Thomas 6530 Traceable Digital Barometer

For the price, I expected the unit would work for longer than three years. At first, the humidity feature stopped registering. Shortly thereafter, the unit started "chewing up" batteries and would barely last a day. My $30 AcuRite unit is four years in and working like a champion.

Product arrived with an unreadable screen. Given the high price, the failure to test is inexcusable.

An excellent barometric pressure instrument. The algorithm for the readout is a little difficult to understand at first.

had to re-calibrate it but works great now

Thomas 6530 Traceable Digital Barometer offers good performance for it's price. It would be nice if the altitude compensation could be done within 5 meters instead of 10 to allow for more precise calibration. After calibration I over 25 consecutive hourly readings had an average difference of -0.352 mbar against my standard. I rate this barometer a solid 4 stars. More info regarding the setup, calibration and performance of this device is given below for those who may find it useful.Important barometric pressure specs for this barometer :Range: 500 to 1030 mbar (absolute pressure)Resolution: 1 mbarAccuracy: +/- 4 mbar (After completing calibration I have yet to have a reading >1 mbar off)Altitude Compensation: -100 to 2500 metersThis barometer displays a bar graph for 0, -1, -3, -6, -12, -24 hours with change in pressure indicated in 2 mbar increments. High and low readings can be viewed for the prior 24 hours. It also displays relative humidity within 3 % (5 to 75% RH), and temperature to within 0.4 C within a 0 to 55 C range. You can set high and low alarms for these and for atmospheric pressure. The defaults if left unchanged are set at points where alarms are unlikely.For a standard to calibrate against I chose Piedmont Triad International Airport which I live about 5 miles from. Googling “barometric pressure at KGSO” led me to a NOAA web page where current barometric and weather data for KGSO is displayed for up to 72 hours. The page displays measurements made 54 minutes after the hour. Mean sea level pressure (MSLP) is displayed to 0.1 millibar precision. Updated readings are also displayed by the Weather Service on their local area forecast page. Be aware pressure displayed in inches of mercury (inHg) is altimeter pressure which I found to be about 0.03 inHg above MSLP for this airport. The NOAA web page confirmed this.When I pulled the slip of plastic allowing for battery power to be supplied, the initial reading on my new barometer was 986 millibars. This was absolute pressure. Checking the WS forecast page showed a barometric MSLP of 1019 millibars. I knew at this point by calibrating I had to increase the reading 33 millibars. I know some have expressed concern about the upper range of this barometer being 1030 millibars, but adjusting for my elevation the barometer would have to see a MSLP of 1063 millibars (31.39 inHg!) to see the top of it's range. That's not going to happen.The setup is straight forward until the barometric pressure calibration step. The altitude compensation is in 100 meter increments. I thought 100 meter steps pretty blunt and left the default at zero, but if I had set it to 3 the setup may have been a little easier. My home is about 1000 feet above sea level, not too different from 300 meters. When I passed the step inputting altitude compensation I was asked to input something the instructions did not mention, current atmospheric pressure in millibars. Here you add or subtract 5 millibars at a time to get to current pressure, and as I recall I had to hit the up button quite a few times to get to 1020 millibars. I take this the price for not having enter 3 in the altitude compensation step.After exiting the setup I found barometric pressure consistently reading about 2 millibars high. I thought if I re-input the MSLP when it was 2 millibars higher (1021 instead of 1019 mbar) the barometer would read spot on. I re-entered the setup when the pressure was higher and when I got to the point where I had to input current MSLP I was surprised to see the number 290. Ah, I thought, this is the elevation correction factor in meters. Here you can subtract or add 10 meter increments and I subtracted 20. After exiting the setup this time I have found the barometer to read correctly to the nearest millibar usually, but not always. This barometer samples barometric pressure every 15 minutes.Once 45 minutes after the hour the Thomas 6530 read 1011 mbar when the WS read was 1012.1 mbar. 15 minutes later the WS updated to 1011.0 mbar. This confirmed to me the NOAA WS measurements are hourly updated to the new reading made 54 minutes after the hour.The calibration certificate that came with my barometer showed the certification test conditions to be 50.53 % RH, 24.52 C, 806.00 mbar, 909.80 mbar, and 1011.22 mbar. Against these conditions the instrument measured 49 % RH, 24.6 C, 805 mbar, 910 mbar, and 1011 mbar. These barometric measurement responses look pretty linear across the measured range. This barometer stores high and low readings for 24 hours. I put them into a table (which I could not paste in here) along with the nearest corresponding reading from the NOAA website for PTI airport. My readings averaged 0.352 mbar lower than the airport's. I can not tweak the altitude compensation to get any closer to airport readings on average as each 10 meter altitude change works out to a little more than a mbar at my altitude. In the end this digital barometer's accuracy is not bad and complies with all it's accuracy specs, and in fact is well within it's specs for barometric pressure. If I get another digital thermometer I will get one that is larger and more suitable to be wall mounted. I would also want one that is warranted to be within a mbar of accuracy, and to have altitude compensation within 5 meters. The 3rd thing is that it should get time from the National Institute of Standards and Technology's (NIST) HF radio station located near Fort Collins, Colorado, or via the internet. For this I expect to pay several times what I paid for the Thomas 6530 Traceable Digital Barometer.

A bit pricey, and the fold out legs are weak, I had to do a little engineering on that. But It is accurate,and supplements my other weather instruments nicely.