I operate in a wastewater treatment plant laboratory and I use respirometric method for BOD5 measurement (hach direct plus). Recently I am obtaining high value for BOD (biological oxygen demand) for example BOD5 581 mg/L when the COD (chemical oxygen demand) of the same sample 895 mg/L. I did multiple assays on another sample and I obtained different values for the same sample using the same initial volume (209 mg/L and 586 mg/L while COD 630 mg/L). I thought that some sensors are not functioning properly. I want to know if these pressure sensors can be calibrated, anyone can help please??