The EPROM eraser I use was purchased from eBay for $20 a number of yers ago. These days you can still find them for as little as $29 plus shipping. Despite it's low price, it's served me well over the years. There's no documentation with it and you have to be careful with getting the timing right to fully erase the EPROM without frying the chip. The time varies based on the eprom type and even between manufacturers of the same eprom.
To determine the amount of time to erase, I place a chip in the eraser and pull it out every minute, place it in my Pocket Programmer and check to see if it reads as erased. If a do a few of one type and some aren't fully erased, I'll bump up the time needed until all are erased.
Setting the dial at 1 keeps the UV light on for about 10 minutes - far longer than is needed for any of the eproms I've erased. Although you can set the dial for less than 1, the timer on the unit is not terribly accurate. I'm using it more as a safety to not go WAY over the needed time if I happen to forget it's on. For the time being I'm keeping track of the time manually using a digital timer as well.
A method I found on the web says once it reads erased, to multiply the time taken by four and use this time to fully erase the eproms. I don't do this. I found this length of time is too much and frys the chips. If a chip is fried, when inserted into the programmer it reads as "the device is not correctly inserted in the socket" - even though it is.
Below is a list of times that have worked for me:
NEC – 5 mins
Fairchild – 5 mins
Motorolla – 8 mins
8516 - 3 mins
Fairchild – 5 to 7.5 mins
Intel – 5 mins.
HN422732G – 5 mins.