Brymen BM257s multimeter

Brymen BM257sI had already bought a digital multimeter in the late 1990s, specifically a Mastech MS8200G. This model is still available today under a different name and with a dark housing instead of a light one as “Testboy 313”, but with an otherwise identical construction. Generally the device still works, but the accuracy is questionable after more than 20 years, the resolution is only 2000 counts (0-1999) and there is no automatic range setting. So I bought a Brymen BM257s to replace it.

Brymen is a Taiwan manufacturer specializing in multimeters and measuring equipment. See also http://www.brymen.com. The devices are not only sold under their own name, but also as an OEM variants by other companies, such as the Metrel MD9016, Greenlee DM-200 or Greenlee DM-510A. There is even an “EEVBlog edition” of the BM235.

The BM257s is a very compact device from the 250 series including capacity measurement, recording of the values for determining and minimum, maximum and CREST mode. In addition it also provides contactless measuring of electric fields.

I bought my copy privately in June 2021 from Welectron, who are a distributor of Bryman devices in Germany. The label also contains the name “Welectron.” but otherwise the device is still labeled as “Brymen BM257s”. I chose the “Edition +” that comes with the BL21S2-T4SC silicone test leads and the BMP-25x bag. I also got the BRUA-20X adapter for data transfer.

Construction and workmanship

The device is in a removable cover in which a stand is also integrated so that it can be set up at an angle. The measuring tips can be clamped to the back if necessary. The original color is red, OEM versions are also offered with a green or blue cover.

The test leads are very handy: the silicone sleeve makes them very flexible and screw-on adapters are included for the tips, with which you can plug them directly into banana sockets. This also enables the use of additional measuring accessories, such as alligator clips.

Two AA batteries are used for the power supply. Such batteries are usually easier to obtain and last longer than 9V block batteries. There are also two ceramic fuses – one 0.4A/1000V and one 11A/1000V. Overall, the structure is very good and the safety classification of the device (300V CAT IV, 600V CAT III, 1000V CAT II) has been tested by the UL.

Also see the report from EEVblog:

Technical data

The resolution is 6000 counts (0-5999), the accuracy depends on the measurand and the measuring range and is between 0.2% and 2%. The exact values can be found in the manual, which is also available online on the Brymen website in the download area.

The multimeter generally works with automatic range setting. However, you can also define the range manually if needed.

The BM257s offers the following options:

  • Direct and alternating voltage up to 1000 V (including measurement of mV).
  • Direct and alternating current up to 10 A (including measurement of μA and mA).
  • Resistance up to 60 MΩ.
  • Capacity up to 3000 μF.
  • Frequency from 5 Hz up to 1 MHz at 5 V or 5 Hz up to 500 kHz at 3 V.
  • Diode test at 1 V.
  • Latched continuitiy testing with acoustic signal.
  • Temperature in °C or °F with an external type K thermocouple from -50 °C up to +1000 °C or -58 °F ip to +1832 °F.
  • Electric field with non-contact measurement (EF).
  • Short-term measurement with automatic holding of the maximum and minimum value (CREST).
  • Recording of the measured values to determine the minimum and maximum (REC).
  • Difference measurement based on the current measured value (REL).
  • Automatic measurement of low impedance (LoZ) for DC voltage, AC voltage and resistance.
  • Display backlight.
  • Acoustic warning if the test leads are plugged into the wrong sockets.
  • Holding the currently displayed measured value (HOLD).

Data transfer

The multimeter can send the display content as a serial data stream via an optical interface. A separate adapter BRUA-20X is required for this. To activate the transfer, the device must be switched on with the HOLD button pressed.

The transfer parameters of the adapter are: 9600 baud, 8 bits, 1 stop bit, no parity. The scope of delivery includes an RS232-USB adapter that provides a virtual COM port.

Fortunately, the protocol is precisely documented by Brymen, so that one does not necessarily have to rely on Brymen’s software. The documentation is supplied as a PDF document together with the software for the adapter. Alternatively, the documents can also be found on the Brymen website in the download area. sigrok also supports the device.

What means “latched continuity testing”?

With a simple continuity test (as it is also implemented in the Mastech MS8200G) you often hear a slight “scratching” which is caused by the fact that the electrical resistance between the measuring tip and the surface to be tested fluctuates minimally.

If there is a latch, switching off of the acoustic signal is minimally delayed and this delay is always restarted as soon as a continuity was detected. This gives you a continuous acoustic signal which can always be heard clearly, even if the contact was only very brief.