacon and Data Logger wireless devices to monitor the humidity inside my guitar cases. All of my instruments have humidity control packs inside the case and as long as I keep the humidity packs in shape I have not had to worry about the humidity inside the case. However, I have kept a digital hygrometer in one case just as a reference along with a hygrometer in the room. The case digital hygrometer broke and while looking to replace it I ran across these Tempo Discs data logger that had temperature, humidity, and dew point. I purchased one about a year ago and tossed it into the storage compartment and it has just been working away without any effort from me. Two nice features I really enjoyed about the device is that I do not have to open the case to get the humidity reading and it has a trend so I can see the high and low humidity readings and note that if the humidity is trending downward it may soon be time to refresh the humidity pack.

The system of one hygrometer and multiple cases worked well for a long time. However, I don't play as often as I used to and would occasionally find that I let my humidity packs go dry. Also, my guitars now reside in multiple places and I loan a few of them out. I have purchased additional devices and now have one Blue Tempo in each case in the storage compartment and I can check the humidity of all the guitars in the same building (specs say 220 feet) with a quick view on my smart phone. Also, if I loan one out I can instruct the person to use the app to keep an eye on the humidity.

My latest devices came nicely packed wrapped in plastic, placed in a nice tin, placed in a velvet like pouch, all within in a cardboard pouch. A big and welcome change since my first one over a year ago. In addition, the tin is a nice size to store picks and the pouch for other small things such as capos, thumb picks, batteries. If you don't have a storage compartment, toss the device in the velvet bag and keep inside the case. The devices are plastic and should not ding any wood, but having them in the velvet case adds another layer of protection.

The devices are about the size of four quarters (USA) and claims to run one year on a CR2032 battery. I have not had my devices long enough to get a good idea of battery life, but it is nice the app shows the battery life of the device. They devices are small and do not have GPS so don't misplace them. With the app, you can at least tell if you are within 200 feet - if you left the device on.

The devices are ready to go, just pull the tab on the device that protects the battery from being in contact and turn it on. There is a push button on/off that is nice if you are not going to use the device for a while. Download the app for your smartphone, bring up the app, and like magic, they all appear. There is no setup needed. So far, no calibration has been needed, they all pretty much match my reference hygrometer (with 0.5%). The logging interval is user defined from 1 minute to 18 hours and I have found a recording period of 15 minutes works well and provides a nice trend. I also named each device the guitar make/model so I know which device is in which guitar. Note the name and any calibration is stored on your smart phone or tablet - not internal to the device. For my loaners, the person only has one guitar so a device name is not important.

Please note that you do need to read the data with the Blue Tempo app, these are not push devices and they do not send to your smart phone. Blue Maestro does offer a internet gateway that connects with your internet router via WiFi and Blue Maestro sensors using Bluetooth and forwards sensor data over the internet to the Blue Maestro Cloud where it can be accessed anywhere. I have not tried this type of access and for my occasional monitoring remote access is not a problem for me since I am generally in the same building.

The application is available for both for both IOS and Android. The app does great job of picking up the signal and displaying realtime information. You can view all of your devices at once along with the current readings and status such as battery level, log count and logging interval and there is no limit to how many devices you can monitor at once. With a few clicks, you can get the history from any device, up to 6000 points, and display charts of temperature, humidity, dew point, and pressure (device dependent). With the graphs you can choose the period to show at the push of a button and quickly scan through the graph's timeline. Each metric measured by the device is graphed both separately and on a combined graph including temperature, humidity, and dew point for comparison purposes. The data also includes high, low, and average readings. You can also export a "˜ csv' file of the log data to a email address or to the cloud. The only time I did this was to compare devices stored in the guitar case storage compartment, in the case itself, and a reference data outside the case. Just for information, I found very little difference if the device in the storage compartment or just in the case. I don't measure the inside of the guitar since that is where my humidity packs live.

The app also lets you set the device name and humidity and temperature calibrations. Again, do note that these are stored with your smartphone or tablet. Be sure to get the latest version of Blue Tempo 2 for your mobil device. You might also try to get the Blue Tempo Utility since it has additional features.

Customer service has been really great. There was a reading issue with my first one and they immediately replaced it. They seem to update the app on a regular basis and the app gets better with each generation. Any email correspondence has been quickly addressed and they are open to suggestions and recommendations.

Blue Maestro offers a wide range of sensing and logging solutions. Look them up on the internet and see what is available. They offer cloud monitoring solutions and are open source and can provide SDKs and APIs to design your own mobile apps or integrate the devices into third party systems.

