How long do boluses last ?
People often ask why boluses don’t last longer when the batteries will keep going for 2 years. This graph illustrates the problem. Our software concatenates the download files into a continuous record and one such is shown here.
This bolus was in a commercial cow and I don’t know what she was being fed. Probably not enough in July given the high pH values. The flat lines are gaps in the data where the computer has just filled in values to keep a continuous record. The daily range all looks about right until November (after 150 days in the cow). Thereafter the range gets narrower until after Christmas at which point the pH tends to drift upward and the daily range gets narrower. I think this indicates reference junction drift. I think rumen liquor has managed to penetrate to the reference electrode which means that the measurement of the millivolt potential difference tends to zero.
In this standard graph of voltage against pH you can see that a signal of 0 volts equates to 7 pH or neutral so as our electrodes begin to measure the same value the potential difference tends to zero and the output looks like 7 pH. So as the bolus ages the pH will tend towards 7 and the dynamic range will get narrower. This is not a problem in fistulates or slaughterhouse recovered boluses that can be washed and recalibrated. We are working on ways to solve this problem called drift in engineering terms but it will take time. For the moment we warrant the bolus to 60 days and advise that 150 days is the maximum length of time to trust the data when the bolus has not been retrieved and recalibrated.
You can read more about detecting drift in a conference presentation given by our founder Prof. Toby Mottram here.