LOGO2 Bridge & OPAMP

Enzyme Assays - Measured by changes in solution conductivity

     Simple, low cost, reliable

            EASY TO BUILD

 

Applicable for blood & urine clinical tests, industrial, school & college projects etc.

Photo 1 whole setup

The instrument employs a square wave (27kHz) AC bridge with two electrochemical cells, a sample & a reference. This allows differential operation with increased sensitivity & stability.

Measures in the micromolar range.

The temperature is controlled to 0.02 deg C. COST ca. £200

 

 

The site is constructed with long pages. Links are underlined in blue. For sub-pages hover to the RHS of Menu items eg. to RHS of HOME is the sub-page Applications

 

            News & Updates will be found further down this Page

 

This site gives a clear explanation as to the design construction and use of              

                                  the conductivity apparatus

 

The Menu items describe the range of content (remember the sub-pages!)

 

 

RESULTS describes kinetic measurements with urease &

                             alkaline phosphatase      

 

 

The Basic Introduction can be skipped by those who are familiar with

                           solution conductivity/enzyme assays.

 

Go to Introduction Basic      Go to Introduction Specific & Technical

 

 

 

                     

                           NEWS & RECENT ADDITIONS

 

 

 

 

 

An important addition is the section on low cost electrodes, constructed using pencil leads. These DIY electrodes allow construction of simple easy to make electrochemical cells. 

GO TO LOW COST ELECTRODES

 

OOPS!!! -- the 1M feedback resistor was initially omitted from the thermosttic control circuit - now present - apologies

 

The original design for the bridge used a long-tailed pair differential amplifier to extract signal. This has been replaced with a 741 OPAMP which gives better common mode signal rejection [important since the input square wave has a transient on the rising edge] & lower noise.

 

The null voltage at balance is not zero but about 25 mV, owing to carbon electrode polarisation. Previously the value was higher but has been reduced by increasing the square wave frequency from 10 kHz to 27 kHz. This decreases the time available for the electrodes to polarise on each cycle and so decreases the impact of polarisation.

Go to Low Cost electrodes

 

 

CALIBRATION, LINEARITY & SENSITIVITY

           see subpage to RHS of RESULTS

 

Following on from detailed studies of electrode polarisation the perfformance of the instrument has been re-assessed. Guide-lines have been evolved that allow conductivity measurements to be made over a 100,000- fold range with a single set of electrodes.

 

IMPROVEMENT TO THE ENERGY EFFICIENCY use of PWM

 

The energy efficiency of the thermostat system could be improved by perhaps 20-30% if pulsed width modulated (PWM) heating was used in place of continuously variable heating. This is currently being tested but has, as yet, not proved able to control the temperature as accurately as the system currently employed.

 

USE of SYSTEM IN A HOT CLIMATE

 

At present when the the apparatus thermostat is set to 25 deg C the system can be used at a temperature up to about 23 deg C. Higher than that and the system starts to drift. The solutions to this are :

1) To increase the working temperature (e.g. to 30 deg C) or 2) To use a cooling method to keep the apparatus below the target temperature.

The former will require calibration at the temperature used, while the latter will add to the cost. PWM thermoelectric cooling is currently being investigated as a viable solution.

 

GO TO RESULTS    GO TO INTRO BASIC   GO TO INTRO SPECIFIC

 

 

 

 

 

 

 

 

 

 

 

 

 

Larger version can be found under

               Equipment Menu