Surprised at stability check on cheap Hanna pH meter

So I took a chance on the HI99104 meter, hoping I could get something workable for less than a hundred bucks. I've used it for about 6 brews now, and after learning how to use it consistently decided to run a stability check. Having sat in storage for the last week it was reading 4.10 and 7.07, so I calibrated to 4.01 and 7.01 at 20C. I did take temperature readings during the tests, but due to the entire spread being 20.0 and 20.6 I am not going to add those to the data table. Please note that the first 20 minutes of readings were done with the probe left in the 7 buffer, and all subsequent readings where done after the probe was rinsed in DI water, placed in the storage cap and rerinsed before being placed in the buffer 5 minutes before the reading was noted. This is because this meter is pretty slow, it consistently takes that long to stabilise at a value. Some of the readings with a 30 minute difference were given 10 minutes to stabilise, just to see if there was a difference between 5 and 10 minutes to read. There was not. On to the data.

pH. Time(minutes from calibration)

0. 7.01
2. 7.01
5. 7.01
7. 7.01
10. 7.01
15. 7.01
20. 7.01

Beginning of 5 minute readings

30. 7.01
40. 7.01-7.02 pulse (would not settle on one or the other,2-5min)
50. 7.01-7.02 pulse
60. 7.01-7.02 pulse
90. 7.01
120. 7.02
150. 7.01-7.02 pulse
180. 7.02
210. 7.02
240. 7.02
270. 7.02-7.03 pulse
300 didn't get measured as I was busy setting up my brew day
330. 7.02

I stopped testing at this point because i was getting busy.
Overall I feel pretty comfortable using this meter for brewing, especially considering I got it for $40 shipped. Even between samples being tested today I have not seen any buffer reading more than .01 off of initial calibration.

 

I don't usually recommend meters like this because of absence of the ATC feature which, while it doesn't make a lot of difference, can add a couple of hundredths unless you take care that the sample and buffer temps are the same and controlled.

Also, while the data you got looks good for sure, you have really only tested half the meter. The reading depends on stability in offset and stabilty in gain. When calibrating in pH 7 buffer you set the offset and when calibrating with 4 buffer you set the gain (slope). When measuring at mash pH you are closer to 4 (1.2) than to 7 (1.8) so slope is a factor. That's why I suggest stability testing with the 4 buffer - you check on slope stability as well as offset stability. So if you wouldn't mind....

 

I can run a new test with the 4 buffer, but it will likely be the weekend before I have time to do so.

 

I now have four hours of data points from the 4 buffer. Readings are every 15 minutes, with each reading allowed 5 minutes to stabilise in the buffer. Probe was rinsed, returned to storage cap and rerinsed between readings. All readings where within .5 degrees Celsius (18.8-19.3)

Time. PH
0. 4.01
15. 4.01
30. 4.01
45. 4.01
60. 4.00
75. 4.01
90. 4.02
105. 4.01
120. 4.02
135. 4.02
150. 4.02
165. 4.02
180. 4.03
195. 4.03
210. 4.03
225. 4.04
240. 4.04
255. 4.04
270. 4.03
285. 4.04
300. 4.03
At this point seven is reading 7.02, I set it at 7.01 before the test

Edit: one more hour of data added

 

I've found that its the 4.01 and 10pH solutions that drift. My end of the day calibration checks for 7 are usually right on the money.

 

That's still not bad but the larger variation at 4 as is expected for the reason touched on in #2.

 

As noted in #2 (and #6) that's to be expected. It isn't the solutions that are drifting it is the slope (gain) of the meter/electrode which isn't a factor at pH 7.