J
April 11th, 2002, 09:23 AM
The APAN thread has prompted me to write this. I made a set of scales accurate to +/- 0.1g in around an hour for virtually nothing.
It consists of an electric motor from an old cassette deck. The two halfs of the beam (just some equal lengths of wooden dowel) are stuck onto the front (oposite end from the spindle) at a downwards angle. The angle determines the accuracy of the scale; the smaller it is from horizontal, the more accurate it is.
A hole is drilled in a length of square wooden dowel around a cm from the top, just smaller than the motor spindle diameter. The motor is pushed in, no need for gluing if you've got a tight fit. The other end of the wooden dowel is screwed onto a base of some kind.
Holes are drilled at each end of the beam, and containers are suspended with thread to act as containers for the weights and stuff to be weighed. I used a couple of plastic scoops that come with tubs of protein powder, with the handles cut off.
A cocktail stick is stuck on to the back of the motor, just below the spindle, and pointed downwards along the wooden stand. With a ruler, a straight line is drawn on the dowel underneath the stick.
When the stick is not directly parallel with the line, you know that the scales are not balanced and to add more weights/chems until it is.
For weights, there have been some good suggestions already. A penny (British) weighs exactly 1/8th of an ounce (3.5g), according to a friend of mine who has a reason to know this <img border="0" title="" alt="[Wink]" src="wink.gif" /> Another idea would be to use .177 airgun pellets, although the weight varies according to the brand.
To make accurate gram weights, you'll need a weighing machine already, but it doesn't have to have the accuracy of this homemade device. I use a set of 'diet scales' with 5g resolution. These are great for measuring out larger quantities of chems, and cost around �5.
Take a length of fairly thick wire (gardening wire will do fine), and keep adding off cuts to the weighing scales until you have exactly 10g (or 100g if you want better accuracy and have plenty of wire). Find out the total length, and calculate the lengths required to make up 1g and 0.1g weights (and even 0.01g if your scales are accurate enough, and you need this accuracy). There are your weights.
The advantage of using an electric motor is that it's a ready made low friction spindle. The tape deck ones are good, because they are large enough to stick the beam to, and they have very low friction because they're designed to be quiet. This is longer lasting than a spring based device (all springs stretch over time), and it's easy to see if it's not calibrated.
If I haven't explained something clearly enough, please let me know.
<small>[ April 11, 2002, 08:25 AM: Message edited by: J ]</small>
It consists of an electric motor from an old cassette deck. The two halfs of the beam (just some equal lengths of wooden dowel) are stuck onto the front (oposite end from the spindle) at a downwards angle. The angle determines the accuracy of the scale; the smaller it is from horizontal, the more accurate it is.
A hole is drilled in a length of square wooden dowel around a cm from the top, just smaller than the motor spindle diameter. The motor is pushed in, no need for gluing if you've got a tight fit. The other end of the wooden dowel is screwed onto a base of some kind.
Holes are drilled at each end of the beam, and containers are suspended with thread to act as containers for the weights and stuff to be weighed. I used a couple of plastic scoops that come with tubs of protein powder, with the handles cut off.
A cocktail stick is stuck on to the back of the motor, just below the spindle, and pointed downwards along the wooden stand. With a ruler, a straight line is drawn on the dowel underneath the stick.
When the stick is not directly parallel with the line, you know that the scales are not balanced and to add more weights/chems until it is.
For weights, there have been some good suggestions already. A penny (British) weighs exactly 1/8th of an ounce (3.5g), according to a friend of mine who has a reason to know this <img border="0" title="" alt="[Wink]" src="wink.gif" /> Another idea would be to use .177 airgun pellets, although the weight varies according to the brand.
To make accurate gram weights, you'll need a weighing machine already, but it doesn't have to have the accuracy of this homemade device. I use a set of 'diet scales' with 5g resolution. These are great for measuring out larger quantities of chems, and cost around �5.
Take a length of fairly thick wire (gardening wire will do fine), and keep adding off cuts to the weighing scales until you have exactly 10g (or 100g if you want better accuracy and have plenty of wire). Find out the total length, and calculate the lengths required to make up 1g and 0.1g weights (and even 0.01g if your scales are accurate enough, and you need this accuracy). There are your weights.
The advantage of using an electric motor is that it's a ready made low friction spindle. The tape deck ones are good, because they are large enough to stick the beam to, and they have very low friction because they're designed to be quiet. This is longer lasting than a spring based device (all springs stretch over time), and it's easy to see if it's not calibrated.
If I haven't explained something clearly enough, please let me know.
<small>[ April 11, 2002, 08:25 AM: Message edited by: J ]</small>