Detection system for measuring absolute emission rate from large-area-coated $\beta$ sources has been indigenously developed. The system consists of a multiwire-based proportional counter with gas flow and a source mounted within the sensitive volume of the detector. Design of the counter enables efficient counting of emissions in $2\pi$ solid angle.A provision is made for change ofthe source and immediate measurement of source activity. These sources are used to calibrate the efficiency of contamination monitors at radiological facilities. Sensitive area of the detector covers 165◦ solid angle nearing $2\pi$ of emission from the source of size $100 \times 150$ mm. Performance of the chamber is tested using collimated $^{55}$Fe X-ray source and $^{90}$Sr / $^{90}$Y coated $\beta$ sources of various activities. The activity measurement system is established as a national primary standard for calibration of coated $\beta$ sources at Radiological Laboratory at BARC. Design and performanceof the chamber are presented.