Converting TDS to Microsiemens

You can't, the two are not necessarily directly related.
Microsiemens is a measurement of conductivity, TDS is total disolved solids...the disolved solids may or may not be conductive.
In water treatment there are many parameters which are interlinked, like ph can effect conductivity but not in a linear way.
I recent measure how the conductivity increased as citric & ascorbic acid was added to water, the conductivity rose sharply as acid was added in a fairly linear way, and then it started going down again!

I'm sure some of the water treatment guys here can tell you more...

Del

Not that I am jealous on the GA that you have received for:

I recent measure how the conductivity increased as citric & ascorbic acid was added to water, the conductivity rose sharply as acid was added in a fairly linear way, and then it started going down again!

...but I find it completely un-orthodox to measure the conductivity of your "five o'clock tea" and, even more, to squeeze a lemon and add C vitamin just to get your kicks

For starters, conductivity varies with temperature and total dissolved solids doesn't.

As a rule-of-thumb, TDS in ppm multiplied by 1.5 gives conductivity in μS/cm for tapwater at room temperature.

dear brothers

there is proper conversion from PPM to microsiemens.

but the often conversion factor used in water treatment is 0.64.if u want convert ppm to microsiemens then Divide the parts per million(ppm) value by 0.64 to convert to a conductivity value

You can only use this conversion if you know something about the constituents of the water. The conductivity will get closer to the TDS value when you measure very high levels of TDS as found in sea water and the value may be 0.9 not the 0.6 value. The 0.6 (+/- 0.1) is used often in values where TDS is less than 3000 mg/L. Still you must be careful. If the dominant anion happens to be SO4^-- it will really throw off this ratio of 0.6 that is often used as a rule of thumb guide.Each constituent parameter contributes to the conductivity to various weighting factors and valances. You can find some of these factors in a Standard Methods book.

If the conductivity is standardized to 25 degrees C some major ion balances contribute these values based on balancing milliequivalents. It is easy to balance a rule of thumb conductivity and I would use it to see if the lab is giving me good data. In fact a good lab will do the calculations and provide them as their own QC. The balance should be equal for both the anion and cation side and I would accept the balance in most water tests if they were within 3% or less. We assume the conductivity is a balance of negative and positive charges. As pointed out there are things that do not contribute or weakly contribute to the conductivity (silicon, organic parts) but can greatly contribute to the TDS. SO4 will only contribute about half its anion real value to the conductivity. If you have purchased a small pen conductivity meter, assure it is temperature compensated. The pen TDS meters are really the same conductivity meter with a compensating factor of 0.6 or somewhat close. A lab conductivity meter applies a standard method procedure and that usually means bring the sample up to the 25 degrees C before testing. TDS is also measured by a standard method procedure and means using an analytic balance to measure residuals after evaporation.