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How to select a proper SD card

WebHMI requires SD to work. On this page we will explain how to select a best card that will suit your needs.

SD cards comparison

There is a wide variety of SD cards. In addition to the obvious differences, such as brand, formfactor and volume, Secure Digital memory cards also differ in the type of Flash-cells (SLC / MLC), class (4, 6, 8, 10) and overall performance.

SLC-memory is more reliable and has a much more significant resource of cells for writing. The typical number of recording cycles is 100,000 versus 3,000 for MLC memory. At the same time, SLC cards are harder to find, their volume is usually less than that of MLC cards, and the price is higher.

Industrial SD cards provide a wider temperature range, uniform deterioration of cells due to wear-leveling, but have a higher price.

So in each specific case it is necessary to find the compromise between volume, reliability, price, availability. That's why WebHMI comes without a memory card. It is assumed that the card will be selected by the integrator based on the requirements that are imposed on the system.

Please note that using a low-quality or fake memory card may result in data loss or unstable WebHMI operation. DO NOT use cards of doubtful origin. Preference is given to industrial memory cards. When you write a large amount of data, the lifetime of commercial cards in WebHMI can be only a few weeks after which the card goes down.

On this photo: Industrial-grade card (left) works for years under heavy load and this commercial-grade card (right) died after 2 months of work. Both cards are from the same manufacturer.


We tested five different memory cards in order to understand their performance differences applicable to WebHMI. The test consists of writing 60,000 rows into a data table for graphs (5 columns).

The test involved such disks:

  • Silicon Power 4GB Class 4 MLC
  • Silicon Power 8GB Class 10 MLC
  • Transcend 8GB Class 10 Industrial MLC TS8GSDHC10M
  • Innodisk 32GB Class 10 Industrial MLC
  • Apacer Industrial 1GB SLC
  • SanDisk Extreme CZ80 16GB USB3.0 Drive MLC
  • RAM Disk

Here it should be noted that the last two devices are not SD cards. SanDisk is a very fast USB drive, which is given for comparison as an external drive. If necessary, WebHMI can be configured to work not with a SD card, but with an external device. RAM-disk is a virtual storage located in the RAM of WebHMI. It is designed to measure the absolute maximum speed of the database, excluding any slow disk operations.

Test results:

Storage deviceTotal time, secRecords/secTime for 1 record, ms
Silicon Power 4GB Class 4 MLC 2243.11 26.7 37.38
Innodisk 32GB Class 10 Industrial MLC 2202.32 27.2 36.71
Silicon Power 8GB Class 10 MLC 911.17 65.8 15.18
Apacer Industrial 1GB SLC 688.90 87.0 11.48
Transcend 8GB Class 10 Industrial MLC 545.04 110.1 9.08
SanDisk Extreme CZ80 16GB USB Drive MLC 182.89 328.0 3.04
RAM Disk 75.89 790.6 1.26

According to the test results, it can be concluded that the Apacer SD Industrial Extended Temp SLC 1GB memory card should be used in most projects where an extra large archive is not required. This card will provide about 80 records per second in the database, which is even higher than the card of the 10th class of Silicon Power.

At the same time, the use of MLC-memory is advisable in systems with higher requirements for recording speed and/or the need to store a large amount of data. Or where the SLC memory does not fit into the budget of the system.

Test your card

How to find a faster card? Test all of your candidates!

WebHMI has a built in benchmark to test performance of a particular card. Go to page Maintenance→Disk Usage→SD card speed test and press Run test button. Test runs up to 1 minute. After test will be completed you will see how many writes to database per second your card can handle.

Here is example of such test run:

2017 Update

After a few years of running various cards we have found that Transcend Industrial 1GB SLC cards suit our needs in most cases. So we can sell these cards with WebHMI by request.

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