Mysterious workings afoot

This is one for those of you who love to solve riddles. I don’t – probably because I’m not all that good at it – so I’m looking to you, gentle readers, to restore my sanity. Because there are mysterious workings afoot and I just can’t figure them out.

So, bear with me briefly, while I give you a bit of background information.

Miss Z is fed bags of perfectly nutrionally balanced formula, using a feed pump that delivers the formula direct to her jejunum (which is part of her intestine). Because the jejunum isn’t designed to accept large amounts of food all at once, like the stomach, she is connected to her feed pump for 16 hours a day as it slowly pumps the formula in at a rate of 60ml per hour. The continuous feed thing is a bit of a pain, but we manage pretty well and it doesn’t seem to prevent Miss Z from going places or doing things.


Feed pump, tubing and bag of formula – all set to go

We see a dietician ever six months to assess how much formula Miss Z is receiving and if the amount needs to be increased based on her growth and activity levels. At the moment, she is on 1,000ml per 24 hours. Since the formula bags are 500ml each, we have divided it up so that she receives one 500ml bag during the day and the second 500ml bag overnight.

That all makes sense right? In fact, it sounds pretty easy, doesn’t it?

But here’s the thing, when I give her the two bags in a 24 hour period, it never, ever totals up to 1,000ml. Ever.

That too, seems completely logical. After all, there is a long tube that connects the bag to Miss Z, so even when the bag is empty, there will be some formula remaining in the tubing. The pump has a safety feature that stops it and sets off an alarm when the bag is entirely empty and air is starting to be pumped into the tubing to prevent air instead of formula being pumped into Miss Z, so she never receives the full amount in the tubing.


A giving set – aka the tubing

However, here is the mystery. Every day I do the same routine with Miss Z: one bag of formula overnight, one bag of formula during the day, change the tubing, and repeat.  And yet, every day she receives a different amount of formula.

Some days, it comes up short. Assuming that the tubing can hold around 10-20ml of formula, you would think that it would be short by around 20ml – so she would have a daily total of 980ml. But no. Some days it only reaches 950ml or even less.

I could account for this shortfall with the formula in the tubing and perhaps a bit of spillage and wastage, except that it gets even more mysterious. Some days she receives over 1,000ml!

This morning, for example. I put on her overnight feed before I went to bed last night. She got new tubing (the tubing, or “giving set” as it is officially known, has to be replaced every 24 hours) and a new bag of formula. I filled the tubing with formula (known as “fill set”) before connecting Miss Z. This morning at around 7am, the pump stopped and the alarm went off because we had reached the point where there was air in the tube. However, when I checked the pump to see how much she had received overnight it said 522ml. So, how did she get 522ml from a 500ml bag?!

The most logical answer to all this would be that the bags of formula contain different amounts. However, I’m doubtful about this because the formula is only available on prescription and provided by a medical company, arriving in sealed 500ml bags, so it would be alarming if they weren’t careful with their volumes. To test this theory, I weighed several of the bags and they all weigh the same amount, so that is unlikely to be the problem.

So, tell me, how can I do the exact same thing when feeding Miss Z every day, but one day I only get 950ml out of two 500ml bags and the next day I get 1,050ml?

Go on then, solve my mystery and put me out of my misery!

(I should probably note that I’m not concerned if Miss Z receives slightly less formula one day and a bit more the next. She is a healthy weight and growing like a weed. Plus, none of us consume the exact same amount of calories every day, so there is no reason for Miss Z to do so. As long as we come close to the target amount, it isn’t a worry. I am just very, very curious as to why it varies so widely!)



2 responses to “Mysterious workings afoot

  1. Catherine Heyd

    Ok, Geoff has two theories.
    1. The scale you are using may not accurate, source of error. 50 ml isn’t very much.

    2. The pump also has a source of error, which may or may not be different for each feeding, therefore could be +/- 50 ml


    • Good theories! I’m a bit skeptical about the scales being off – surely if they were off, then all the formula bags I’ve weighed would be off by the same amount? But I agree about the potential for the pump to be a source of error. I hadn’t thought of that! And the fact that it is turned off and on throughout the day seems like it would increase the risk of error.


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