Pros
Easy to connect to device, no setup required.
Easy to adjust data collection interval
Quick to read multiple devices at the same time
Small and lightweight
Quick to respond to temperature and humidity changes.
Have on/off button and airplane mode
Great customer service

Wishes
Save name and calibration on the device and not on your smart phone or tablet.
You can select temperature units (C or F) but it would be nice to have the ability to select barometric units if the device monitors pressure.
Sometime on the tablet the trend does not take up the full display in landscape mode. This issue seems to go away when you get a few thousand data points.
Built in GPS - well, not really but it might be handy when you misplace a device.

So now all I need is more guitars...

I'm using this system to monitor a trailer/RV to keep an eye on the pipes and furnace in subzero temperatures, and prevent mold. I think anyone with a mobile application, mold, or EMF concerns should consider this product. For me it works best simply as a local Bluetooth monitoring system, with graphing and historical data ability.

Be careful when you open your order--they're just tiny things wrapped in twisted bubble wrap, without any logos, boxes, or flyers to differentiate the sensors from package filler.

Pros:
-Outside of weather station systems like Lacrosse, this is the only wireless sensor I've found which offers a dew point calculation. Knowing the dew point is essential if you're concerned about high humidity, and particular mold growth. Relative humidity fluctuates with temperature and can be very misleading if read alone.
-My sensors didn't require calibration, and were within a degree or percentage point from each other straight from the package.
-You can customize your broadcast strength, interval and recording interval through the app to impact battery life, frequency strength, and how much data you want. You can also put them in airplane mode where they record but don't broadcast.
-Graphing in the app is very intuitive and allows you to see dew point and temperature data overlayed on top of each other, which shows you at a glance when condensation occurs
-The sensors transmit via Bluetooth so you don't have to have internet or wifi to monitor them, just a mobile phone or other device with Bluetooth and their app.
-It was easy to setup assuming you download the correct app Tempo Utility (which I didn't at first-I was trying to use a different Blue Maestro app)
-Support is pretty responsive if you call during U.K. Business hours

Cons:
-I've yet to figure out how to get the app to show me anything more than the number of times a custom alert event has happened. I can't see the alert event data or get real time notifications, though I think I should. I've got a support request in on this at the moment.
-Remote monitoring (out of Bluetooth range) requires a wifi gateway (currently only sold through their website) and obviously wifi, and a cloud account
-The cloud monitoring system doesn't have much documentation and is a bit buggy.

I bought this to measure temperature and humidity conditions in my crawlspace - I didn't want to spend almost this much for a 1990's era 915mhz "weather station" when I could get something this cute and simple.

Overall it works well, though the companion app could use some work. Some recommendations would be to improve the graphing function (why doesn't it auto-range when you open it?), and improve the explanation of the "reference date and time" and how it relates to clearing the stored data. I had a few rounds of clearing data, reseting the reference time, and other fiddling before I finally got it logging time-accurate data. Maybe a scripted action to "clear data, set the time, and begin logging now" in one action?

The hardware itself seems good so far - no change in battery life over the first few days, and the battery arrived at 100%. Range is good, about as much as I expected from such a tiny device. As long as I'm in a room roughly over the crawlspace my iPhone 6 connects easily.

The other offerings from Blue Maestro look interesting - if the user experience was tuned up a bit, I could see myself buying a few more of their sensor units for other purposes at home and work.

I think I wiped Amazon out of these for a bit. Ordered dozens - no complaints from those I have distributed them to.

As previous reviews have noted they needed to be restarted after setup to ensure they gather more than a few data points. The two units I have personal experience with agreed with my Fluke thermometer for temperature measurements (+/- .2ºC) and were fairly close to the weather service readings when used outdoors for %RH. They were within the accuracy for %RH which, admittedly can be hard to measure in high humidity environments. (This is the reason for 4 stars for accuracy but it is more a limitation of how %RH is measured than any fault of the device.)

Customer support, after an initial delay due to being very busy, has been very good and all of my technical questions have been answered. Also, they have been super responsive to my business oriented needs.

Apart from the one issue with needing to restart the devices once so they would generate continuous data logs there have been no issues so far. We are collecting data by the minute which, honestly, is too much data unless you are actively analyzing it. The units I have ordered are being used to monitor for conditions that would create condensation and are seeing fairly close agreement with real world condensation.

Will continue to order as needed.